Particle Physics Planet

November 30, 2015

Christian P. Robert - xi'an's og


WariseWhile in Warwick this week, I borrowed a recent issue (Oct. 08, 2015) of Nature from Tom Nichols and read it over diners in a maths house. Its featured topic was reproducibility, with a long initial (or introductory) article about “Fooling ourselves”, starting with an illustration from Andrew himself who had gotten a sign wrong in one of those election studies that are the basis of Red State, Blue State. While this article is not bringing radically new perspectives on the topic, there is nothing shocking about it and it even goes on mentioning Peter Green and his Royal Statistical Society President’s tribune about the Sally Clark case and Eric-Jan Wagenmakers with a collaboration with competing teams that sounded like “putting one’s head on a guillotine”. Which relates to a following “comment” on crowdsourcing research or data analysis.

I however got most interested by another comment by MacCoun and Perlmutter, where they advocate a systematic blinding of data to avoid conscious or unconscious biases. While I deem the idea quite interesting and connected with anonymisation techniques in data privacy, I find the presentation rather naïve in its goals (from a statistical perspective). Indeed, if we consider data produced by a scientific experiment towards the validation or invalidation of a scientific hypothesis, it usually stands on its own, with no other experiment of a similar kind to refer to. Add too much noise and only noise remains. Add too little and the original data remains visible. This means it is quite difficult to calibrate the blinding mechanisms in order for the blinded data to remain realistic enough to be analysed. Or to be different enough from the original data for different conclusions to be drawn. The authors suggest blinding being done by a software, by adding noise, bias, label switching, &tc. But I do not think this blinding can be done blindly, i.e., without a clear idea of what the possible models are, so that the perturbed datasets created out of the original data favour more one of the models under comparison. And are realistic for at least one of those models. Thus, some preliminary analysis of the original or of some pseudo-data from each of the proposed models is somewhat unavoidable to calibrate the blinding machinery towards realistic values. If designing a new model is part of the inferential goals, this may prove impossible… Again, I think having several analyses run in parallel with several perturbed datasets quite a good idea to detect the impact of some prior assumptions. But this requires statistically savvy programmers. And possibly informative prior distributions.

Filed under: Books, Statistics Tagged: blinding, data privacy, maths house, Nature, Red State Blue State, reproducible research, Royal Statistical Society, Sally Clark, University of Warwick

by xi'an at November 30, 2015 11:15 PM

astrobites - astro-ph reader's digest

Sustainable Climates on Greenhouse Super-Earths?

Title: A proposal for climate stability on H2-greenhouse planets
Author: Dorian S. Abbot
First author’s institution: Department of the Geophysical Sciences, University of Chicago
Status: Accepted for publication in Astrophysical Journal Letters

Preserving Life-Sustaining Climates Through Biological Feedback

Recent astronomical surveys have detected many so-called Super-Earths — planets up to 10 Earth masses and potentially with a rocky interior. Many of them orbit their respective stars either far away or very close and some seem to harbour thick molecular hydrogen atmospheres. From an astrobiological point of view this is both a blessing and a curse. At first sight — if you think about Venus, our hellish neighbour with its acidic atmosphere — mainly hydrogen atmospheres seem to be problematic because they would easily enable runaway greenhouse processes and it might be hard to achieve a stable climate as on Earth, which is in general regarded as a requirement for the development of even the most basic life forms. However, the massive amounts of hydrogen could also enable the presence of liquid water on the surface if the planet is outside the zone (planet-star distance) which is traditionally regarded as ‘healthy’!

The latter idea is further investigated in Abbot’s paper, in which he is especially interested in whether the presence of simple biological feedback can enable a stable climate over millions of years. For such a process to occur the chemical back coupling of the biological process (for example, the respiration or metabolism of simple life forms) needs to have a temperature dependence which increases up to a certain temperature and then drops again, like in Figure 1.

Fig 1: Biological feedback depending on planetary surface temperature. Imagine the metabolism of basic life such as bacteria, which process certain molecules via chemical reactions to gain energy. The solid curve is the function which the author uses for this mathematical considerations, whereas the dashed curve can be regarded as the more realistic assumption, where the biological feedback increases slowly with temperature and then abruptly decreases when a certain temperature is reached (imagine the bacteria dying from too much heat). Abbot (2015)

Hydrogen Gain vs. Loss

For a long-term stability coupling of life and atmosphere which enables a roughly constant temperature over millions of years, it is important to understand what happens when there is more hydrogen gain than loss. If there is more and more hydrogen pumped into the system, for example by outgassing from the planetary interior, such as volcanism, then the biology of the planet has to act in a way such that it gains energy from a hydrogen eliminating process, to prevent dramatic increase in hydrogen in the long term (this would increase greenhouse effects, eventually leading to a runaway effect). Now, if any perturbation of the system occurs, like changes in stellar radiation or heat from the interior by radioactive decay, the coupled system needs to be able to somehow compensate these changes. In order to achieve this, the gain in hydrogen cannot be higher than what the biological process can compensate for (see Figure 2).


Fig 2: The temperature change dΦ/dτ of the planet caused by external perturbation versus non-dimensional planet surface temperature Φ. β is a measure of the amount of hydrogen gain. The solid lines indicate the temperature behaviour due to biological feedback. The arrows indicate the direction of the temperature change at a certain perturbed starting temperature. For example, a start at Φ = 0.1 would lead to an increase of temperature by the biological feedback. Top panel: If the environment conditions are right, the system can reach a stable point (circles) where the biological process consumes just enough hydrogen to keep the equilibrium temperature. Bottom panel: If there is too much hydrogen gain the system ‘overshoots’ to an indefinite increase in temperature. Abbot (2015)

Ok, what does this mean? The setting must be ‘friendly’ enough, such that the biology can compensate for all changes to the usual temperature. For example, if we have simple bacteria metabolising on a planet with active volcanism and out of a sudden the radiation from the star increases, the bacteria have more energy to proliferate. The increased number of bacteria can then account for the increased energy budget and compensate for the changes by lowering the abundances of hydrogen, which in turn decreases the greenhouse effect.

The story for a net hydrogen loss is the exact oppposite. An example would be a planet from which the atmosphere is stripped away by extreme ultraviolet radiation (XUV) from the central star, which for example can be emitted by young M dwarfs. If the planet in sum loses hydrogen, the active biological process on the planet must gain energy from a chemical reaction which adds hydrogen to the system. However, if there is a change and hydrogen is somehow stripped away from the planet the biological process must add more and more hydrogen to keep the equilibrium.

Climate Stability in Numbers

The most important outcome of this study is to show that it is in principle possible to sustain a stable climate at very unfavourable conditions. For extreme planet-star distances the presence of liquid water on the surface is usually regarded as unlikely. But even under these conditions the biological back coupling of any form of life and the atmosphere can lead to sustainable life-friendly conditions. However, for this to happen the environment conditions (for example nutrients) need to be balanced and the temperature changes to the system (for example massive volcanism or increased stellar radiation) cannot be too extreme.

With upcoming planet surveys it might actually be possible to observe this effect and understand if this feedback loop is happening out there! If we could measure the surface pressure of hydrogen on many hydrogen dominated Super-Earths (which is hard enough) and find that an unexpectedly large number of planets far away from their stars have the correct amount of molecular hydrogen, this would mean that some sort of climate-stabilizing feedback could act on these strange and far-away worlds.


by Tim Lichtenberg at November 30, 2015 07:08 PM

Peter Coles - In the Dark

Away Days

No time to blog today as I am at yet another Awayday. In fact I will be Away for Two Days.


Can anyone name my location (in the photograph above)?

by telescoper at November 30, 2015 06:51 PM

Emily Lakdawalla - The Planetary Society Blog

Fall issue of The Planetary Report is Here!
At last! The fall issue of The Planetary Report is off the press—or ready for Planetary Society members to download now.

November 30, 2015 04:03 PM

Tommaso Dorigo - Scientificblogging

CERN And LIP Openings For Graduate Students In Physics - Good $$$
Have you recently obtained a Masters degree in a scientific discipline ? Are you fascinated by particle physics ? Do you have an interest in Machine Learning developments, artificial intelligence, and all that ? Or are you just well versed in Statistical Analysis ? Do you want to be paid twice as much as I am for attending a PhD ? If the above applies to you, you are certainly advised to read on. 

read more

by Tommaso Dorigo at November 30, 2015 02:53 PM

Emily Lakdawalla - The Planetary Society Blog

Favorite Astro Plots #3: The rate of lunar cratering
The third entry in my series of blog posts about Favorite Astro Plots contains one of the biggest discoveries from the Apollo program -- as well as one of the biggest questions in planetary science. The chart was nominated by planetary scientist Barbara Cohen. It has to do with the ages of surfaces on the Moon.

November 30, 2015 01:03 PM

Lubos Motl - string vacua and pheno

Leptoquarks may arrive: LHC to prove \(E_6\) SUSY GUT?
The most conservative stringy scenario to explain all the anomalies

The LHC has glimpsed numerous small anomalies. Some of them may be easily related to leptoquarks.

For our purposes, we define a leptoquark as a new elementary spinless particle that is capable of decaying to a lepton and a quark. So it is not a bound state of a lepton and a quark, it is a genuinely new elementary particle, but it carries the same quantum numbers as such a bound state would carry. We want the decay to be allowed by statistics (and by all other possible constraints) – so the new particle has to be a boson.

In Summer 2014, the CMS has observed a 2.5-sigma excess in the search of leptoquarks, suggesting that a leptoquark of the mass around \(650\GeV\) might exist.

But there exist more well-known anomalies that have been used as evidence in favor of the existence of leptoquarks.

In particular, it has been known for years that the observed muon's magnetic moment or, equivalently, its \((g-2)\) is slightly different than the Standard Model predicts. The expected relative precision is poorer than for the electron counterpart of this quantity – the most accurately verified prediction of science – and the prediction is off, anyway.

But as this blog has discussed in detail, the LHCb Collaboration – a smaller brother of ATLAS and CMS – has revealed numerous experimental results that could be resolved if leptoquarks exist, too.

In August, I described the apparent violation of the lepton universality observed when the LHCb measured the B-mesons' decay rates. (See also a June 2014 blog post.) That paper had D-mesons in the final state.

The LHCb has also claimed some anomalies in decays that involve K-mesons in the final state; see also a July 2013 blog post. And the CMS has possibly seen (with the fuzzy vision that a 2.5-sigma deviation represents) a flavor-violating decay of the Higgs, \(h\to\mu^\pm\tau^\mp\), which seemed to take place in 1% of the Higgs boson's decays. (See Jester's summary of these anomalies, too.)

As some phenomenology papers such as the fresh Bauer-Neubert paper point out, all or almost all these anomalies may be easily explained if you add a leptoquark to the Standard Model. If such a new particle is capable of decaying to the right-handed (singlet) up-quark and a right-handed (singlet) charged lepton, it can explain the correction to the muon magnetic moment as well the LHCb anomalies both with K-mesons and D-mesons.

There are obviously tons of papers about leptoquarks and (recently) their application as explanations of the possibly emerging LHC anomalies. See e.g. Hewett-Rizzo 1997, Reuter-Wiesler 2010, Freytsis et al. June 2015, Crivelin et al. July 2015, and Baek-Nishiwaki September 2015.

Now, what would be the bigger message?

If the experiments proved the existence of leptoquarks that have no other reasons to exist and no ancestry or relatives or purpose, we would have to accept this fact. But should we actually expect something like that to take place in the future? Well, I mostly don't. I think that in isolation, they're artificial purpose-less new particles. Occam's razor is a reason to favor a simpler model without such arbitrary additions.

However, they don't have to be arbitrary and optional. In grand unified theories, they (or at least some of their variations – depending on the choice of GUT groups and representations) may become unavoidable. They may become mandatory because they may be relatives of the fermions we know.

Leptoquarks are scalars so they can't be related to fermions by an old-fashioned, "bosonic" symmetry. But supersymmetry may change it. Leptoquarks may be superpartners of new fermions that are members of the same multiplets with the fermions we know so well.

\(SO(10)\) and \(E_6\) multiplets

Because of the excesses that look like a new gauge boson of mass \(2\TeV\) and perhaps also other signals at higher masses, I have repeatedly discussed the left-right-symmetric models (extending the Standard Model) whose gauge group is ideally\[

SU(3)_c \times SU(2)_L \times SU(2)_R \times U(1)_{B-L}.

\] The minimum GUT gauge group is \(SU(5)\) but it needs two different representations, basically \({\bf 5}\oplus\bar{\bf 10}\), to account for the known 15 two-spinors in one generation of fermions (three colors times two from doublet/flavor times two from Dirac-is-left-and-right, which is 12, plus 3 leptonic from a Weyl/Majorana neutrino and a Dirac charged lepton).

If you want to unify these reps as well, you surely prefer the \(SO(10)\) gauge group. And you actually do need at least \(SO(10)\) if you want the \(U(1)_{B-L}\) generator to be a part of the gauge group. The \(SU(5)\) GUTs simply don't contain the left-right-symmetric (or non-really-unified Pati-Salam) models.

With \(SO(10)\), things are nicer. All the leptons of one generation transform as \({\bf 16}\) which is a chiral spinor of \(SO(10)\) that decomposes under \(SU(5)\) as\[

{\bf 5}\oplus\bar{\bf 10} \oplus{\bf 1}

\] Note that \(5+10+1=16\) – but just to be sure (beginners listen now), the claims about the representations say much more than that the dimensions work. ;-) Aside from the \(5+10\) two-spinors from \(SU(5)\) GUT that are exactly enough for the Standard Model fermions, there is an extra singlet that is capable of completing the neutrino to a Dirac particle (the right-handed neutrino is what is added).

If you feel more familiar with the orthogonal groups than the unitary groups, \(SO(10)\) must be simpler for you. You may obviously embed \(SO(6)\times SO(4)\) group to it – by dividing the 10-dimensional vector to two segregated pieces \(6+4\) – and \(SO(6)\sim SU(4)\) [thanks Bill Z. for the fix] while \(SO(4)\sim SU(2)\times SU(2)\). It's enough to see that you have all the required groups for the left-right-symmetric extension of the Standard Model.

Xindl X recorded a "relative song" for the 100th anniversary of general relativity although it mostly says that life in Czechia is relatively OK unless you compare it with the civilized countries. ;-)

The left-right-symmetric models predict the new gauge bosons. Those from \(SU(2)_R\) which is new may have been seen by the LHC – the \(2\TeV\) and \(3\TeV\) gauge bosons – and (all?) the other gauge bosons of the GUT group almost certainly have to be much heavier, close to the GUT scale. However, the matter spectrum – away from the gauge multiplets – may always be partly or completely accessible.

This is particularly important if we try to increase the gauge group \(SO(10)\) to a larger and potentially cooler gauge group, \(E_6\). It's one of the five compact exceptional Lie groups, the others are \(E_7\),\(E_8\),\(F_4\),\(G_2\). The group \(E_6\) is the only one among these five that has complex representations (which are not equivalent to their complex conjugates). This is needed for the chiral and CP-violating spectrum of the Standard Model.

The \(E_6\) Dynkin diagram is the only exceptional group Dynkin diagram with an exact left-right symmetry - the only other simple compact Lie groups with this property are \(SU(\geq 3)\) – which is equivalent to its having complex (not real, not pseudoreal) representations because the reflection of the diagram indeed nontrivially acts on the set of irreps as well and a reflection, generating \(\ZZ_2\), has to be a mirroring operation and the complex conjugation of these reps is the only possibility.

The group is 78-dimensional but the smallest nontrivial, fundamental representations are \({\bf 27}\) and \(\bar{\bf 27}\). Those extend and replace the 16-dimensional spinor of \(SO(10)\). The decomposition under the \(SO(10)\) subgroup is obviously\[

{\bf 27} = {\bf 16}_{+1} + {\bf 10}_{-2} + {\bf 1}_{+4}

\] up to some possible bars that partially depend on your conventions (but one must be careful about certain correlations between bars and nonbars at different places). I've included some subscripts that actually express, in a certain simple normalization, the charge of the representations under a \(U(1)\): the maximum subgroup we may embed to \(E_6\) is actually \(SO(10)\times U(1)\). Note that the trace of the new \(U(1)\) vanishes over the 27-dimensional representation because\[

16\times 1 - 10\times 2 + 1\times 4 = 0,

\] as expected from traces of generators in non-Abelian simple groups. So it is interesting. Aside from the well-known 16 fermionic two-spinors, we also have an extra representation \({\bf 10}\) of \(SO(10)\). It decomposes as \({\bf 5}\oplus \bar{\bf 5}\) under the \(SU(5)\) subgroup which is why you shouldn't confuse it with the representation \({\bf 10}\) of \(SU(5)\) which is an antisymmetric tensor, \(5\times 4/2\times 1 = 10\).

There is a difference between the new 10 components of the matter fields and the previous 16 that we already had in \(SO(10)\). In SUSY GUT, particles have superpartners and because SUSY is broken, one of these two partners is expected to be lighter or the "normal one". It's the one that is R-parity-even. Evil tongues say that 1/2 of the particles predicted by SUSY have already been discovered – it's the particles with the even R-parity. But for all known particle species, the R-parity may be defined or written as\[

P_R = (-1)^{B-3L +2J}

\] and we may extend this definition to all new particle species as well if we assume that \(B,L,P_R\) remain well-defined and conserved. So if \(B-3L\) is even, the R-parity-even (\(P_R=+1\)) particle is a boson (\(j\in\ZZ\)) which is the case for the gauge bosons and the Higgs. But if \(B-3L\) is odd, which is the case for leptons and quarks because exactly one of the terms in \(B-3L\) is nonzero and \(\pm 1\), then the normal R-parity-even parts of the supermultiplet are the fermions with \(j\in \ZZ+1/2\). It works.

Now, the funny thing is that \(B-3L\) of the new 10 components of the 27-dimensional multiplet is even, so the "normal", R-parity-even parts of the supermultiplet that we can see are bosons. SUSY predicts new bosonic as well as fermionic particles from the 10-dimensional representation of \(SO(10)\) but it's the bosons that may be lighter because their R-parity is even. All of the things have to work in this way, obviously, because we postulated that the leptoquarks must be allowed to decay to known R-parity-even fermions and no LSP (lightest R-parity-odd) particle is ever found among the decay products.

A funny thing is that it is possible that it will turn out to be harder or hard to find any R-parity-odd particles at the LHC. But even if that were the case, the discovery of the R-parity-even leptoquarks would be significant evidence in favor of SUSY because those particles could be relatives of the leptons and quarks via a combination of SUSY and a new gauge symmetry. Some consistency checks could work.

If you think about "all possible spectra" left by string/M-theory to field theory to deal with, I do think that the SUSY \(E_6\) GUT is the single most likely possibility offered by string/M-theory. Recall that one of the two ten-dimensional heterotic string vacua, conventionally known as the \(E_8\times E_8\) heterotic string theory (the outdated terminology involving "many string theories" is from the 1980s but people don't want to rename things all the time), is the only class of models in string theory that has a sufficient gauge group for particle physics that is also "bounded from above" and bounded from above by a unique and preferred choice of the group.

To get down to \(\NNN=1\) supersymmetry, we need to compactify heterotic string theory on a Calabi-Yau manifold. This 6-real-dimensional manifold has to have the holonomy of \(SU(3)\), a subgroup of the generic (unoriented) potato manifold's holonomy \(O(6)\), and the simplest bundle to choose is one that identifies the gauge connection with the gravitational connection on the manifold.

This forces us to embed an \(SU(3)\) to \(E_8\). And the unbroken group we are left with is unavoidably an \(E_6\) because \(E_6\times SU(3)\) is a maximum subgroup of \(E_8\). I still believe that these are the most beautiful relationships between groups that may be related to the gauge groups in particle physics. Note that \(E_8\) is the largest exceptional group but it is no good for grand unified theory model building. The only viable exceptional group is \(E_6\) because of its complex representations.

And string theory contains an explanation why the "best" group \(E_8\) gets broken exactly to the "viable" \(E_6\): it's because the latter is the centralizer of \(SU(3)\) within \(E_8\) and we pick \(SU(3)\) because we basically have "three complex extra dimensions" predicted by the \(D=10\) string theory. All these things make so much sense. Many other structures of a similar degree of beauty (and maybe sometimes prettier) have been found in string theory during the following 3 decades after 1985 when heterotic string theory was born and proven to be at least semi-realistic.

But I believe that this path from \(E_8\) to \(E_6\) GUTs etc. remains the most persuasive scenario offered by string theory and telling us "what happens with the gauge groups" before the extra dimensions and stringy stuff may be forgotten.

We may be looking at early hints of all this new wonderful physics that is waiting to be found in coming years. We've been used to "nothing beyond the Standard Model" all the time and we tend to be very humble and shy. It's normal that phenomenologists are only willing to add one or two new particle species (components).

But if and when the LHC starts to see physics beyond the Standard Model, I am sure that the atmosphere will change or should change. Model builders should immediately become generous again. Many models with lots of new component fields are way more natural and justifiable than models where you humbly add one or two unmotivated extra components to your field content. If new physics is found at the LHC, we will have to think big again because the new experimental findings may tell us much more about the super-fundamental stringy architecture of Nature than we may believe at this moment.

By the way, I've encountered lots of models in my life, about \(10^{272,000}\) LOL. But if I had to choose one, I would still choose a \(T_6/\ZZ_3\) orbifold compactification of heterotic string theory; it's even simpler than the generically curved low-Hodge-number manifolds, it seems to me. I believe that it may actually explain three generations most sensibly, along with something like the \(S_4\) family symmetry. I am spending some time with the phenomenology of this model now.

by Luboš Motl ( at November 30, 2015 08:17 AM

November 29, 2015

Christian P. Robert - xi'an's og

the philosophical importance of Stein’s paradox

I recently came across this paper written by three philosophers of Science, attempting to set the Stein paradox in a philosophical light. Given my past involvement, I was obviously interested about which new perspective could be proposed, close to sixty years after Stein (1956). Paper that we should actually celebrate next year! However, when reading the document, I did not find a significantly innovative approach to the phenomenon…

The paper does not start in the best possible light since it seems to justify the use of a sample mean through maximum likelihood estimation, which only is the case for a limited number of probability distributions (including the Normal distribution, which may be an implicit assumption). For instance, when the data is Student’s t, the MLE is not the sample mean, no matter how shocking that might sounds! (And while this is a minor issue, results about the Stein effect taking place in non-normal settings appear much earlier than 1998. And earlier than in my dissertation. See, e.g., Berger and Bock (1975). Or in Brandwein and Strawderman (1978).)

While the linear regression explanation for the Stein effect is already exposed in Steve Stigler’s Neyman Lecture, I still have difficulties with the argument in that for instance we do not know the value of the parameter, which makes the regression and the inverse regression of parameter means over Gaussian observations mere concepts and nothing practical. (Except for the interesting result that two observations make both regressions coincide.) And it does not seem at all intuitive (to me) that imposing a constraint should improve the efficiency of a maximisation program…

Another difficulty I have with the discussion of the case against the MLE is not that there exist admissible estimators that dominate the MLE (when k≥5, as demonstrated by Bill Strawderman in 1975), but on the contrary that (a) there is an infinity of them and (b) they do not come out as closed-form expressions. Even for James’ and Stein’s or Efron’s and Morris’, shrinkage estimators, there exists a continuum of them, with no classical reason for picking one against the other.

Not that it really matters, but I also find rechristening the Stein phenomenon as holistic pragmatism somewhat inappropriate. Or just ungrounded. It seems to me the phenomenon simply relates to collective decision paradoxes, with multidimensional or multi-criteria utility functions having no way of converging to a collective optimum. As illustrated in [Lakanal alumni] Allais’ paradox.

“We think the most plausible Bayesian response to Stein’s results is to either reject them outright or to adopt an instrumentalist view of personal probabilities.”

The part connecting Stein with Bayes again starts on the wrong foot, since it is untrue that any shrinkage estimator can be expressed as a Bayes posterior mean. This is not even true for the original James-Stein estimator, i.e., it is not a Bayes estimator and cannot be a Bayes posterior mean. I also do neither understand nor relate to the notion of “Bayesians of the first kind”, especially when it merges with an empirical Bayes argument. More globally, the whole discourse about Bayesians “taking account of Stein’s result” does not stand on very sound ground because Bayesians automatically integrate the shrinkage phenomenon when minimising a posterior loss. Rather than trying to accommodate it as a special goal. Laughing (in the paper) at the prior assumption that all means should be “close” to zero or “close together” does not account for the choice of the location (zero) or scale (one) when measuring quantities of interest. And for the fact that Stein’s effect holds even when the means are far from zero or from being similar, albeit as a minuscule effect. That is, when the prior disagrees with the data, because Stein’s phenomenon is a frequentist occurrence. What I find amusing is instead to mention a “prior whose probability mass is centred about the sample mean”. (I am unsure the authors are aware that the shrinkage effect is irrelevant for all practical purposes unless the true means are close to the shrinkage centre.) And to state that improper priors “integrate to a number larger than 1” and that “it’s not possible to be more than 100% confident in anything”… And to confuse the Likelihood Principle with the prohibition of data dependent priors. And to consider that the MLE and any shrinkage estimator have the same expected utility under a flat prior (since, if they had, there would be no Bayes estimator!). The only part with which I can agree is, again, that Stein’s phenomenon is a frequentist notion. But one that induces us to use Bayes estimators as the only coherent way to make use of the loss function. The paper is actually silent about the duality existing between losses and priors, duality that would put Stein’s effect into a totally different light. As expressed e.g. in Herman Rubin’s paper. Because shrinkage both in existence and in magnitude is deeply connected with the choice of the loss function, arguing against an almost universal Bayesian perspective of shrinkage while adhering to a single loss function is rather paradoxical. Similarly, very little of substance can be found about empirical Bayes estimation and its philosophical foundations.

While it is generally agreed that shrinkage estimators trade some bias for a decrease in variance, the connection with AIC is at best tenuous. Because AIC or other model choice tools are not estimation devices per se. And because they force infinite shrinkage, namely to have some components of the estimator precisely equal to zero. Which is an impossibility for Bayes estimates. A much more natural (and already made) connection would be to relate shrinkage and LASSO estimators, since the difference can be rephrased as the opposition between Gaussian and Laplace priors.

I also object at the concept of “linguistic invariance”, which simply means (for me) absolute invariance, namely that the estimate of the transform must be the transform of the estimate for every and all transforms. Which holds for the MLE. But also, contrary to the author’s assertion, for Bayes estimation under my intrinsic loss functions.

“But when and how problems should be lumped together or split apart remains an important open problem in statistics.”

The authors correctly point out the accuracy of AIC (over BIC) for making predictions, but shrinkage does not necessarily suffer from this feature as Stein’s phenomenon also holds for prediction, if predicting enough values at the same time… I also object to the envisioned possibility of a shrinkage estimator that would improve every component of the MLE (in a uniform sense) as it contradicts the admissibility of the single component MLE! And the above quote shows the decision theoretic part of inference is not properly incorporated.

Overall, I thus clearly wonder at the purpose of the paper, given the detailed coverage of many aspects of the Stein phenomenon provided by Stephen Stigler and others over the years. Obviously, a new perspective is always welcome, but this paper somewhat lacks enough appeal. While missing essential features making the Stein phenomenon look like a poor relative of Bayesian inference. In my opinion, Stein’s phenomenon remains an epi-phenomenon, which rather signals the end of the search for a golden standard in frequentist estimation than the opening of a new era of estimation. It pushed me almost irresistibly into Bayesianism, a move I do not regret to this day! In fine, I also have trouble seeing Stein’s phenomenon as durably impacting the field, more than 50 years later, and hence think it remains of little importance for epistemology and philosophy of science. Except maybe for marking the end of an era, where the search for “the” ideal estimator was still on the agenda.

Filed under: Books, pictures, Statistics, University life Tagged: Bayesian Analysis, Bayesian Choice, Charles Stein, decision theory, frequentist inference, James-Stein estimator, loss functions, philosophy of sciences, Stein effect, Stein's phenomenon, Stephen Stigler

by xi'an at November 29, 2015 11:15 PM

Peter Coles - In the Dark

Einstein’s Legacy

Yesterday I braved the inclement weather and the perils of weekend travel on Southern Trains to visit Queen Mary College, in the East End of London, for the following event:


I used to work at  Queen Mary, but haven’t been back for a while. The college and environs have been smartened up quite a lot since I used to be there, as seems to be the case for the East End generally. I doubt if I could afford to live there now!

Owing to a little local difficulty which I won’t go into, I was running a bit late so I missed the morning session. I did, however, arrive in time to see my former colleague Bangalore Sathyaprakash from Cardiff talking about gravitational waves, Jim Hough from Glasgow talking about experimental gravity – including gravitational waves but also talking about the puzzling state of affairs over “Big G” – and Pedro Ferreira from Oxford whose talk on “Cosmology for the 21st Century” gave an enjoyable historical perspective on recent developments.

The talks were held in the Great Hall in the People’s Palace on Mile End Road, a large venue that was pretty full all afternoon. I’m not sure whether it was the District/Hammersmith & City Line or the Central Line (or both) that provided the atmospheric sound effects, especially when Jim Hough described the problems of dealing with seismic noise in gravitational experiments and a train rumbled underneath right on cue.

UPDATE: Thanks to Bryn’s comment (below) I looked at a map: the Central Line goes well to the North whereas the District and Hammersmith & City Line go directly under the main buildings adjacent to Mile End Road.


Anyway, the venue was even fuller for the evening session, kicked off by my former PhD supervisor, John Barrow:

Einstein's Legacy

This session was aimed at a more popular audience and was attended by more than a few A-level students. John’s talk was very nice, taking us through all the various cosmological models that have been developed based on Einstein’s theory of General Relativity.

Finally, topping the bill, was Sir Roger Penrose whose talk was engagingly lo-tech in terms of visual aids but aimed at quite a high level. His use of hand-drawn transparencies was very old-school, but a useful side-effect was that he conveyed very effectively how entropy always increases with time.

Penrose covered some really interesting material related to black holes and cosmology, especially to do with gravitational entropy, but my heart sank when he tried at the end to resurrect his discredited “Circles in the Sky” idea. I’m not sure how much the A-level students took from his talk, but I found it very entertaining.

The conference carries on today, but I couldn’t attend the Sunday session owing to pressure of work. Which I should be doing now!

P.S. I’ll say it before anyone else does: yes, all the speakers I heard were male, as indeed were the two I missed in the morning. I gather there was one cancellation  of a female speaker (Alessandra Buonanno), for whom Sathya stood in.  But still.


by telescoper at November 29, 2015 02:15 PM

November 28, 2015

ZapperZ - Physics and Physicists

What Good Is Particle Physics?
I've tackled this issue a few times on here, such as in this blog post. In this video, Don Lincoln decides to address this issue.


by ZapperZ ( at November 28, 2015 11:57 PM

Christian P. Robert - xi'an's og

seveneves [book review]

As the latest Neal Stephenson’s novel, I was waiting most eagerly to receive Seveneves (or SevenEves ). Now I have read it, I am a bit disappointed by the book. It is a terrific concept, full of ideas and concepts, linking our current society and its limitations with what a society exiled in space could become, and with a great style as well, but as far as the story itself goes I have trouble buying it! In short, there is too much technology and not enough psychology, too many details and not enough of a grand scheme… This certainly is far from being the best book of the author. When compared with Snow Crash, Cryptonomicon, Anathem, or Reamde for instance. Even the fairly long and meandering Baroque Cycle comes on top of this space opera à la Arthur Clarke (if only for the cables linking Earth and space stations at 36,000 kilometres…).

 The basis of Seveneves is a catastrophic explosion of our Moon that leads to the obliteration of live on Earth within a range of two years. The only way out is to send a small number of people to a space station with enough genetic material to preserve the diversity of the Human species. Two-third of the book is about the frantic scramble to make this possible. Then Earth is bombarded by pieces of the Moon, while the inhabitants of the expanded space station try to get organised and to get more energy from iced asteroids to get out of the way, while badly fighting for power. This leads the crowd of survivors to eventually reduce to seven women, hence the seven Eves. Then, a five thousand year hiatus, and the last part of the book deals with the new Human society, hanging up in a gigantic sphere of space modules around the regenerated Earth, where we follow a team of seven (!) characters whose goal is not exactly crystal clear.

While most books by Stephenson manage to produce a good plot on top of fantastic ideas, with some characters developed with enough depth to be really compelling, this one is missing at the plot level and even more at the character level, maybe because we know most characters are supposed to die very early in the story. But they do look like caricatures, frankly! And behave like kids astray on a desert island. Unless I missed the deeper message… The construction of the spatial mega-station is detailed in such details that it hurts!, but some logistic details on how to produce food or energy are clearly missing. And missing is also the feat of reconstituting an entire Human species out of seven women, even with a huge bank of human DNAs. The description of the station five thousand years later is even more excruciatingly precise. At a stage where I have mostly lost interest in the story, especially to find very little differences in the way the new and the old societies operate. And to avoid spoilers, gur er-nccnevgvba bs gur gjb tebhcf bs crbcyr jub erznvarq ba Rnegu, rvgure uvqqra va n qrrc pnir be ng gur obggbz bs gur qrrcrfg gerapu, vf pbzcyrgryl vzcynhfvoyr, sbe ubj gurl pbhyq unir fheivirq bire gubhfnaqf bs lrnef jvgu ab npprff gb erfbheprf rkprcg jung gurl unq cnpxrq ng gur ortvaavat… It took me some effort and then some during several sleepless nights to get over this long book and I remain perplexed at the result, given the past masterpieces of the author.

Filed under: Books, Kids Tagged: Anathem, echidna, Neal Stephenson, ROT13, Seveneves, Snow Crash, space opera

by xi'an at November 28, 2015 11:15 PM

November 27, 2015

Christian P. Robert - xi'an's og

superintelligence [book review]

“The first ultraintelligent machine is the last invention that man need ever make, provided that the machine is docile enough to tell us how to keep it under control.” I.J. Good

I saw the nice cover of Superintelligence: paths, dangers, strategies by Nick Bostrom [owling at me!] at the OUP booth at JSM this summer—nice owl cover that comes will a little philosophical fable at the beginning about sparrows—and, after reading an in-depth review [in English] by Olle Häggström, on Häggström hävdar, asked OUP for a review copy. Which they sent immediately. The reason why I got (so) interested in the book is that I am quite surprised at the level of alertness about the dangers of artificial intelligence (or computer intelligence) taking over. As reported in an earlier blog, and with no expertise whatsoever in the field, I was not and am not convinced that the uncontrolled and exponential rise of non-human or non-completely human intelligences is the number One entry in Doom Day scenarios. (As made clear by Radford Neal and Corey Yanovsky in their comments, I know nothing worth reporting about those issues, but remain presumably irrationally more concerned about climate change and/or a return to barbarity than by the incoming reign of the machines.) Thus, having no competence in the least in either intelligence (!), artificial or human, or in philosophy and ethics, the following comments on the book only reflect my neophyte’s reactions. Which means the following rant should be mostly ignored! Except maybe on a rainy day like today…

“The ideal is that of the perfect Bayesian agent, one that makes probabilistically optimal use of available information.  This idea is unattainable (…) Accordingly, one can view artificial intelligence as a quest to find shortcuts…” (p.9)

Overall, the book stands much more at a philosophical and exploratory level than at attempting any engineering or technical assessment. The graphs found within are sketches rather than outputs of carefully estimated physical processes. There is thus hardly any indication how those super AIs could be coded towards super abilities to produce paper clips (but why on Earth would we need paper clips in a world dominated by AIs?!) or to involve all resources from an entire galaxy to explore even farther. The author envisions (mostly catastrophic) scenarios that require some suspension of belief and after a while I decided to read the book mostly as a higher form of science fiction, from which a series of lower form science fiction books could easily be constructed! Some passages reminded me quite forcibly of Philip K. Dick, less of electric sheep &tc. than of Ubik, where a superpowerful AI(s) turn humans into jar brains satisfied (or ensnared) with simulated virtual realities. Much less of Asimov’s novels as robots are hardly mentioned. And the third laws of robotics dismissed as ridiculously too simplistic (and too human).

“These occasions grace us with the opportunity to abandon a life of overconfidence and resolve to become better Bayesians.” (p.130)

Another level at which to read the book is as a deep reflection on the notions of intelligence, ethics, and morality. In the human sense. Indeed, before defining and maybe controlling such notions for machines, we should reflect how they are defined or coded for humans. I do not find the book very successful at this level (but, again, I know nothing!), as even intelligence does not get a clear definition, maybe because it is simply impossible to do so. The section on embryo selection towards more intelligent newborns made me cringe, not only because of the eugenic tones, but also because I am not aware of any characterisation so far of gene mutations promoting intelligence. (So far, of course, and the book generally considers that any technology or advance that is conceivable now will eventually be conceived. Presumably thanks to our own species’ intelligence.) And of course the arguments get much less clear when ethics and morality are concerned. Which brings me to one question I kept asking myself when going through the book, namely why would we be interested in replicating a human brain and its operation, or creating a superintelligent and self-enhancing machine, except for the sake of proving we can do it? With a secondary question, why would a superintelligent AI necessarily and invariably want to take over the world, a running assumption throughout the book?

“Within a Bayesian framework, we can think of the epistemology as a prior probability function.” (p.224)

While it is an easy counter-argument (and thus can be easily countered itself), notions that we can control the hegemonic tendencies of a powerful AI by appealing to utility and game theory are difficult to accept. This formalism hardly works for us (irrational) humans, so I see no reason why an inhuman form of intelligence could be thus constrained, as it can as well pick another form of utility or game theory as it evolves, following a inhuman logic that we cannot even fathom. Everything is possible, not even the sky is a limit… Even the conjunction of super AIs and of nano-technologies, from which we should be protected by the AI(s) if I follow the book (p.131). The difference between both actually is a matter of perspective as we can envision a swarm of nano-particules endowed with a collective super-intelligence…

“At a pre-set time, nanofactories producing nerve gas or target-seeking-mosquito-like robots might then burgeon forth simultaneously from every square metre of the globe.” (p.97)

Again, this is a leisurely read with no attempt at depth. If you want a deeper perspective, read for instance Olle Häggstöom’s review. Or ask Bill Gates, who “highly recommend this book” as indicated on the book cover. I found the book enjoyable in its systematic exploration of “all” possible scenarios and its connections with (Bayesian) decision theory and learning. As well as well-written, with a pleasant style, rich in references as well as theories, scholarly in its inclusion of as many aspects as possible, possibly lacking some backup from a scientific perspective, and somehow too tentative and exploratory. I cannot say I am now frightened by the emergence of the amoral super AIs or on the contrary reassured that there could be ways of keeping them under human control. (A primary question I did not see processed and would have liked to see is why we should fight this emergence. If AIs are much more intelligent than us, shouldn’t we defer to this intelligence? Just like we cannot not fathom chicken resisting their (unpleasant) fate, except in comics like Chicken Run… Thus completing the loop with the owl.)

Filed under: Books, Statistics, Travel, University life Tagged: 2001: A Space Odyssey, AIs, artificial intelligence, Bill Gates, Chicken Run, doomsday argument, ethics, HAL, intelligence, Isaac Asimov, JSM 2015, morality, Nick Bostrom, Philip K. DIck, Seattle

by xi'an at November 27, 2015 11:15 PM

Clifford V. Johnson - Asymptotia

The New Improved Scooby-Gang? (Part 2)

usc_dornsife_frontline_scholars_medium(Click for larger view.) The answer's still no, but I still amuse myself with the joke. (Alternative forms would have been "The New Expendables Poster?" or "Sneak peek at the Post-Infinity Wars Avengers Lineup?"...) This is a photo that I was thinking would not make it out to the wider world, but that's probably because I was not paying attention. We spent a lot of time on that rooftop getting that right - no it was not photoshopped, the city is right behind us there - as part of the "Frontline Scholars" campaign for USC's Dornsife College of Letters Arts and Sciences... and then the next thing I heard from our Dean (pictured three from the right) is that he is now our ex-Dean. So I figured the campaign they were planning would not feature him anymore, and hence this picture would not be used. But I think the photo was actually in use for the campaign for a while and I did not know since I don't actually pay attention to [...] Click to continue reading this post

The post The New Improved Scooby-Gang? (Part 2) appeared first on Asymptotia.

by Clifford at November 27, 2015 10:22 PM

John Baez - Azimuth

Relative Entropy in Biological Systems

Here’s a draft of a paper for the proceedings of a workshop on Information and Entropy in Biological System this spring:

• John Baez and Blake Pollard, Relative Entropy in Biological Systems.

We’d love any comments or questions you might have. I’m not happy with the title. In the paper we advocate using the term ‘relative information’ instead of ‘relative entropy’—yet the latter is much more widely used, so I feel we need it in the title to let people know what the paper is about!

Here’s the basic idea.

Life relies on nonequilibrium thermodynamics, since in thermal equilibrium there are no flows of free energy. Biological systems are also open systems, in the sense that both matter and energy flow in and out of them. Nonetheless, it is important in biology that systems can sometimes be treated as approximately closed, and sometimes approach equilibrium before being disrupted in one way or another. This can occur on a wide range of scales, from large ecosystems to within a single cell or organelle. Examples include:

• A population approaching an evolutionarily stable state.

• Random processes such as mutation, genetic drift, the diffusion of organisms in an environment or the diffusion of molecules in a liquid.

• A chemical reaction approaching equilibrium.

An interesting common feature of these processes is that as they occur, quantities mathematically akin to entropy tend to increase. Closely related quantities such as free energy tend to decrease. In this review, we explain some mathematical results that make this idea precise.

Most of these results involve a quantity that is variously known as ‘relative information’, ‘relative entropy’, ‘information gain’ or the ‘Kullback–Leibler divergence’. We’ll use the first term. Given two probability distributions p and q on a finite set X, their relative information, or more precisely the information of p relative to q, is

\displaystyle{ I(p\|q) = \sum_{i \in X} p_i \ln\left(\frac{p_i}{q_i}\right) }

We use the word ‘information’ instead of ‘entropy’ because one expects entropy to increase with time, and the theorems we present will say that I(p\|q) decreases with time under various conditions. The reason is that the Shannon entropy

\displaystyle{ S(p) = -\sum_{i \in X} p_i \ln p_i }

contains a minus sign that is missing from the definition of relative information.

Intuitively, I(p\|q) is the amount of information gained when we start with a hypothesis given by some probability distribution q and then learn the ‘true’ probability distribution p. For example, if we start with the hypothesis that a coin is fair and then are told that it landed heads up, the relative information is \ln 2, so we have gained 1 bit of information. If however we started with the hypothesis that the coin always lands heads up, we would have gained no information.

We put the word ‘true’ in quotes here, because the notion of a ‘true’ probability distribution, which subjective Bayesians reject, is not required to use relative information. A more cautious description of relative information is that it is a divergence: a way of measuring the difference between probability distributions that obeys

I(p \| q) \ge 0


I(p \| q) = 0 \iff p = q

but not necessarily the other axioms for a distance function, symmetry and the triangle inequality, which indeed fail for relative information.

There are many other divergences besides relative information, some of which we discuss in Section 6. However, relative information can be singled out by a number of characterizations, including one based on ideas from Bayesian inference. The relative information is also close to the expected number of extra bits required to code messages distributed according to the probability measure p using a code optimized for messages distributed according to q.

In this review, we describe various ways in which a population or probability distribution evolves continuously according to some differential equation. For all these differential equations, I describe conditions under which relative information decreases. Briefly, the results are as follows. We hasten to reassure the reader that our paper explains all the jargon involved, and the proofs of the claims are given in full:

• In Section 2, we consider a very general form of the Lotka–Volterra equations, which are a commonly used model of population dynamics. Starting from the population P_i of each type of replicating entity, we can define a probability distribution

p_i = \frac{P_i}{\displaystyle{\sum_{i \in X} P_i }}

which evolves according to a nonlinear equation called the replicator equation. We describe a necessary and sufficient condition under which I(q\|p(t)) is nonincreasing when p(t) evolves according to the replicator equation while q is held fixed.

• In Section 3, we consider a special case of the replicator equation that is widely studied in evolutionary game theory. In this case we can think of probability distributions as mixed strategies in a two-player game. When q is a dominant strategy, $I(q|p(t))$ can never increase when p(t) evolves according to the replicator equation. We can think of I(q\|p(t)) as the information that the population has left to learn.
Thus, evolution is analogous to a learning process—an analogy that in the field of artificial intelligence is exploited by evolutionary algorithms.

• In Section 4 we consider continuous-time, finite-state Markov processes. Here we have probability distributions on a finite set X evolving according to a linear equation called the master equation. In this case I(p(t)\|q(t)) can never increase. Thus, if q is a steady state solution of the master equation, both I(p(t)\|q) and I(q\|p(t)) are nonincreasing. We can always write q as the Boltzmann distribution for some energy function E : X \to \mathbb{R}, meaning that

\displaystyle{ q_i = \frac{\exp(-E_i / k T)}{\displaystyle{\sum_{j \in X} \exp(-E_j / k T)}} }

where T is temperature and k is Boltzmann’s constant. In this case, I(p(t)\|q) is proportional to a difference of free energies:

\displaystyle{ I(p(t)\|q) = \frac{F(p) - F(q)}{T} }

Thus, the nonincreasing nature of I(p(t)\|q) is a version of the Second Law of Thermodynamics.

• In Section 5, we consider chemical reactions and other processes described by reaction networks. In this context we have populations P_i of entities of various kinds i \in X, and these populations evolve according to a nonlinear equation called the rate equation. We can generalize relative information from probability distributions to populations by setting

\displaystyle{ I(P\|Q) = \sum_{i \in X} P_i \ln\left(\frac{P_i}{Q_i}\right) - \left(P_i - Q_i\right) }

If Q is a special sort of steady state solution of the rate equation, called a complex balanced equilibrium, I(P(t)\|Q) can never increase when P(t) evolves according to the rate equation.

• Finally, in Section 6, we consider a class of functions called f-divergences which include relative information as a special case. For any convex function f : [0,\infty) \to [0,\infty), the f-divergence of two probability distributions p, q : X \to [0,1] is given by

\displaystyle{ I_f(p\|q) = \sum_{i \in X} q_i f\left(\frac{p_i}{q_i}\right)}

Whenever p(t) and q(t) are probability distributions evolving according to the master equation of some Markov process, I_f(p(t)\|q(t)) is nonincreasing. The f-divergence is also well-defined for populations, and nonincreasing for two populations that both evolve according to the master equation.

by John Baez at November 27, 2015 05:48 PM

astrobites - astro-ph reader's digest

We’ll be counting galaxies…

Title: The galaxy luminosity function at z ~ 6 and evidence for rapid evolution in the bright end from z  ~ 7 to 5
Authors: R. A. A. Bowler et al.
First Author Institution: SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
Status: Published in MNRAS, 451, 1817

A lot of the time Astrophysicists might use some fancy computing and equations to figure out things about the Universe – but sometimes just counting the number of galaxies you can see, can tell you a whole lot more. You can turn the number of galaxies you can see in a certain area of sky into a number density and then look how this value changes with increasing stellar mass (or luminosity) of a galaxy (i.e. how bright a galaxy is and therefore how much mass it has in stars). Astronomers call this the luminosity function (LF) and it’s really important if you want to get a handle on how galaxies have been built up over time.

We can also get an estimate of the LF from simulations that try to emulate the Universe. Comparing both the observed and simulated galaxy number densities tells us how well our models of what shapes the Universe perform. Our most accepted model of the Universe is λ Cold Dark Matter (λCDM) where the Universe contains a cosmological constant, λ (i.e. dark energy) and cold dark matter (cold to represent their minimal energy). However λCDM does not give us the same number density of galaxies as observed at all stellar galaxy masses. It overestimates the numbers of galaxies at the extremely low mass and extremely high mass end of the LF. People have tried to explain this discrepancy in many different ways. For example at the low mass end it’s thought that this is caused by what astronomers call a selection effect – that the galaxies should be there, but they’re so faint that there’s no way we can currently detect them. We don’t have that excuse at the high mass end though, as these galaxies are really bright, so they’d be hard to miss.  But one thing that isn’t included in the simulations is feedback of energy from AGN (active galactic nuclei – i.e. active black holes in the centres of galaxies) which can throw out or heat gas needed for star formation in a galaxy. This feedback can stop a galaxy from growing overly large in mass, so people have argued that the inclusion of this in the simulations removes the discrepancy with observations.

Studying this problem locally with galaxies that are nearby is relatively easy compared to trying to get this LF for galaxies at high redshift, because even the brightest galaxies at huge distances are hard to detect. Getting accurate estimates for their number counts is therefore really difficult, especially if we want to study how the LF changes with increasing redshift (i.e. increasing distance and therefore looking further back into the Universe’s past). The author’s of this paper have been tackling this problem of getting accurate number counts for galaxies at z > 6 (in the first billion years of the Universe’s life) by using two ultra deep surveys of the sky in the infra-red: COSMOS and UDS. They found 266 candidate galaxies (after removing possible contaminant sources that look like galaxies at big distances, such as brown dwarf stars in our galaxy) all within a redshift of 5.5 < z < 6.5. Using these galaxies and other results from previous publications, they looked at how the LF changes from z= 5 to z = 7 (over 500 million years of the Universe’s history) , which is shown below in Figure 1.



Figure 1; The y-axis shows the number density of galaxies at a given infra-red magnitude on the x-axis. Bright, massive galaxies are on the left of the plot and faint, small galaxies are on the right of the plot. The different coloured lines show how the LF changes with increasing redshift.


What’s cool about Figure 1 is that it shows that, at the faint end (M_{1500} \sim -18), the LFs at the three different redshifts are extremely similar; whereas at the bright end(M_{1500} \sim -22) the three differ quite significantly. The authors argue that this could be because the AGN feedback that’s thought to affect the bright end of the LF in the local Universe has less of an effect with increasing redshift (i.e. decreasing age of the Universe). At such early times in the Universe’s life (z=7  is only ~700  million years after the Big Bang) black holes haven’t managed to grow and accrete sufficient material to be powerful enough to output enough energy to feedback on the star formation of a galaxy. As the Universe ages, black holes keep growing, eventually become active, so that AGN feedback becomes a big influence on the numbers of super massive galaxies at the bright end of the LF. This is a really intriguing idea, one that needs to be looked at further with the advent of deeper and wider surveys of the sky in the future, such as those using ALMA and the JWST. So watch this space!

by Becky Smethurst at November 27, 2015 03:02 PM

Tommaso Dorigo - Scientificblogging

Single Top Production At The LHC
As an editor of the new Elsevier journal "Reviews in Physics" I am quite proud to see that the first submissions of review articles are reaching publication stage. Four such articles are going to be published in the course of the next couple of months, and more are due shortly thereafter. 

read more

by Tommaso Dorigo at November 27, 2015 03:01 PM

CERN Bulletin

LHC Report: plumbing new heights

Following the end of the arduous 2015 proton run on 4 November, the many teams working on the LHC and its injector complex are naturally entitled to a calmer period before the well-earned end-of-year break. But that is not the way things work.    


The CCC team after stable heavy-ion beams are declared in the LHC

Instead, the subdued frenzy of setting up the accelerators for a physics run has started again, this time for heavy-ion beams, with a few additional twists of the time-pressure knob. In this year’s one-month run, the first week was devoted to colliding protons at 2.51 TeV per beam to provide reference data for the subsequent collisions of lead nuclei (the atomic number of lead is Z=82, compared to Z=1 for protons) at the unprecedented energy of 5.02 TeV in the centre of mass per nucleon pair.

The chain of specialised heavy-ion injectors, comprising the ECR ion source, Linac3 and the LEIR ring, with its elaborate bunch-forming and cooling, were re-commissioned to provide intense and dense lead bunches in the preceding weeks. Through a series of exquisite RF gymnastics, the PS and SPS assemble these into 24-bunch trains for injection into the LHC. The beam intensity delivered by the injectors is a crucial determinant of the luminosity of the collider.

Commissioning of the LHC’s 2.51 TeV proton cycle had to be interleaved with that of the new heavy-ion optics in the LHC, resulting in many adjustments to the schedule on the fly and specialist teams being summoned at short notice to the CCC. Besides the overall energy shift compared to the 6.5 TeV proton optics, there is an additional squeeze of the optics and manipulations of crossing angles and the interaction point position for the ALICE experiment. Rapid work by the LHC’s optics measurements and correction team allowed the new heavy-ion magnetic cycle to be implemented from scratch (using proton beams) over the weekend of 14-15 November. Members of the collimation team also spent many hours on careful aperture measurements. At every step, one must be mindful of the strict requirements of machine protection.  

The first lead-ion beams were injected on the evening of Monday, 16 November and brought into collision in all four experiments, by a bleary-eyed team, 10 hours later in the early morning.  

The proton reference run resumed that Tuesday evening. After some unnerving down time, its luminosity target was comfortably attained on Sunday morning and the ion commissioning resumed with more aperture measurements and the process of verifying the “loss maps” to confirm that errant beam particles fetch up where they can do the least harm. These are very different from those of protons because of the many ways in which the lead nuclei can fragment as they interact with the collimators. A penultimate switch of particle species provided a bonus of proton reference data to the experiments overnight.

Finally, on 23 November the lead ions had the LHC to themselves and commissioning resumed with tuning of injection, RF and feedback systems. And many more loss maps.

Stable beams for physics with 10 bunches per beam was finally declared at 10:59 on 25 November and spectacular event displays started to flow from the experiments. Further fills should increase the number of bunches beyond 400.

The remaining weeks of the run will continue to be eventful with physics production interrupted by ion-source oven refills, van der Meer scans, solenoid polarity reversals and studies of phenomena that may limit future performance. These include tests of magnet quench levels with collimation losses and the use of crystals as collimators. We also plan to test strategies for controlling the secondary beams emerging from the collision point due to ultraperipheral (“near miss”) interactions. 

November 27, 2015 02:11 PM

CERN Bulletin

From the CERN web: knowledge transfer, sustainability, CERN openlab and more

This section highlights articles, blog posts and press releases published in the CERN web environment over the past weeks. This way, you won’t miss a thing...


Previous successful Knowledge Transfer enterprises have helped to develop several useful technologies, such as these photonic crystals, which glow when high-energy charged particles pass through, and are used for medical imaging.

New Knowledge Transfer website to grow CERN’s industry links
23 November – by Harriet Jarlett 

CERN’s Knowledge Transfer Group has just launched a new tool to encourage CERN researchers and businesses to share their technologies, ideas and expertise. It’s hoped that by facilitating these exchanges the tool will inspire new ways to apply CERN technologies commercially, to help benefit industry and society.

Continue to read…


 The power station at CERN's Prévessin site. (Image: Margot Frenot/CERN).

CERN and research institutes discuss energy sustainability
18 November – by Harriet Jarlett

On 29 October, CERN attended the third “Energy for Sustainable Science at Research Infrastructures” workshop at DESY in Germany. The bi-annual workshop, which was established in 2011, with ESS in Sweden and the European association of national research facilities (ERF), brought together delegates from research institutes worldwide to discuss energy consumption, strategies to improve energy awareness and plans for energy sustainability.

Continue to read…


From over 1500 applicants, 40 students were selected to take part in the 2015 CERN openlab summer student programme.

Faster research code wins student CERN openlab internship
16 November – by Harriet Jarlett

CERN openlab and its partner company Intel jointly announced the winners of the Modern Code Developer Challenge on 14 November, at the annual Intel HPC Developer Conference. The overall winner, Mathieu Gravey from École des Mines d’Alès in France, was awarded the grand prize after he reduced the time it took to run a large dataset of code simulating brain development from 45 hours to just under eight and a half minutes. He’ll join CERN openlab as a summer student next year.

Continue to read…


 The Sanford Underground Research Facility, where DUNE will study neutrinos produced 1300 km away at Fermilab. (Image: Sanford Underground Research Facility.)

DUNE and its CERN connection
13 November – CERN Courier

With almost 800 scientists and engineers from 145 institutes in 26 nations, the DUNE experiment is gaining global interest from the neutrino-physics community.

Continue to read…


An accelerating cavity from CERN’s Large Electron Positron Collider is part of the Collider exhibition, now in Singapore. (Image: ArtScience Museum).

LHC arrives in Singapore
13 November – by Harriet Jarlett

The Large Hadron Collider has reached Asia. On 14 November, the “Collider” exhibition opens at Singapore’s ArtScience Museum. This exhibition, which began life at London’s Science Museum back in 2013, has already travelled to Manchester and Paris. It showcases CERN's activities through theatre, video and sound art. Visitors are guided through a digital control room and detector cavern, and interact with objects such as LHC magnets and parts of detector systems.

Continue to read…

November 27, 2015 11:11 AM

CERN Bulletin

Innovation meets entrepreneurship

On Thursday 26 November, CERN openlab hosted an event on innovation and entrepreneurship. It was organised in collaboration with the CERN Knowledge Transfer Group and IdeaSquare.


Attended by 80 people, the event featured talks on commercialisation, public-private partnership, intellectual property, and other related topics. The participants also had the opportunity to discuss their own business ideas one-to-one with invited experts, who provided tailored advice.

The event was supported by CERN openlab partner company Intel as part of a joint project on innovation and entrepreneurship.

More information about the event is available here.

November 27, 2015 11:11 AM

CERN Bulletin

2015 Five-Yearly Review : one last formal step, with the implementation to follow
Taking into account the arbitration by the Director General the Staff Council decided that it did not oppose the Management proposals for the 2015 Five-Yearly Review (see Echo 234). Consequently, at the TREF meeting of Thursday 26 November, Management presented its consolidated proposals taking into account the outcome of the arbitration. The Staff Association was invited to express its point of view (the text of our declaration follows). After the Member States’ delegates got satisfactory answers to their questions for clarification, none of the 14 delegations represented opposed the proposals nor were there any abstentions. The Chair of TREF, B. Dormy, will thus report to Finance Committee and Council on 16 and 17 December that TREF recommends that these committees approve the Management proposals. A huge amount of work by many CERN colleagues, representatives of the Management, the Sectors, and the Staff Association has come to a successful conclusion. Now we move into the important implementation phase of the various measures. This process will occupy 2016 and beyond and will need mutual trust, confidence, and a collaborative spirit of all those involved to guarantee that the talent management tools for the operation of the new career system, will allow CERN to match its excellence in science and technology with an excellence in social policy, and thus motivating its staff. Staff Association declaration at the TREF meeting of 26 November 2015 Following the procedure set out in Annex A1 of the Staff Rules and Regulations, CERN Management and the Staff Association in the framework of the SCC and its sub-committees, and all of us in this Committee, have been working for over two years for the preparation of the 2015 Five-Yearly Review, whose outcome we are debating today. In Spring 2014 we had reports in TREF on the recruitment markets, recruitment and retention, comparator institutes for fellows, we got an explanation of the data collection process for salary comparisons (with an explanation by the OECD of the methodology used), and we discussed the Management proposal identifying the financial and social conditions to be reviewed. As a result, Council approved in June 2014 the proposal of the Director-General for the programme of the 2015 Five-Yearly Review, with CERN’s career structure and diversity-related social and financial conditions as optional items. In October 2014, the OECD presented a study of the comparators for the salary review and the methodology for the benchmark study on diversity. In March 2015, the OECD came back with the results of this benchmark study, where it was found that CERN’s family-related policies are amongst the most restrictive compared to other international organizations. In May 2015, the OECD presented the results of the international salary survey, which showed that on average, the Swiss high-technology market, used as key comparator, offered salaries 31% above those at CERN. The differences found are in line with those found in 2010. Finally, in TREF’s previous meeting, on 13 October, the Management orally presented its preliminary proposals for the 2015 Five-Yearly Review, including a first discussion of the new career structure. At that meeting the Staff Association pointed out in its preliminary comments that serious concerns were raised by staff delegates regarding the forecast “cost containment” worth 55 MCHF over 10 years since it resulted in most staff receiving less remuneration, and hence a lower retirement pension, over that period and beyond. A particular area of concern related to the 7% of staff members whose salaries were currently above the maxima of the new grades, and who would no longer be eligible for advancement under the new system unless they were promoted due to a change in functions. We organized several public meetings during October, which allowed us to receive direct feedback from the staff we represent. The analysis of this feedback gave rise to very animated discussions in the Staff Council of Thursday 29 October, and resulted in the adoption of a resolution and a declaration in the SCC meeting of 9 November with a series of demands. A meeting of the Staff Council later that week decided to ask for arbitration by the Director-General on a series of six remaining points, a process that came to a fruitful conclusion on Friday 13 November. Let me briefly recall the six points subject to the arbitration: The elimination of tracks, which we considered superfluous. Indeed, they divided staff in four categories, introducing structural complexity without adding value. Having no tracks also lets us correct a kind of injustice, which we consider contrary to the CERN policy of diversity. Indeed, staff in the lower grades, when they come from afar, e.g. the Nordic Countries, the UK, Germany, Central or Southern Europe, do not have access to the international indemnity. And that even though they face the same problems with lower financial resources as their colleagues higher up in the pay scale, wanting to keep contact with relatives at home, or being unable to count on a helping hand from parents when living in the Geneva area. The extension of the transition measures until the next Five-Yearly review in 2020 to mitigate the negative effect of the new system for staff (about 7 % of the current staff population) who are mapped into a personal position where advancement is no longer possible. In the interest of diversity a technical change was also requested to allow, in very specific and well-defined circumstances, for the extension of a female fellowship beyond the maximum duration of three years to cover the period of the maternity leave. Inclusion of the effect of the new career system in future actuarial studies for the CERN health insurance scheme (CHIS). In order to gauge its effect of the new system over several years a fixed merit recognition budget for at least three years. Guarantees on the implementation of the talent management tools necessary for the operation of the new career system, including detailed monitoring using milestones defined and agreed to by the SCC. In the end, thanks to the long efforts during the whole of 2015 of all partners in the concertation process to find a consensus, the Staff Council in office agreed, without enthusiasm, not to oppose the Management proposals for the outcome of the 2015 Five-Yearly Review. Let me now come back to the various measures proposed. Concerning the obligatory component: basic salaries for staff members. The staff is aware that one cannot ignore the difficult economic and financial climate prevailing in several Member States. Yet, the staff is still very disappointed by the proposal of the Management to leave salary levels unchanged, in particular in view of the size of the gap observed in the salary comparisons. One of the purposes of a Five-Yearly review is to ensure that CERN can attract, retain, and motivate staff of the highest competence and integrity coming from all Member States. Can we be sure that the Organization can still attract those with the highest competence from all over Europe and beyond? We doubt that the attractiveness and competitiveness of the Organization can be maintained solely with the measures proposed in the optional part of this Five-Yearly Review. Concerning the optional part, diversity, the staff welcomes the proposed measures since they align CERN with societal changes in our Member States and with good practices observed in most international organizations. Yet they benefit only a minority of staff and are auto-funded by limiting the advancement budget for all staff with respect to the current situation. Finally, let us look at the proposals concerning the CERN career structure. Of course we welcome the advantages claimed by the new system, namely enhanced motivation, transparency, consistency, and rationalization of the career and salary scale structure. But, as pointed out above, we regret the decision of Management to propose a major cost containment measure, i.e., a cut of 20% in the annual advancement budget, coupled to the introduction of non-recurrent performance payments. Whilst the latter payments mitigate the loss in take-home pay, they do not contribute to our social security systems. In the new system, a large fraction of the staff will have their advancement prospects, and consequently the level of their pension, reduced with respect to the current MARS system. For instance, over a career of thirty years this slow-down in advancement will translate, on average, into a loss of pension of some ten per cent. The proposed cost containment measures save an estimated 55 MCHF over the coming ten years with respect to the current situation, and provide cost-neutrality by the next Five-Yearly Review in 2020. Thus this Five-Yearly Review becomes quite a hard sell to the staff, who considers that money is taken from all to give some extra benefits to a few. We already had quite some difficulties to contain a revolt by a non-negligible part of the current Staff Council to obtain a vote not to oppose the Management proposals. The recently elected new Staff Council, whose mandate starts in January 2016, has a much larger fraction of delegates who are critical of the outcome of the current Review. Therefore, it was agreed in the SCC after arbitration that all implementation tools and procedures for the new career structure be discussed in the framework of the concertation process. In particular, a detailed roadmap for the introduction and follow-up of these tools will be defined, and the outcome of the annual advancement and promotion exercises will be monitored quantitatively. CERN has seen the discovery of the higgs in 2012, the extremely successful consolidation work during LS1 in 2013 and 2014 for the LHC, its injectors, the LHC experiments and elsewhere, and, this year, a remarkably effective restart of the LHC at 13 TeV, the other accelerators, and the experiments. Moreover, CERN has a potential for scientific discoveries and technological developments for several decades to come, with HL-LHC, proposals for several non-LHC projects, and, why not, with a future linear or circular accelerator.  The excellence of CERN in basic science and front-line technologies, and its essential role in the training of future specialists has been recognized on numerous occasions. But, excellence has a cost. Yet, this cost is an essential investment in the future, it funds progress, it creates high-quality jobs for our children and grandchildren. All together we must defend the Organization’s success in basic science, an example of peaceful collaboration through equitable cost sharing between Member States that has enabled Europe to have a physics laboratory, unique in the world, and of which we are all proud. Short-term considerations that are often purely financial have no future. We need adequate human and material resources that can ensure the proper functioning of the Organization. We owe it to the future generations; they will be thankful that we have not sacrificed them.

by Staff Association at November 27, 2015 10:13 AM

CERN Bulletin

Lev Borisovich Okun (1929 - 2015)

Soviet and Russian theoretical physicist Lev Borisovich Okun passed away on 23 November, 2015, after a long illness.



Lev Okun was born in 1929, in western Russia and graduated from the Moscow Institute for Physics and Engineering in the early 1950s under the supervision of Arkady Migdal. Lev Okun came to the Institute of Theoretical and Experimental Physics (ITEP) in 1954 as a graduate student of Isaac Pomeranchuk, the head of the ITEP Theory Department. In 1956 he was instrumental in the proof of the Okun–Pomeranchuk theorem, establishing the asymptotic equality of cross sections of certain scattering processes.

A number of Okun’s pioneering works were devoted to weak interactions of elementary particles. In particular, he was among the first to explain the special features of CP preserving neutral kaon decays, and his results on the false vacuum decay and domain walls in cosmology are of paramount importance. His many textbooks on physics are well-known and cherished worldwide.

For many years Lev Okun was the heart of the ITEP Theory Department. He was devoted to physics beyond limits, believing that there can be nothing more noble in the world than theoretical physics. He taught his students to be as committed to physics as he was himself. Many of them became outstanding theorists, now scattered all over the world.

Lev was also a great supporter of the CERN programme. He regularly visited the Theory Division for many years, and was a member of the Scientific Policy Committee. His advice was always extremely valuable. We will miss the great scientist and also the kind, warm and wonderful friend.

His colleagues and friends

November 27, 2015 10:11 AM

November 26, 2015

Sean Carroll - Preposterous Universe


This year we give thanks for an area of mathematics that has become completely indispensable to modern theoretical physics: Riemannian Geometry. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, and the Fourier Transform. Ten years of giving thanks!)

Now, the thing everyone has been giving thanks for over the last few days is Albert Einstein’s general theory of relativity, which by some measures was introduced to the world exactly one hundred years ago yesterday. But we don’t want to be everybody, and besides we’re a day late. So it makes sense to honor the epochal advance in mathematics that directly enabled Einstein’s epochal advance in our understanding of spacetime.

Highly popularized accounts of the history of non-Euclidean geometry often give short shrift to Riemann, for reasons I don’t quite understand. You know the basic story: Euclid showed that geometry could be axiomatized on the basis of a few simple postulates, but one of them (the infamous Fifth Postulate) seemed just a bit less natural than the others. That’s the parallel postulate, which has been employed by generations of high-school geometry teachers to torture their students by challenging them to “prove” it. (Mine did, anyway.)

It can’t be proved, and indeed it’s not even necessarily true. In the ordinary flat geometry of a tabletop, initially parallel lines remain parallel forever, and Euclidean geometry is the name of the game. But we can imagine surfaces on which initially parallel lines diverge, such as a saddle, or ones on which they begin to come together, such as a sphere. In those contexts it is appropriate to replace the parallel postulate with something else, and we end up with non-Euclidean geometry.


Historically, this was first carried out by Hungarian mathematician János Bolyai and the Russian mathematician Nikolai Lobachevsky, both of whom developed the hyperbolic (saddle-shaped) form of the alternative theory. Actually, while Bolyai and Lobachevsky were the first to publish, much of the theory had previously been worked out by the great Carl Friedrich Gauss, who was an incredibly influential mathematician but not very good about getting his results into print.

The new geometry developed by Bolyai and Lobachevsky described what we would now call “spaces of constant negative curvature.” Such a space is curved, but in precisely the same way at every point; there is no difference between what’s happening at one point in the space and what’s happening anywhere else, just as had been the case for Euclid’s tabletop geometry.

Real geometries, as takes only a moment to visualize, can be a lot more complicated than that. Surfaces or solids can twist and turn in all sorts of ways. Gauss thought about how to deal with this problem, and came up with some techniques that could characterize a two-dimensional curved surface embedded in a three-dimensional Euclidean space. Which is pretty great, but falls far short of the full generality that mathematicians are known to crave.

Georg_Friedrich_Bernhard_Riemann.jpeg Fortunately Gauss had a brilliant and accomplished apprentice: his student Bernard Riemann. (Riemann was supposed to be studying theology, but he became entranced by one of Gauss’s lectures, and never looked back.) In 1853, Riemann was coming up for Habilitation, a German degree that is even higher than the Ph.D. He suggested a number of possible dissertation topics to his advisor Gauss, who (so the story goes) chose the one that Riemann thought was the most boring: the foundations of geometry. The next year, he presented his paper, “On the hypotheses which underlie geometry,” which laid out what we now call Riemannian geometry.

With this one paper on a subject he professed not to be all that interested in, Riemann (who also made incredible contributions to analysis and number theory) provided everything you need to understand the geometry of a space of arbitrary numbers of dimensions, with an arbitrary amount of curvature at any point in the space. It was as if Bolyai and Lobachevsky had invented the abacus, Gauss came up with the pocket calculator, and Riemann had turned around a built a powerful supercomputer.

Like many great works of mathematics, a lot of new superstructure had to be built up along the way. A subtle but brilliant part of Riemann’s work is that he didn’t start with a larger space (like the three-dimensional almost-Euclidean world around us) and imagine smaller spaces embedded with it. Rather, he considered the intrinsic geometry of a space, or how it would look “from the inside,” whether or not there was any larger space at all.

Next, Riemann needed a tool to handle a simple but frustrating fact of life: “curvature” is not a single number, but a way of characterizing many questions one could possibly ask about the geometry of a space. What you need, really, are tensors, which gather a set of numbers together in one elegant mathematical package. Tensor analysis as such didn’t really exist at the time, not being fully developed until 1890, but Riemann was able to use some bits and pieces of the theory that had been developed by Gauss.

Finally and most importantly, Riemann grasped that all the facts about the geometry of a space could be encoded in a simple quantity: the distance along any curve we might want to draw through the space. He showed how that distance could be written in terms of a special tensor, called the metric. You give me segment along a curve inside the space you’re interested in, the metric lets me calculate how long it is. This simple object, Riemann showed, could ultimately be used to answer any query you might have about the shape of a space — the length of curves, of course, but also the area of surfaces and volume of regions, the shortest-distance path between two fixed points, where you go if you keep marching “forward” in the space, the sum of the angles inside a triangle, and so on.

Unfortunately, the geometric information implied by the metric is only revealed when you follow how the metric changes along a curve or on some surface. What Riemann wanted was a single tensor that would tell you everything you needed to know about the curvature at each point in its own right, without having to consider curves or surfaces. So he showed how that could be done, by taking appropriate derivatives of the metric, giving us what we now call the Riemann curvature tensor. Here is the formula for it:


This isn’t the place to explain the whole thing, but I can recommend some spiffy lecture notes, including a very short version, or the longer and sexier textbook. From this he deduced several interesting features about curvature. For example, the intrinsic curvature of a one-dimensional space (a line or curve) is alway precisely zero. Its extrinsic curvature — how it is embedded in some larger space — can be complicated, but to a tiny one-dimensional being, all spaces have the same geometry. For two-dimensional spaces there is a single function that characterizes the curvature at each point; in three dimensions you need six numbers, in four you need twenty, and it goes up from there.

There were more developments in store for Riemannian geometry, of course, associated with names that are attached to various tensors and related symbols: Christoffel, Ricci, Levi-Civita, Cartan. But to a remarkable degree, when Albert Einstein needed the right mathematics to describe his new idea of dynamical spacetime, Riemann had bequeathed it to him in a plug-and-play form. Add the word “time” everywhere we’ve said “space,” introduce some annoying minus signs because time and space really aren’t precisely equivalent, and otherwise the geometry that Riemann invented is the same we use today to describe how the universe works.

Riemann died of tuberculosis before he reached the age of forty. He didn’t do bad for such a young guy; you know you’ve made it when you not only have a Wikipedia page for yourself, but a separate (long) Wikipedia page for the list of things named after you. We can all be thankful that Riemann’s genius allowed him to grasp the tricky geometry of curved spaces several decades before Einstein would put it to use in the most beautiful physical theory ever invented.

by Sean Carroll at November 26, 2015 05:24 PM

Peter Coles - In the Dark

Why is General Relativity so difficult?

Just a brief post following yesterday’s centenary of General Relativity, after which somebody asked me what is so difficult about the theory. I had two answers to that, one mathematical and one conceptual.


The Field Equations of General Relativity are written above. In the notation used they don’t look all that scary, but they are more complicated than they look. For a start it looks like there is only one equation, but the subscripts μ and ν can each take four values (usually 0, 1, 2 or 3), each value standing for one of the dimensions of four-dimensional space time. It therefore looks likes there are actually 16 equations. However, the equations are the same if you swap μ  and ν around. This means that there are “only” ten independent equations. The terms on the left hand side are the components of the Einstein Tensor which expresses the effect of gravity through the curvature of space time and the right hand side describes the energy and momentum of “stuff”, prefaced by some familiar constants.

The Einstein Tensor is made up of lots of partial derivatives of another tensor called the metric tensor (which describes the geometry of space time), which relates, through the Field Equations, to how matter and energy are distributed and how these components move and interact. The ten equations that need to be solved simultaneously are second-order non-linear partial different equations. This is to be compared with the case of Newtonian gravity in which only ordinary different equations are involved.

Problems in Newtonian mechanics can be difficult enough to solve but the much greater mathematical complexity in General Relativity means that problems in GR can only be solved in cases of very special symmetry, in which the number of independent equations can be reduced dramatically.

So that’s why it’s difficult mathematically. As for the conceptual problem it’s that most people (I think) consider “space” to be “what’s in between the matter” which seems like it must be “nothing”. But how can “nothing” possess an attribute like curvature? This leads you to conclude that space is much more than nothing. But it’s not a form of matter. So what is it? This chain of thought often leads people to think of space as being like the Ether, but that’s not right either. Hmm.

I tend to avoid this problem by not trying to think about space or space-time at all, and instead think only in terms of particle trajectories or ligh rays and how matter and energy affect them. But that’s because I’m lazy and only have a small brain…



by telescoper at November 26, 2015 03:07 PM

arXiv blog

Why Ball Tracking Works for Tennis and Cricket but Not Soccer or Basketball

Following the examples of tennis and cricket, a new generation of ball-tracking algorithms is attempting to revolutionize the analysis and refereeing of soccer, volleyball, and basketball.

When it comes to ball sports, machine-vision techniques have begun to revolutionize the way analysts study the game and how umpires and referees make decisions. In cricket and tennis, for example, these systems routinely record ball movement in three dimensions and then generate a virtual replay that shows exactly where a ball hit the ground and even predicts its future trajectory (to determine whether it would have hit the wicket, for example).

November 26, 2015 05:30 AM

Clifford V. Johnson - Asymptotia

Happy Centennial, General Relativity!

general_relativity_centennial_kip_thorne(Click for larger view.) Well, I've already mentioned why today is such an important day in the history of human thought - One Hundred years of Certitude was the title of the post I used, in talking about the 100th Anniversary (today) of Einstein completing the final equations of General Relativity - and our celebration of it back last Friday went very well indeed. Today on NPR Adam Frank did an excellent job expanding on things a bit, so have a listen here if you like.

As you might recall me saying, I was keen to note and celebrate not just what GR means for science, but for the broader culture too, and two of the highlights of the day were examples of that. The photo above is of Kip Thorne talking about the science (solid General Relativity coupled with some speculative ideas rooted in General Relativity) of the film Interstellar, which as you know [...] Click to continue reading this post

The post Happy Centennial, General Relativity! appeared first on Asymptotia.

by Clifford at November 26, 2015 03:38 AM

November 25, 2015

Emily Lakdawalla - The Planetary Society Blog

How Can We Write About Science When People Are Dying?
Stories about exploration and wonder can be powerful antidotes to seemingly endless suffering and destruction.

November 25, 2015 11:42 PM

Emily Lakdawalla - The Planetary Society Blog

In Pictures: LightSail Cameras Prepped for Flight
LightSail's flight cameras are being prepped for installation after receiving a software upgrade and checkout from their manufacturer.

November 25, 2015 07:13 PM

astrobites - astro-ph reader's digest

KIC 8462852 – What’s the Fuss?

Kepler light curve of the star

Four years of monitoring this star reveals erratic events when more than 20% of the light, or flux, is missing. The small numbers at the top of the figure correspond to the 17 quarters of Kepler‘s primary operations. This light curve graph shows the fraction of this star’s maximum brightness over time, measured in days.

You’ve probably heard of the star in today’s paper. The “WTF star” (WTF stands for “Where’s the flux?” of course), also informally known as “Tabby’s star,” for the paper’s first author, has been in the media since its discovery and two followup papers hit astro-ph. Today, a group of astrobiters pool our expertise to bring you a comprehensive look at KIC 8462852 and what new observations may reveal.

An otherwise normal star

By nearly all accounts, KIC 8462852 is a normal star. It is one of over 150,000 stars observed by the Kepler space telescope during its initial four-year mission and looks like a run-of-the-mill F-type star, a little more massive than our Sun. It has no companion star yanking it around and no out-of-the-ordinary rotation or magnetic activity. It was passed over by algorithms that search for transiting exoplanets. The only reason this star stood out is thanks to Planet Hunters, a citizen science project that harnesses humans’ pattern recognition skills. Trained volunteers pored over data from Kepler and noted that KIC 8462852 dimmed significantly about two years into Kepler‘s mission, as shown above. They kept an eye on it until a huge fraction of light suddenly went missing again, nearly two years later, but differently this time. The huge dimming and irregular pattern made this star noteworthy.

What could be blocking the flux?

So what could be causing these unusual dips in flux? First, the authors did a careful analysis of their dataset and ruled out any glitches due to things like cosmic ray events and electronic errors within the instrument, concluding that these dips are astrophysically “real.” With glitches ruled out, another possibility is inherent stellar variability, but the shape of the light curve and other characteristics of this star rule out any known type of variable star.

A more likely possibility is that the star is orbited by clumps of dust, which are spread out in an area larger than the size of a planet, and can therefore block more light. But where would this dust come from? There would have to be enough dust to block up to 20% of the star’s visible light, yet not enough dust to produce a telltale infrared glow. The authors suggest that dust near the star could have been produced in a collision between planets, or it might be orbiting large planetesimals, which in turn orbit the star. However, these scenarios both predict a bright infrared signal, which was not detected when WISE and Spitzer observed the system in 2010 and 2015, respectively.

Finally, the authors suggest that the dips may be caused by chunks of some kind giant comet, which is breaking up as it approaches the star. This would provide an explanation for the dips in brightness without the system being bright in the infrared. Though the comet scenario seems to fit the data best, it is still not perfect, and more observations and modeling are needed to show that a comet breakup could produce the light curve of KIC 8462852.

How will we ever know?

Now that we have some ideas of what may be causing the anomalous signal, the next task is to eliminate or verify hypotheses with follow-up observations. Probably the most important piece will be long-term monitoring to look for more dips in brightness. This will answer a multitude of questions to help determine the true cause of the signal: Are the dips in brightness periodic? How much does the depth of the dips vary? Do they change in shape or duration? Do they disappear entirely?

Disintegration of a comet in our Solar System caught by the eye of the Hubble Space Telescope. This comet, named 73P/Schwassmann-Wachmann 3, fragmented off many pieces as it plummeted toward the Sun in 2006. As the radiation from a star heats a comet, the ices that hold it together sublimate, releasing large chunks of rock into space. Something similar may be happening near KIC 8462852.

Disintegration of a comet in our Solar System caught by the eye of the Hubble Space Telescope. This comet, named 73P/Schwassmann-Wachmann 3, fragmented off many pieces as it plummeted toward the Sun in 2006. As the radiation from a star heats a comet, the ices that hold it together sublimate, releasing large chunks of rock into space. Something similar may be happening near KIC 8462852.

Discovering that the dips are periodic would add credence to the dust cloud scenario, though the lack of infrared light would still be a problem. If we measure color information of future dips, that could constrain the size of any dust in the vicinity. On the other hand, if the comet scenario is correct, we would expect to find weaker dips or no future dips as chunks of the fragmented comet spread out, no longer eclipsing the star. There is a small star about 1000 AU from KIC 8462852 which may have provoked a barrage of comets, so measuring the motion of this nearby star could provide insights into the timings of “comet showers” near KIC 8462852.

If future observations manage to rule out all of these hypotheses, the mystery of the “WTF star” will grow stranger still.

The elephant alien in the room

Of course, much of the interest in this star has to do with a follow-up paper by Wright et al. They suggest a more esoteric reason for the huge drops in flux. Over the past few decades, some astronomers have speculated that advanced civilizations could build structures so large that they would block some of the light from their star. The most extreme of these is the Dyson sphere, a vast globe that could theoretically surround a star and harvest its light as a power source. But explaining KIC 8462852’s flux dips in this way doesn’t need something quite as dramatic. Instead, Wright et al. propose a swarm of alien-built objects sequentially passing in front of the star, with variously sized structures causing different dips in the light curve. These so-called megastructures would need to be enormous—up to half the size of the star.

Although this explanation is extremely speculative (starting, as it does, with “suppose an alien civilization exists”), it is consistent with the observations, so Wright et al. suggest searching for artificial radio signals coming from the system. An initial survey has drawn a blank, although only for very powerful signals. So what will we do next? Though further observations will surely take place, for now we need to wait; KIC 8462852 has “moved” into Earth’s daytime sky, making most follow-up observations impossible for several months.

The last word

Here at astrobites, the consensus is that the “WTF” light curve is almost certainly a natural phenomenon. Frequent readers will recognize a common astrobite narrative: The authors of this paper observed something new and unusual! None of our theoretical models explain it very well, so we’re going to get more observations and keep working on simulations!

That said, finding clear signs of an extraterrestrial civilization would be one of the most important discoveries of all time. According to some random guy on twitter, the WTF light curve could clearly be the Milky Way’s own Death Star:


Aliens or not, KIC 8462852 is certainly worth a closer look.


This post was written by Erika Nesvold, Meredith Rawls, David Wilson, and Michael Zevin.

by Astrobites at November 25, 2015 06:40 PM

David Berenstein, Moshe Rozali - Shores of the Dirac Sea

GR turns 100

For those of you who have free time to read on the history of gravitation, here is a good link to many famous papers on the subject:

Happy anniversary GR!


Filed under: gravity, relativity

by dberenstein at November 25, 2015 06:24 PM

John Baez - Azimuth

Regime Shift?

There’s no reason that the climate needs to change gradually. Recently scientists have become interested in regime shifts, which are abrupt, substantial and lasting changes in the state of a complex system.

Rasha Kamel of the Azimuth Project pointed us to a report in Science Daily which says:

Planet Earth experienced a global climate shift in the late 1980s on an unprecedented scale, fueled by anthropogenic warming and a volcanic eruption, according to new research. Scientists say that a major step change, or ‘regime shift,’ in Earth’s biophysical systems, from the upper atmosphere to the depths of the ocean and from the Arctic to Antarctica, was centered around 1987, and was sparked by the El Chichón volcanic eruption in Mexico five years earlier.

As always, it’s good to drill down through the science reporters’ summaries to the actual papers.  So I read this one:

• Philip C. Reid et al, Global impacts of the 1980s regime shift on the Earth’s climate and systems, Global Change Biology, 2015.

The authors of this paper analyzed 72 time series of climate and ecological data to search for such a regime shift, and found one around 1987. If such a thing really happened, this could be very important.

Here are some of the data they looked at:

Click to enlarge them—they’re pretty interesting! Vertical lines denote regime shift years, colored in different ways: 1984 blue, 1985 green, 1986 orange, 1987 red, 1988 brown, 1989 purple and so on. You can see that lots are red.

The paper has a lot of interesting and informed speculations about the cause of this shift—so give it a look.  For now I just want to tackle an important question of a more technical nature: how did they search for regime shifts?

They used the ‘STARS’ method, which stands for Sequential t-Test Analysis of Regime Shifts. They explain:

The STARS method (Rodionov, 2004; Rodionov & Overland, 2005) tests whether the end of one period (regime) of a certain length is different from a subsequent period (new regime). The cumulative sum of normalized deviations from the hypothetical mean level of the new regime is calculated, and then compared with the mean level of the preceding regime. A shift year is detected if the difference in the mean levels is statistically significant according to a Student’s t-test.

In his third paper, Rodionov (2006) shows how autocorrelation can be accounted for. From each year of the time series (except edge years), the rules are applied backwards and forwards to test that year as a potential shift year. The method is, therefore, a running procedure applied on sequences of years within the time series. The multiple STARS method used here repeats the procedure for 20 test-period lengths ranging from 6 to 25 years that are, for simplicity (after testing many variations), of the same length on either side of the regime shift.

Elsewhere I read that the STARS method is ‘too sensitive’. Could it be due to limitations of the ‘statistical significance’ idea involved in Student’s t-test?

You can download software that implements the STARS method here. The method is explained in the papers by Rodionov.

Do you know about this stuff?  If so, I’d like to hear your views on this paper and the STARS method.

by John Baez at November 25, 2015 05:13 PM

Peter Coles - In the Dark

Autumn Statement – Summary for Science

I’ve been in meetings all afternoon so far so I missed the live broadcast of the Chancellor’s Autumn Statement.

Now that I’ve caught up a little it seems that there’s much to be relieved about. Yet again it seems the Government has deployed the tactic of allowing scare stories of dire cuts to spread in order that the actual announcement  appears much better than people feared, even if it is mediocre.

You can find the overall key results of the spending review and autumn statement here, but along with many colleagues who work in research and higher education I went straight to the outcome for the Department of Business, Innovation and Skills (BIS) which you can find here.

The main results for me – from the narrow perspective of a scientist working in a university –  are:

  1. The overall budget for BIS will be cut by 17% in cash terms between now and 2020.
  2. Most of the above cut will happens from 2018 onwards by, among other things, “asking universities to take more responsibility for student access”.
  3. In more detail (quoted from here) “In this context, the government will reduce the teaching grant by £120 million in cash terms by 2019 to 2020, but allow funding for high cost subjects to be protected in real terms. The government will work with the Director of Fair Access to ensure universities take more responsibility for widening access and social mobility, and ask the Higher Education Funding Council for England to retarget and reduce by up to half the student opportunity fund, focusing funding on institutions with the most effective outcomes. The government will also make savings in other areas of the teaching grant.”
  4. My current employer, the University of Sussex, has done extremely well on widening participation so this is good news locally. Many big universities have achieved nothing in this area so, frankly, deserve this funding to be withdrawn.
  5. It is also to be welcomed that the premium for high cost subjects (i.e. STEM disciplines) is to be protected in real terms, although it still does not affect the actual cost of teaching these subjects.
  6. Contrary to many expectations it seems that HEFCE will not be scrapped immediately. That is significant in itself.
  7. The level of science funding will increase from £4.6 billion to £4.7 billion next year, and will thereafter be protected in real terms over the Parliament.
  8. The real terms protection sounds good but of course we currently have a very low rate of inflation, so this is basically five more years of almost flat cash.
  9. There is supposed to be an additional £500m by 2020 which George Osborne didn’t mention in his speech. I don’t know whether this is extra money or just the cash increase estimated by inflation-proofing the £4.7bn.
  10. The above two points sound like good news….
  11. …but the total budget  will include a £1.5 billion new “Global Challenges Fund” which will build up over this period. This suggests that there may be a significant transfer of funds into this from existing programmes. There could be big losers in this process, as it amounts to a sizeable fraction of the total research expenditure.
  12. In any event the fraction of GDP the UK spends on science is not going to increase, leaving us well behind our main economic competitors.
  13. The Government is committed to implementing the Nurse Review, which will give it more direct leverage to reprioritise science spending.
  14. It isn’t clear to me how  “pure” science research will fare as a result of all this. We will have to wait and see….

The Autumn Statement includes only a very high level summary of allocations so we don’t know anything much about how these decisions will filter down to specific programmes at this stage. The Devil is indeed in the Detail. Having said that, the overall settlement for HE and Research looks much better than many of us had feared so I’d give it a cautious welcome. For now.

If anyone has spotted anything I’ve missed or wishes to comment in any other way please use the box below!


by telescoper at November 25, 2015 04:18 PM

ZapperZ - Physics and Physicists

Hot Cocoa Physics
Just in time for the cold weather, at least here in the upper northern hemisphere, APS Physics Central has a nice little experiment that you can do at home with your friends and family. Using just a regular mug, hot water/milk, cocoa mix, and a spoon, you can do a demo that might elicit a few questions and answers.

For those celebrating Thanksgiving this week, I wish you all a happy and safe celebration.


by ZapperZ ( at November 25, 2015 04:01 PM

Symmetrybreaking - Fermilab/SLAC

Revamped LHC goes heavy metal

Physicists will collide lead ions to replicate and study the embryonic universe.

“In the beginning there was nothing, which exploded.”

~ Terry Pratchett, author

For the next three weeks physicists at the Large Hadron Collider will cook up the oldest form of matter in the universe by switching their subatomic fodder from protons to lead ions.

Lead ions consist of 82 protons and 126 neutrons clumped into tight atomic nuclei. When smashed together at extremely high energies, lead ions transform into the universe’s most perfect super-fluid: the quark gluon plasma. Quark gluon plasma is the oldest form of matter in the universe; it is thought to have formed within microseconds of the big bang.

“The LHC can bring us back to that time,” says Rene Bellwied, a professor of physics at the University of Houston and a researcher on the ALICE experiment. “We can produce a tiny sample of the nascent universe and study how it cooled and coalesced to make everything we see today.”

Scientists first observed this prehistoric plasma after colliding gold ions in the Relativistic Heavy Ion Collider, a nuclear physics research facility located at the US Department of Energy’s Brookhaven National Laboratory.

“We expected to create matter that would behave like a gas, but it actually has properties that make it more like a liquid,” says Brookhaven physicist Peter Steinberg, who works on both RHIC and the ATLAS heavy ion program at the LHC. “And it’s not just any liquid; it’s a near perfect liquid, with a very uniform flow and almost no internal resistance."

The LHC is famous for accelerating and colliding protons at the highest energies on Earth, but once a year physicists tweak its magnets and optimize its parameters for lead-lead or lead-proton collisions.

The lead ions are accelerated until each proton and neutron inside the nucleus has about 2.51 trillion electronvolts of energy. This might seem small compared to the 6.5 TeV protons that zoomed around the LHC ring during the summer. But because lead ions are so massive, they get a lot more bang for their buck.

“If protons were bowling balls, lead ions would be wrecking balls,” says Peter Jacobs, a scientist at Lawrence Berkeley National Laboratory working on the ALICE experiment. “When we collide them inside the LHC, the total energy generated is huge; reaching temperatures around 100,000 times hotter than the center of the sun. This is a state of matter we cannot make by just colliding two protons.”

Compared to the last round of LHC lead-lead collisions at the end of Run I, these collisions are nearly twice as energetic. New additions to the ALICE detector will also give scientists a more encompassing picture of the nascent universe’s behavior and personality.

“The system will be hotter, so the quark gluon plasma will live longer and expand more,” Bellwied says. “This increases our chances of producing new types of matter and will enable us to study the plasma’s properties more in depth.”

The Department of Energy, Office of Science, and the National Science Foundation support this research and sponsor the US-led upgrades the LHC detectors.

Bellwied and his team are particularly interested in studying a heavy and metastable form of matter called strange matter. Strange matter is made up of clumps of quarks, much like the original colliding lead ions, but it contains at least one particularly heavy quark, called the strange quark.

“There are six quarks that exist in nature, but everything that is stable is made only out of the two lightest ones,” he says. “We want to see what other types of matter are possible. We know that matter containing strange quarks can exist, but how strange can we make it?”

Examining the composition, mass and stability of ‘strange’ matter could help illuminate how the early universe evolved and what role (if any) heavy quarks and metastable forms of matter played during its development.

by Sarah Charley at November 25, 2015 02:00 PM

Lubos Motl - string vacua and pheno

Does dark matter clump to radial filaments?
Earth's dark matter hair?

Lots of media including The Washington Post, Popular Science, Space Daily, Christian Science Monitor, Russia Today, and Fox News bring us the happy news that Nude Socialist already hyped in August.

The Earth is sprouting hair – radial filaments of dark matter.

This claim is taken from the July 2015 paper by Gary Prézeau, an experimenter at JPL NASA in Pasadena and a member of Planck,
Dense Dark Matter Hairs Spreading Out from Earth, Jupiter and Other Compact Bodies (arXiv)
which has just appeared in the Astrophysical Journal (which produced the new wave of interest). He claims that the ordinary cold dark matter (CDM) is organizing itself in such a way that compact objects including the Earth or other planets develop radial thick enough filaments of dark matter, the hair.

Does it make any sense? I spent about 5 minutes by efforts to understand why would such an anthropomorphic structure completely differing from the usual distributions develop. After some failed attempts to understand what this guy is talking about, I looked at the citation count and it remains at zero at this point. So I am surely not the only one who has problems.

Prézeau claims to have used some computer models but one shouldn't need computer models to explain the qualitative character of the "shape of ordinary dark matter", should he? After 10 minutes, I finally began to understand why he would believe such a thing. It's an interesting point but I still don't believe the conclusion.

A priori, you could think that for the dark matter to organize to well-defined filaments like that, it would need to be rather strongly interacting. After all, powerful molecular forces boiling down to electromagnetism have to act within the human hair for the hair to remain compact. There are no strong forces like that in CDM. So what is responsible for the clumping?

But as I understood, his reasoning is exactly the opposite one. The dark matter is supposed to be clumped into these filaments because it has almost no interactions. And interactions are needed for thermalization etc. So Prézeau claims that at the last scattering surface, the dark matter particles only live at/near a 3-dimensional submanifold of the 6-dimensional phase space.

And the subsequent evolution preserves the regularity and peaky character of the distribution. Only if the dark matter manages to orbit around the galaxy several times, the position and momentum became "chaotic" – more or less Maxwell-Boltzmann-distributed – as the particles are perturbed by various local gravitational fields. But the WIMPs are only flying at 220 kilometers per second. With a circumference over \(10^{18}\) kilometers, it may take some \(10^{16}\) seconds or 0.3 billion years to orbit the galaxy. Those numbers say some 50 orbits since the Big Bang which seems enough to randomize but maybe it is not.

So he claims – while referring to the authority of some computers – that because of the concentrated character at the last scattering surface and "too short and simple" evolution of the phase space in the subsequent 14 billion years, there will be easy to detect clumps. And because of the Earth's or other planetary gravitational fields, there will be hair that starts at a "root" with a very high density and goes outwards.

I seem to have problems with too many statements in the paper. First, I don't really see why the dark matter particle should start at a 3-dimensional manifold in the phase space only. It was spatially everywhere and only the magnitude of the momentum could have been constrained, approximately, right? And the kinetic energy was nonzero so it's still a 5-dimensional space.

Also, he talks about the general relativistic metric. I don't see why he would need general relativity to discuss the hypothetical clumping of matter particles near the weak Earth's gravitational field. Also, he admits that the focal points are \(2\times 10^{15}\) meters, some 10 AU, away from the Earth for the dark matter speed. But why doesn't he agree that this huge distance means that the Earth's gravity is way too weak to modify the distribution of the dark matter at nearby distances – thousands or tens of thousands of kilometers from our planet?

And where does the hypothetical clumping to "preferred angular locations" of the hair come from? The thickness of these filaments is supposed to be vastly smaller than the Earth's radius. Where would such a hugely accurate localization come from? He even proposes these "soon-to-be-discovered" filaments as probes to study geological layers of the Earth!

Also, even his claims about the Kepler problem seem to be wrong to me. When an "unbound" particle moves in the Earth's gravitational field, the trajectory is a hyperbola. At infinity, the hyperbola approaches two lines – in a plane that crosses the center of the Earth. But he seems to claim that the lines themselves go through the Earth's center but they don't. Well, the asymptotic lines are become "close" to lines through the center visually, in the spherical coordinates, but the distance remains nonzero (and much greater than the Earth's radius) in the absolute sense. Prézeau seems to use his wrong idea about the asymptotics to claim that there is some focusing that doesn't actually exist.

And so on and so on. The paper offers lots of technically sounding claims and even elegant equations but it does seem to do almost nothing to explain the extraordinary claim about the shape of the dark matter. At this point, the paper seems to make almost no sense to me. Obviously, this detail doesn't prevent the journalists from selling this 0-citation paper as a scientific fact. For example, Forbes used the title "Strange But True: Dark Matter Grows Hair Around Stars And Planets".

Oh really? Wow, this text is actually by Ethan Siegel.

Does someone understand the paper more well than I do so that she could make me think that the paper is less nonsensical than I thought?

by Luboš Motl ( at November 25, 2015 09:14 AM

Clifford V. Johnson - Asymptotia

The New Improved Scooby-Gang? (Part 1)

This is a group shot from an excellent event I mentioned on here only briefly:


(Click for larger view. Photo from album linked below.) It was on Back to the Future Day... the date (October 21st 2015) that Marty McFly came forward in time to in the second of the BTTF movies... where we found hover boards and so forth, if you recall. The Science and Entertainment Exchange hosted a packed event at the Great Company (in downtown LA) which had several wonderful things and people, including some of the props from the films, the designer of lots of the props from the films, a ballroom done up like the high school prom of the first film, the actor who played George McFly (in the second two films), an actual DeLorean, and so much more. Oh! Also four experts who talked a bit about aspects of the science and other technical matters in the movies, such as [...] Click to continue reading this post

The post The New Improved Scooby-Gang? (Part 1) appeared first on Asymptotia.

by Clifford at November 25, 2015 01:56 AM

astrobites - astro-ph reader's digest

Zooming in on Betelgeuse
  • Title: The close circumstellar environment of Betelgeuse – III. SPHERE/ZIMPOL imaging polarimetry in the visible
  • Authors: P. Kervella, E. Lagadec, M. Montargès, S. T. Ridgway, A. Chiavassa, X. Haubois, H.-M. Schmid, M. Langlois, A. Gallenne, G. Perrin
  • First Author’s Institution: Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU, France & Departamento de Astronomía, Universidad de Chile and LESIA, Observatoire de Paris, PSL, CNRS, UPMC, Univ. Paris-Diderot
  • Paper Status: In press at Astronomy & Astrophysics

Have you ever wondered how to tell the difference between a bright star and a planet in the night sky? Astronomers have a trick: see if the Earth’s atmosphere makes it twinkle. Planets don’t twinkle because, even with a small telescope, they appear as little circles in the sky. It takes a lot of atmospheric turbulence to distort the image of a circular disk. But if you point that same telescope at a star and zoom all the way in, you’ll never zoom far enough to turn that dot into a disk. A star-dot is a “point source” in astro-speak, and dots twinkle.

In recent years, however, advances that pair adaptive optics with powerful telescopes have started resolving real images of the closest, biggest stars. Adaptive optics essentially cancels out the turbulence of twinkling, allowing a telescope to see the disks of stars.

Today’s paper uses this technique to study Betelgeuse. You know Betelgeuse—it’s the bright red one in Orion’s shoulder that’s bound to explode “soon” (so, maybe in the next 100,000 years). It’s an immense red supergiant; the type of star that regularly pushes the envelope of stellar physics. The first paper in this series used images in near-infrared wavelengths to partially resolve Betelgeuse’s photosphere, or visible surface, and begin to characterize the asymmetric material surrounding it. A second paper discovered that Betelgeuse’s circumstellar material is clumpy in the infrared, likely due to dust formation, and extends to tens of solar radii. (That sounds huge, but keep in mind Betelgeuse itself is nearly 1000 times larger than the Sun!).

As it turns out, adaptive optics and polarized visible light are a great way to probe Betelgeuse’s secrets, and that is the focus of today’s paper. This light has lots to tell us about interactions among the star’s surface, the closest and most-recently-ejected clumps of gas, and brand new polarized dust.


Asymmetric Betelgeuse and its environment imaged in visible light (top) and polarized visible light (bottom). Each column is a different filter. The red dashed circle indicates Betelgeuse’s infrared photospheric radius. The light dashed circle is three times this.

You have to squint really hard to make Betelgeuse spherical

As you can see in the images above, Betelgeuse is not symmetric, and neither is its circumstellar material. The top row shows brightness in different visible-light filters while the bottom row shows degree of polarization (light colors are more polarized than dark).

Most of the imaged polarized light is far from the star’s photosphere, and is probably polarized due to dust scattering. However, bits of this dust are close to the star, too! It’s well known that red supergiants like Betelgeuse lose significant amounts of mass. Mass loss seems to be connected to the huge convective cells inside supergiants, because they too are not spherically symmetric, but we don’t know precisely how. We do know the lost mass forms a circumstellar envelope around the star and and provides the material from which dust can form. It follows that if the dust was all far away or all close-in, that would tell us something about how it got there. Instead, at any single distance away from the star, we find different amounts of dust and gas in a range of different temperatures and densities.


Left: A map of hydrogen emission (red) and absorption (blue) in the vicinity of Betelgeuse, with the same dashed lines as before for reference. Right: Color composite of three of the filters from the first figure (the narrow hydrogen alpha filter is excluded).

Hydrogen clues

Two of the filters used to image Betelgeuse are sensitive to the familiar red hydrogen alpha spectral feature. Because one filter is broader than the other, subtracting the light in the narrow filter from the light seen with the broad filter yields a map of where hydrogen gas is emitting or absorbing light. It also turns out to be highly asymmetric. Most of the hydrogen emission is confined within a distance of three times Betelgeuse’s near-infrared radius. It’s a similar distance from the star as most of the polarized dust, but the spatial distributions are different.

The main result of the paper is that Betelgeuse’s asymmetries persist in both in dust and gas, with a major interface between the two located around three times the near-infrared stellar radius. These asymmetries agree with different types of past observations and also strongly point toward a connection between supergiant mass loss and vigorous convection.

Unlike many cosmic blobs, we should be able to witness Betelgeuse change shape. The authors close by suggesting we study how the inner edge of Betelgeuse’s circumstellar envelope evolves with time. So far we only have static images, but if there’s one thing astronomers like more than pictures, it’s movies. Besides, who knows—maybe we’ll catch a supernova in the making!

by Meredith Rawls at November 25, 2015 12:18 AM

November 24, 2015

Lubos Motl - string vacua and pheno

Point-like QFTs in the bulk can't be a consistent theory of QG
Dixon's research is impressive applied science using deep insights by others, mainly string theorists

Lance Dixon is a prominent particle theorist at SLAC. A few days ago, he gave an interview about quantum gravity.
Q&A: SLAC Theorist Lance Dixon Explains Quantum Gravity
He's been most tightly associated with multiloop calculations in quantum field theory (including some calculations at four loops, for example) and various tricks to climb over the seemingly "insurmountably difficult" technical obstacles that proliferate as you are adding loops to the Feynman diagrams. However, as a Princeton graduate student in the 1980s, he's done important research in string theory as well. Most famously, he is one of the co-fathers of the technique of the "orbifolds".

Also, most of his claims in the interview are just fine. But some of his understanding of the big picture is so totally wrong that you could easily post it at one of the crackpots' forums on the Internet.

To answer the question "what is quantum gravity?", he begins as follows:
With the exception of gravity, we can describe nature’s fundamental forces using the concepts of quantum mechanics.
Well, one needs to be more specific about the meaning of "can". In this form, the sentence pretty much says that as we know it, gravity is inconsistent with quantum mechanics. But this isn't right. Frank Wilczek's view is the opposite extreme. Frank says that gravity is so compatible with the quantum field theory (The Standard Model) that he already clumps them into one theory he calls "The Core Theory".

The truth is somewhere in between. Gravity as a set of phenomena is demonstrably consistent with quantum mechanics – we observe both of them in Nature while the gravitational (and other) phenomena simply can't escape the quantum logic of the Universe. And in fact, even our two old-fashioned theories are "basically consistent" for all practical and many of the impractical purposes. We can derive the existence of the Hawking radiation, gravitons, and even their approximate cross sections at any reasonable accuracy from the quantized version of general relativity. Using a straightforward combination of GR and QFT, we may even calculate the primordial gravitational fluctuations that have grown into galaxies and patterns in the CMB.

The "only" problem is that those theories can't be fully compatible or the predictions can't be arbitrarily precise, at least if we want to avoid the complete loss of predictivity (the need to measure infinitely many continuous parameters before any calculation of a prediction may be completed).

OK, Dixon says lots of sane things about the similarities and differences between electromagnetism and gravity, the character of difficulties we encounter when we apply the QFT methods to gravity, and some new hard gravitational phenomena such as the Hawking radiation. But things become very strange when he is asked:
Why is it so difficult to find a quantum theory of gravity?

One version of quantum gravity is provided by string theory, but we’re looking for other possibilities.
You may also look for X-Men in New York. You may spend lots of time with this search which doesn't mean that you will have a reasonable chance to find them. There are no X-Men! The case of "other possible" theories of quantum gravity aside from string theory is fully analogous.

Moreover, I don't really think that any substantial part of Dixon's own work could be described as this meaningless "search for other theories of quantum gravity". Whenever gravity enters his papers at all, he is researching well-known i.e. old approximate theories of quantum gravity – such as various supergravity theories.

Dixon says that gravitons' spin is two, the force is weak, and universally attractive. But the next question is:
How does this affect the calculations?

It makes the mathematical treatment much more difficult.

We generally calculate quantum effects by starting with a dominant mathematical term to which we then add a number of increasingly smaller terms.
He's describing "perturbative calculations" – almost all of his work may be said to be about "perturbative calculations". However, it is simply not true that this is the right way to do research of quantum mechanics "in general". Perturbation theory is just an important method.

It is true that if we talk about "quantum effects", in the sense of corrections, we must start with a "non-quantum effect" i.e. the classical approximation and calculate the more accurate result by adding the "quantum corrections". But it is simply not always the case that a chosen "classical result" is the dominant contribution. Sometimes, physics is so intrinsically quantum that one must try to make the full-fledged quantum calculations right away.

Even more importantly, he tries to obscure the fact that the perturbative – power law – corrections are not the only effects of quantum mechanics. When he does these power-law perturbative calculations, and his papers arguably never do anything else, he is not getting the exact result. There almost always exist infinitely many non-perturbative corrections, instantons etc. The existence of the non-perturbative effects is actually related to the divergence of the perturbative series as a whole.

To summarize, he is just vastly overstating the importance of the perturbative – and especially multiloop – calculations, the kind of calculations his work has focused on. You know, these multiloop terms are only important relatively to the classical term if the quantum effects are sufficiently strong. But if they are strong enough to contribute \(O(100)\) percent of the result, then the non-perturbative terms neglected by Dixon will contribute \(O(100)\) percent, too. In other words, the multiloop terms are "in between" two other types of contributions, classical and nonperturbative, which is why they generally aren't the key terms.

In practice, Dixon's work has been about the question "up to how many loops do all the divergences cancel" in a given supersymmetric theory. Does \(d=11\) supergravity cancel all divergences even at seven loops? True experts have to care about this question but ultimately, it is a technical detail. Supersymmetry allows the theory to "get rather far" but at the end, this theory and its toroidal compactifications can't be consistent and have to be completed to the full string/M-theory for consistency.

If you click the link in the previous sentence, you may remind yourself that nonperturbatively, the \(\NNN=8\) \(d=4\) SUGRA theory simply isn't OK. It wouldn't be OK even if the whole perturbative expansion of SUGRA were convergent (which I am not self-confidently excluding at all even though I do tend to believe those who say that there are divergences at 7 loops). This is why all the hard technical work in Dixon's multiloop papers consists of irrelevant technical details that simply don't affect the answers to the truly important questions. You don't need to know anything about Dixon's papers but you may still comprehend and verify the arguments in the following paragraph.

The theory has the noncompact continuous symmetry but the symmetry has to be broken because the spectum of charged black hole microstates has to be discrete thanks to the Dirac quantization rule (the minimum electric and the minimum magnetic charge are "inverse" to one another if the Dirac string is invisible). That's why the \(E_{k(k)}(\RR)\) symmetry is unavoidably broken to a discrete subgroup of it, \(E_{k(k)}(\ZZ)\), the subgroup that preserves the lattice of the charges, just like in string/M-theory, and all the other "purely stringy phenomena" that go beyond SUGRA (starting with the existence of low-tension/light strings in a weakly coupled, stringy limit of the moduli space we just identified) may then be proven to be there, too.

Also, the \(\NNN=8\) SUGRA is too constrained because it's too supersymmetric. To get more realistic spectra, you need to reduce the SUSY and then the divergences unavoidably appear at a small number of loops. So effective gravitational QFTs are either realistic or relatively convergent at the multiloop level but not both. There is a trade-off here. Again, string/M-theory is the only way to make the theories realistic while preserving the convergence properties. In some sense, all the SUSY breaking in string theory may be said to be spontaneous (the compactification on a complicated manifold is a spontaneous symmetry breaking of symmetries that would be present for other manifolds).

SUGRA-like quantum field theories are wrong for other, perhaps more qualitative reasons. They can't really agree with holography or, more immediately, with the high-mass spectrum of the excitations. High mass excitations must be dominated by black hole microstates with the entropy scaling like the area. But the high energy density behavior of a QFT in a pre-existing background always sees the entropy scale like the volume. The real problem is that the background just can't be assumed to be fixed in any sense if we get to huge (Planckian) energy or entropy densities. It follows that the causal structure is heavily non-classical in the quantum gravity regime as well, and this is what makes the bulk QFT inapplicable as a framework.

This was an example but I want to stress a very general point that makes Dixon's argumentation totally weird:
Dixon uses all the self-evidently pro-string arguments as if they were arguments in favor of "another theory".
This paradox manifests itself almost in every aspect of Dixon's story. Let me be more specific. There are several paragraphs saying things like
We’ve succeeded in using this discovery to calculate quantum effects to increasingly higher order, which helps us better understand when divergences occur.
And these comments are implicitly supposed to substantiate Dixon's previous claim that "he is looking for other theories of quantum gravity". Except that virtually all the good surprises he has encountered exists thanks to insights discovered in string theory!

First of all, the cancellations of divergences in his SUGRA papers depend on supersymmetry – plus other structures but SUSY is really needed. (All known cancellations of divergences in \(\NNN=8\) SUGRA may be fuly derived from SUSY and the non-compact \(E_{7(7)}(\RR)\) symmetry!) In the West, SUSY was discovered when people were trying to find a better string theory than the old \(d=26\) bosonic string theory. The world sheet supersymmetry was found to be necessary to incorporate fermions. And the spacetime supersymmetry emerged and seemed necessary to eliminate the tachyon in the spacetime. The ability of SUSY to cancel lots of (mostly divergent) terms was quickly revealed and became established. It was clear that SUSY is capable of cancelling the divergences; the only remaining questions were "which ones" and "how accurately".

You know, this kind of "silence" about the importance of SUSY for the cancellation of divergences; and about SUSY's role within string theory is unavoidably inviting some insane interpretations. In the past, the notorious "Not Even Wrong" crackpot forum has often promoted the ludicrous story – implicitly encouraged by Dixon's comments – that maybe we don't need string theory because field theories might cancel the divergences.

The following blog post on that website would attack supersymmetry.

The irony is that the good news in the first story are primarily thanks to supersymmetry which is trashed in the second story. So the two criticisms of string-theory-related physics directly contradict one another! You may either say that SUSY should be nearly banned in the search for better theories of Nature; or you may celebrate results that depend on SUSY. But you surely shouldn't do both at the same moment, should you?

But it's not just supersymmetry and the reasons behind the cancellation of divergences where Dixon's story sounds ludicrously self-contradictory. What about the relationship between gravitons and gluons?
What have you learned about quantum gravity so far?

Over the past decades, researchers in the field have made a lot of progress in better understanding how to do calculations in quantum gravity. For example, it was empirically found that in certain theories and to certain orders, we can replace the complicated mathematical expression for the interaction of gravitons with the square of the interaction of gluons – a simpler expression that we already know how to calculate.
So the insight that gravitons behave like squared gluons is also supposed to be an achievement of the "search for other, non-stringy theories of quantum gravity"? Surely you're joking, Mr Dixon. You know, this "gravitons are squared gluons" relationship is known as the KLT (Kawai-Lewellen-Tye) relationship. Their 1986 paper was called A Relation Between Tree Amplitudes of Closed and Open Strings. Do you see any strings in that paper? ;-) It is all about string theory – and the characteristic stringy properties of the particle spectrum and interactions (including the detailed analysis of the topologies of different strings).

The point is that an open string – a string with two endpoints – has the \(n\)-th standing wave and the corresponding modes to be excited, \(\alpha_n^\mu\). A closed string – topologically a circle – has the \(n\)-th left-moving wave and \(n\)-th right-moving wave. The operators capable of exciting the closed string come from left-movers and right-movers, \(\alpha_n^\mu\) and \(\tilde \alpha_n^\mu\). So the closed string has twice as many operators that may excite it – it looks like a pair of open strings living together (its Hilbert space is close to a tensor product of two open string Hilbert spaces). Similarly, the amplitudes for closed strings look like (combinations of) products of analogous amplitudes from two copies of open strings. That's the basic reason behind all these KLT relationships. And now, in 2015, Dixon indirectly suggests that this relationship is an achievement of the search for non-stringy theories of quantum gravity?

This relationship was found purely within string theory and it only remains valid and non-vacuous to the extent to which you preserve a significant portion of the string dynamics. The relationship tells you lots about the dynamics of the massless states as well. But you can't really find any good quantitative explanation why the relationship works in so many detailed situations that would be non-stringy. It's only in string theory where the graviton \(\alpha^{\mu}_{-1}\tilde\alpha^\nu_{-1}\ket 0\) is "made of" two gluons – because it has these two creation operators which are analogous to the one creation operator in the gluon open string state \(\alpha^{\mu}_{-1}\ket 0\). The point-like graviton has the spin two, as two times the spin of a gluon, but you can't "see" the two gluons inside the graviton because all the particles are infinitely small.

And this kind of irony goes on and on and on. He has used SUSY and KLT relationships as evidence for a "non-stringy" theory of quantum gravity. Is there something else that he can use against strings? Sure, dualities! ;-)
We were also involved in a recent study in which we looked at the theory of two gravitons bouncing off each other. It was shown over 30 years ago that divergences occurring on the second order of these calculations can change under so-called duality transformations that replace one description of the gravitational field with a different but equivalent one. These changes were a surprise because they could mean that the descriptions are not equivalent on the quantum level. However, we’ve now demonstrated that these differences actually don’t change the underlying physics.
This is about equally amazing. You know, this whole way of "duality" reasoning – looking for and finding theories whose physics is the same although superficially, there seem to be serious technical differences between two theories or vacua – has spread purely because of the research done by string theorists in the early and mid 1990s. The first paper that Dixon et al. cite is a 1980 SUGRA paper by Duff and Nieuwenhuizen and the duality is meant to be "just" an electromagnetic duality for the \(p\)-forms. But before string theory, people indeed believed that such dualities weren't exact symmetries of the theories. Only within the string-theory-based research, many such dualities were shown to be surprisingly exact. They are just claiming a similar phenomenon in a simpler theory. They would probably never dare to propose such a conjecture if there were no stringy precedents for this remarkably exact relationship. The previous sentence may be a speculation but what is not a speculation is that they're far from the first ones who have brought evidence for the general phenomenon, a previously disbelieved exact equivalence (duality). Tons of examples of this phenomenon has previously been found by string theorists.

Most of the examples of dualities arose in the context of string theory but even the cases of dualities that apply to field theories, like insights about the Seiberg-Witten \(\NNN=2\) gauge theories etc., were found when the authors were thinking about the full stringy understanding of the physical effects. They may have tried to hide their reasoning in their paper to make the paper more influential even among the non-stringy researchers but you can't hide the truth forever. Most experts doing this stuff today are thinking in terms of the embeddings to string/M-theory anyway because those embeddings are extremely natural if not paramount.

So what Dixon was doing was just trying to apply a powerful tool discovered in the string theory research to a situation that is less rich than the situations dealt with in string theory.

Near the end, Dixon joined the irrational people who don't like that string theory has many solutions:
However, over the years, researchers have found more and more ways of making string theories that look right. I began to be concerned that there may be actually too many options for string theory to ever be predictive, when I studied the subject as a graduate student at Princeton in the mid-1980s. About 10 years ago, the number of possible solutions was already on the order of \(10^{500}\). For comparison, there are less than \(10^{10}\) people on Earth and less than \(10^{12}\) stars in the Milky Way. So how will we ever find the theory that accurately describes our universe?

For quantum gravity, the situation is somewhat the opposite, making the approach potentially more predictive than string theory, in principle. There are probably not too many theories that would allow us to properly handle divergences in quantum gravity – we haven’t actually found a single one yet.
I had to laugh out loud. So Dixon wants one particular theory. He has zero of them so he's equally far from a theory of everything as string theorists who have a theory with many solutions. Is that meant seriously? Zero is nothing! In Czech, when you have zero of something, we say that you have "a šit". Nothing is just not too much.

Moreover, Dixon's comment about "making string theories" has been known to be totally wrong since the mid 1990s. There is only one string theory which has many solutions – just like the equation for the hydrogen energy eigenstates has many solutions. There are not "many string theories". This fact wasn't clear before the mid 1990s but it became totally clear afterwards. When Dixon continues to talk about "many string theories", it's just like someone who talks about evolution but insists that someone created many species at the same moment. The whole point of evolution is that this isn't the case. Even though the species look different, they ultimately arose from the same ancestors.

To talk about very many ancestors of known species means to seriously misunderstand or distort the very basics of biology and Dixon is doing exactly the same thing with the different string vacua. What he's saying is as wrong as creationism. A professional theoretical physicist simply shouldn't embarrass himself in this brutal way in 2015.

Dixon wants to say that we want "one right theory" and he has "zero" why string theorists have "\(10^{500}\)" which is also far from the number he wants, one. But even if this "distance" were measuring the progress, the whole line of reasoning would be totally irrational because the number "one" is pure prejudice with zero empirical or rational support. You may fool yourself by saying that a theory of nuclei predicting 1 or 50 possible nuclei (or a theory of biology predicting that there should be 1 or 50 mammal species) is "more predictive" and therefore "better" but this rhetorical sleight-of-hand won't make the number 1 or 50 right. The right number of nuclei or mammal species or vacuum-like solutions to the underlying equations is much higher than 1 or 50. Emotional claims about a "better predictivity" can never beat or replace the truth! It's too bad that Dixon basically places himself among the dimwits who don't understand this simple point.

What we observe is that there exists at least one kind of a Universe, or one string vacuum if you describe physics by string theory. There is no empirical evidence whatever that the number isn't greater than one or much greater than one. Instead, there is a growing body of theoretical evidence that the right number almost certainly exceeds one dramatically.

At some moment, Lance Dixon decided to study the heavily technical multiloop questions and similar stuff. It's a totally serious subset of work in theoretical physics but it simply lacks the "wow" factor. Maybe he wants to fool others as well as himself into thinking that the "wow" factor is there. But it isn't there. A cancellation of 4-loop divergences in a process described by a theory is simply a technicality. A physicist who can calculate such things is surely impressively technically powerful and that is what will impress fellow physicists. But the result itself is unlikely to be a game-changer. Most of such results are entries in a long telephone directory of comparable technical results and non-renormalization theorems.

The true game-changers in the recent 40 years were concepts like supersymmetry, duality, KLT relations, holography and AdS/CFT, Matrix theory, ER-EPR or entanglement-glue correspondence, and perhaps things like the Yangian, recursive organization of amplitudes sorted by the helicities etc. Many of them have been use in Dixon's technically impressive research. But this research has been an application of conceptually profound discoveries made by others, not a real source of new universally important ideas, and it's just very bad if Dixon tries to pretend something else.

And that's the memo.

by Luboš Motl ( at November 24, 2015 06:54 PM

Emily Lakdawalla - The Planetary Society Blog

Blue Origin Lands Spent Suborbital Rocket Stage in Texas
Secretive spaceflight company Blue Origin flew its New Shepard launch vehicle to the edge of space, deployed a suborbital spacecraft and returned the spent booster rocket to Earth for an upright landing.

November 24, 2015 04:24 PM

Symmetrybreaking - Fermilab/SLAC

Charge-parity violation

Matter and antimatter behave differently. Scientists hope that investigating how might someday explain why we exist.

One of the great puzzles for scientists is why there is more matter than antimatter in the universe—the reason we exist.

It turns out that the answer to this question is deeply connected to the breaking of fundamental conservation laws of particle physics. The discovery of these violations has a rich history, dating back to 1956.

Parity violation

It all began with a study led by scientist Chien-Shiung Wu of Columbia University. She and her team were studying the decay of cobalt-60, an unstable isotope of the element cobalt. Cobalt-60 decays into another isotope, nickel-60, and in the process, it emits an electron and an electron antineutrino. The nickel-60 isotope then emits a pair of photons. 

The conservation law being tested was parity conservation, which states that the laws of physics shouldn’t change when all the signs of a particle’s spatial coordinates are flipped. The experiment observed the decay of cobalt-60 in two arrangements that mirrored one another. 

The release of photons in the decay is an electromagnetic process, and electromagnetic processes had been shown to conserve parity. But the release of the electron and electron antineutrino is a radioactive decay process, mediated by the weak force. Such processes had not been tested in this way before.

Parity conservation dictated that, in this experiment, the electrons should be emitted in the same direction and in the same proportion as the photons. 

But Wu and her team found just the opposite to be true. This meant that nature was playing favorites. Parity, or P symmetry, had been violated.

Two theorists, Tsung Dao Lee and Chen Ning Yang, who had suggested testing parity in this way, shared the 1957 Nobel Prize in physics for the discovery.

Charge-parity violation

Many scientists were flummoxed by the discovery of parity violation, says Ulrich Nierste, a theoretical physicist at the Karlsruhe Institute of Technology in Germany. 

“Physicists then began to think that they may have been looking at the wrong symmetry all along,” he says. 

The finding had ripple effects. For one, scientists learned that another symmetry they thought was fundamental—charge conjugation, or C symmetry—must be violated as well. 

Charge conjugation is a symmetry between particles and their antiparticles. When applied to particles with a property called spin, like quarks and electrons, the C and P transformations are in conflict with each other.

“Physicists then began to think that they may have been looking at the wrong symmetry all along.”

This means that neither can be a good symmetry if one of them is violated. But, scientists thought, the combination of the two—called CP symmetry—might still be conserved. If that were the case, there would at least be a symmetry between the behavior of particles and their oppositely charged antimatter partners. 

Alas, this also was not meant to be. In 1964, a research group led by James Cronin and Val Fitch discovered in an experiment at Brookhaven National Laboratory that CP is violated, too. 

The team studied the decay of neutral kaons into pions; both are composite particles made of a quark and antiquark. Neutral kaons come in two versions that have different lifetimes: a short-lived one that primarily decays into two pions and a long-lived relative that prefers to leave three pions behind.  

However, Cronin, Fitch and their colleagues found that, rarely, long-lived kaons also decayed into two instead of three pions, which required CP symmetry to be broken. 

The discovery of CP violation was recognized with the 1980 Nobel Prize in physics. And it led to even more discoveries. 

It prompted theorists Makoto Kobayashi and Toshihide Maskawa to predict in 1973 the existence of a new generation of elementary particles. At the time, only two generations were known. Within a few years, experiments at SLAC National Accelerator Laboaratory found the tau particle—the third generation of a group including electrons and muons. Scientists at Fermi National Accelerator Laboratory later discovered a third generation of quarks—bottom and top quarks.

Digging further into CP violation

In the late 1990s, scientists at Fermilab and European laboratory CERN found more evidence of CP violation in decays of neutral kaons. And starting in 1999, the BaBar experiment at SLAC and the Belle experiment at KEK in Japan began to look into CP violation in decays of composite particles called B mesons

By analyzing dozens of different types of B meson decays, scientists on BaBar and Belle revealed small differences in the way B mesons and their antiparticles fall apart. The results matched the predictions of Kobayashi and Maskawa, and in 2008 their work was recognized with one half of the physics Nobel Prize.

“But checking if the experimental data agree with the theory was only one of our goals,” says BaBar spokesperson Michael Roney of the University of Victoria in Canada. “We also wanted to find out if there is more to CP violation than we know.”

This is because these experiments are seeking to answer a big question: Why are we here?

When the universe formed in the big bang 14 billion years ago, it should have generated matter and antimatter in equal amounts. If nature treated both exactly the same way, matter and antimatter would have annihilated each other, leaving nothing behind but energy. 

And yet, our matter-dominated universe exists.

CP violation is essential to explain this imbalance. However, the amount of CP violation observed in particle physics experiments so far is a million to a billion times too small.

Current and future studies

Recently, BaBar and Belle combined their data treasure troves in a joint analysis1First observation of CP violation in B0->D(*)CP h0 decays by a combined time-dependent analysis of BaBar and Belle data. It revealed for the first time CP violation in a class of B meson decays that each experiment couldn't have analyzed alone due to limited statistics.

This and all other studies to date are in full agreement with the standard theory. But researchers are far from giving up hope on finding unexpected behaviors in processes governed by CP violation.

The future Belle II, currently under construction at KEK, will produce B mesons at a much higher rate than its predecessor, enabling future CP violation studies with higher precision.

And the LHCb experiment at CERN’s Large Hadron Collider is continuing studies of B mesons, including heavier ones that were only rarely produced in the BaBar and Belle experiments. The experiment will be upgraded in the future to collect data at 10 times the current rate.

To date, CP violation has been observed only in particles like these ones made of quarks.

“We know that the types of CP violation already seen using some quark decays cannot explain matter’s dominance in the universe,” says LHCb collaboration member Sheldon Stone of Syracuse University. “So the question is: Where else could we possibly find CP violation?”

One place for it to hide could be in the decay of the Higgs boson. Another place to look for CP violation is in the behavior of elementary leptons—electrons, muons, taus and their associated neutrinos. It could also appear in different kinds of quark decays. 

“To explain the evolution of the universe, we would need a large amount of extra CP violation,” Nierste says. “It’s possible that this mechanism involves unknown particles so heavy that we’ll never be able to create them on Earth.”

Such heavyweights would have been produced last in the very early universe and could be related to the lack of antimatter in the universe today. Researchers search for CP violation in much lighter neutrinos, which could give us a glimpse of a possible large violation at high masses.

The search continues.

by Manuel Gnida and Kathryn Jepsen at November 24, 2015 02:00 PM

arXiv blog

Data Mining Reveals How Smiling Evolved During a Century of Yearbook Photos

By mining a vast database of high-school yearbook photos, a machine-vision algorithm reveals the change in hairstyles, clothing, and even smiles over the last century.

Data mining has changed the way we think about information. Machine-learning algorithms now routinely chomp their way through data sets of Twitter conversations, travel patterns, phone calls, and health records, to name just a few. And the insights this brings is dramatically improving our understanding of communication, travel, health, and so on.

November 24, 2015 11:27 AM

November 23, 2015

astrobites - astro-ph reader's digest

A 1500 Year Old Explosion (maybe)

Title: Discovery of an eclipsing dwarf nova in the ancient nova shell Te 11

Authors: Brent Miszalski, P. A. Woudt, S. P. Littlefair et al.

First author’s institution: South African Astronomical Observatory

Status: Accepted for publication in MNRAS


The nebula Te 11. Observations of the interacting star at its core suggest it may have been formed by a giant explosion, matching one spotted by astronomers over 1500 years ago.

On the 16th of November in 483 CE, astronomers in China recorded the appearance of “a guest star east of Shen, as large as a peck measure, and like a fuzzy star”. The new celestial light shone brightly for just under a month, then faded to nothing. Over 1500 years later, the authors of today’s paper may have found the source.

The suspect is a nebula known as Te 11, a cloud of expanding, glowing gas around half a light-year across at its widest point. Te 11 was originally thought to be a planetary nebula. These are, confusingly, nothing to do with planets, but are instead made out of material thrown off a red giant star as it shrinks into a white dwarf.

But although visually Te 11 looks like a planetary nebula, many of its characteristics don’t quite fit. It’s moving too slowly, and has much less mass than other, confirmed examples.

To search for alternative ways in which the nebula could have formed, the authors obtained a light curve, shown in the figure below, and spectroscopy of the object lurking in Te 11’s centre. They found a white dwarf,  just as the planetary nebula hypotheses predicted. But it wasn’t alone.


Light curve of the dwarf nova at the core of Te 11, showing an eclipse of the white dwarf by its companion star. The purple, red, yellow and green lines show the contribution from the white dwarf, disc, a bright spot on the disc, and elliptical shape of the system, respectively, adding up to make the blue line.

The white dwarf is accompanied by an M dwarf star, so close together that they orbit around their centre of mass in less than three hours. At such close proximity, the gravity of the white dwarf draws material off its companion, forming a ring of gas known as an accretion disc. The material in the disc then gradually spirals down onto the white dwarf.


Artist’s impression of a dwarf nova. The gravity of the white dwarf draws material off the companion star, forming an accretion disc. Image: NOAA

In a number of these systems, the disc becomes unstable every few years, probably due to a change in the viscosity of the gas caused by a rise in temperature (no one is exactly sure how it works). The material falling onto the white dwarf briefly turns from a gentle trickle into a raging torrent, releasing huge amounts of gravitational energy as light. The regular mini-explosions give the systems their name: Dwarf novae, after the larger cosmic explosions called novae and supernovae.

The authors’ observations of Te 11 had been prompted by five novae-like events in the last ten years, spotted by the Catalina Real-Time Transit Survey. The new observations both confirmed that the system was a dwarf nova, and provided exact measurements of some of the characteristics of the two stars, such as their masses and radii.

Te11 hosts an was unusually massive white dwarf, 1.18 times the mass of the Sun (a typical white dwarf is around 0.6). This meant that, as well as dwarf novae, bigger classical novae could also occur. Classical novae take place when the mass building up on the white dwarf becomes so dense that the hydrogen begins to fuse, releasing huge amounts of energy and blowing apart the (newly added) outer layers of the star.

Such a high mass white dwarf means that a novae could reasonably have occurred recently, within a time scale of hundreds of years. The material from the novae would have slammed into the unusually dense interstellar medium in the area, creating the Te 11 nebula. The authors postulate that this huge explosion was the source of the “fuzzy star” spotted in 483 CE.

Miszalski et al. finish by suggesting that more novae could have occurred since then, and high resolution imagining might reveal shells of material nestled inside the nebula. Observing these would give unprecedented insight into the physics of novae and the structures they leave behind.


by David Wilson at November 23, 2015 03:27 PM

The Great Beyond - Nature blog

Archived newsblog

Nature’s news team is no longer updating this newsblog: all articles below are archived and extend up to the end of 2014.

Please go to for the latest breaking news from Nature.

by Richard Van Noorden at November 23, 2015 10:20 AM

Tommaso Dorigo - Scientificblogging

Supersymmetry Is About To Be Discovered, Kane Says
While in the process of fact-checking information that is contained in the book I am finalizing, I had the pleasure to have a short discussion with Gordon Kane during the weekend. A Victor Weisskopf distinguished professor at the University of Michigan as well as a director emeritus of the Michigan Center for Theoretical Physics, Gordon is one of the fathers of Supersymmetry, and has devoted the last three decades to its study.

read more

by Tommaso Dorigo at November 23, 2015 09:50 AM

November 21, 2015

Sean Carroll - Preposterous Universe

Long-Term Forecast

This xkcd cartoon is undeniably awesome as-is, but the cosmologist in me couldn’t resist adding one more row at the bottom.


Looks like the forecast calls for Boltzmann Brains! I guess Hilbert space is finite-dimensional after all.

by Sean Carroll at November 21, 2015 02:00 AM

November 20, 2015

Symmetrybreaking - Fermilab/SLAC

Physicists get a supercomputing boost

Scientists have made the first-ever calculation of a prediction involving the decay of certain matter and antimatter particles.

Sometimes the tiniest difference between a prediction and a result can tell scientists that a theory isn’t quite right and it’s time to head back to the drawing board.

One way to find such a difference is to refine your experimental methods to get more and more precise results. Another way to do it: refine the prediction instead. Scientists recently showed the value of taking this tack using some of the world’s most powerful supercomputers.

An international team of scientists has made the first ever calculation of an effect called direct charge-parity violation—or CP violation—a difference between the decay of matter particles and of their antimatter counterparts.

They made their calculation using the Blue Gene/Q supercomputers at the RIKEN BNL Research Center at Brookhaven National Laboratory, at the Argonne Leadership Class Computing Facility at Argonne National Laboratory, and at the DiRAC facility at the University of Edinburgh.

Their work took more than 200 million supercomputer core processing hours—roughly the equivalent of 2000 years on a standard laptop. The project was funded by the US Department of Energy’s Office of Science, the RIKEN Laboratory of Japan and the UK Science and Technology Facilities Council.

The scientists compared their calculated prediction to experimental results established in 2000 at European physics laboratory CERN and Fermi National Accelerator Laboratory.

Scientists first discovered evidence of indirect CP violation in a Nobel-Prize-winning experiment at Brookhaven Lab in 1964. It took them another 36 years to find evidence of direct CP violation.

“This so-called ‘direct’ symmetry violation is a tiny effect, showing up in just a few particle decays in a million,” says Brookhaven physicist Taku Izubuchi, a member of the team that performed the calculation.

Physicists look to CP violation to explain the preponderance of matter in the universe. After the big bang, there should have been equal amounts of matter and antimatter, which should have annihilated with one another. A difference between the behavior of matter and antimatter could explain why that didn’t happen.

Scientists have found evidence of some CP violation—but not enough to explain why our matter-dominated universe exists.

The supercomputer calculations, published in Physical Review Letters 1 Standard Model prediction for direct CP violation in K→ππ decay, so far show no statistically significant difference between prediction and experimental result in direct CP violation.

But scientists expect to double the accuracy of their calculated prediction within two years, says Peter Boyle of the University of Edinburgh. “This leaves open the possibility that evidence for new phenomena… may yet be uncovered.”

by Kathryn Jepsen at November 20, 2015 07:09 PM

Tommaso Dorigo - Scientificblogging

Anomaly! - A Different Particle Physics Book
I was very happy today to sign a contract with an international publisher that will publish a book I have written. The book, titled "Anomaly! - Scientific Discoveries and the Quest for the Unknown", focuses on the CDF experiment, a particle detector that operated at the Tevatron collider for 30 years. 
The Tevatron was the highest-energy collider until the turn-on of the LHC. The CDF and DZERO experiments there discovered the sixth quark, the top, and produced a large number of world-class results in particle physics. 

read more

by Tommaso Dorigo at November 20, 2015 02:02 PM

Matt Strassler - Of Particular Significance

An Overdue Update

A number of people have asked why the blog has been quiet. To make a long story short, my two-year Harvard visit came to an end, and my grant proposals were turned down. No other options showed up except for a six-week fellowship at the Galileo Institute (thanks to the Simons Foundation), which ended last month.  So I am now employed outside of science, although I maintain a loose affiliation with Harvard as an “Associate of the Physics Department” (thanks to Professor Matt Schwartz and his theorist colleagues).

Context: U.S. government cuts to theoretical high-energy physics groups have been 25% to 50% in the last couple of years. (Despite news articles suggesting otherwise, billionaires have not made up for the cuts; and most donations have gone to string theory, not particle physics.) Spare resources are almost impossible to find. The situation is much better in certain other countries, but personal considerations keep me in this one.

News from the Large Hadron Collider (LHC) this year, meanwhile, is optimistic though not without worries. The collider itself operated well despite some hiccups, and things look very good for next year, when the increased energy and high collision rate will make the opportunities for discoveries the greatest since 2011. However, success depends upon the CMS experimenters and their CERN lab support fixing some significant technical problems afflicting the CMS detector and causing it to misbehave some fraction of the time. The ATLAS detector is working more or less fine (as is LHCb, as far as I know), but the LHC can’t run at all while any one of the experimental detectors is open for repairs. Let’s hope these problems can be solved quickly and the 2016 run won’t be much delayed.

There’s a lot more to say about other areas of the field (gravitational waves, neutrinos, etc.) but other bloggers will have to tell those tales. I’ll keep the website on-line, and will probably write some posts if something big happens. And meanwhile I am slowly writing a book about particle physics for non-experts. I might post some draft sections on this website as they are written, and I hope you’ll see the book in print sometime in the next few years.

Filed under: Housekeeping, Uncategorized

by Matt Strassler at November 20, 2015 05:02 AM

November 19, 2015

Symmetrybreaking - Fermilab/SLAC

Shrinking the accelerator

Scientists plan to use a newly awarded grant to develop a shoebox-sized particle accelerator in five years.

The Gordon and Betty Moore Foundation has awarded $13.5 million to Stanford University for an international effort, including key contributions from the Department of Energy’s SLAC National Accelerator Laboratory, to build a working particle accelerator the size of a shoebox. It’s based on an innovative technology known as “accelerator on a chip.”

This novel technique, which uses laser light to propel electrons through a series of artfully crafted glass chips, has the potential to revolutionize science, medicine and other fields by dramatically shrinking the size and cost of particle accelerators.

“Can we do for particle accelerators what the microchip industry did for computers?” says SLAC physicist Joel England, an investigator with the 5-year project. “Making them much smaller and cheaper would democratize accelerators, potentially making them available to millions of people. We can’t even imagine the creative applications they would find for this technology.”

Robert L. Byer, a Stanford professor of applied physics and co-principal investigator for the project who has been working on the idea for 40 years, says, “Based on our proposed revolutionary design, this prototype could set the stage for a new generation of ‘tabletop’ accelerators, with unanticipated discoveries in biology and materials science and potential applications in security scanning, medical therapy and X-ray imaging.”

The chip that launched an international quest

The international effort to make a working prototype of the little accelerator was inspired by experiments led by scientists at SLAC and Stanford and, independently, at Friedrich-Alexander University Erlangen-Nuremberg (FAU) in Germany. Both teams demonstrated the potential for accelerating particles with lasers in papers published on the same day in 2013.

In the SLAC/Stanford experiments, published in Nature 1 Demonstration of electron acceleration in a laser-driven dielectric microstructure, electrons were first accelerated to nearly light speed in a SLAC accelerator test facility. At this point they were going about as fast as they could go, and any additional acceleration would boost their energy, not their speed.

The speeding electrons then entered a chip made of silica and traveled through a microscopic tunnel that had tiny ridges carved into its walls. Laser light shining on the chip interacted with those ridges and produced an electrical field that boosted the energy of the passing electrons.

In the experiments, the chip achieved an acceleration gradient, or energy boost over a given distance, roughly 10 times higher than the existing 2-mile-long SLAC linear accelerator can provide. At full potential, this means the SLAC accelerator could be replaced with a series of accelerator chips 100 meters long, roughly the length of a football field. 

In a parallel approach, experiments led by Peter Hommelhoff of FAU and published in Physical Review Letters 2 Laser-based acceleration of nonrelativistic electrons at a dielectric structure demonstrated that a laser could also be used to accelerate lower-energy electrons that had not first been boosted to nearly light speed. Both results taken together open the door to a compact particle accelerator.

A tough, high-payoff challenge

For the past 75 years, particle accelerators have been an essential tool for physics, chemistry, biology and medicine, leading to multiple Nobel prize-winning discoveries. They are used to collide particles at high energies for studies of fundamental physics, and also to generate intense X-ray beams for a wide range of experiments in materials, biology, chemistry and other fields. This new technology could lead to progress in these fields by reducing the cost and size of high-energy accelerators. 

The challenges of building the prototype accelerator are substantial. Demonstrating that a single chip works was an important step; now scientists must work out the optimal chip design and the best way to generate and steer electrons, distribute laser power among multiple chips and make electron beams that are 1000 times smaller in diameter to go through the microscopic chip tunnels, among a host of other technical details.

“The chip is the most crucial ingredient, but a working accelerator is way more than just this component,” says Hommelhoff, a professor of physics and co-principal investigator of the project. “We know what the main challenges will be, and we don’t know how to solve them yet. But as scientists we thrive on this type of challenge. It requires a very diverse set of expertise, and we have brought a great crowd of people together to tackle it.”

The Stanford-led collaboration includes world-renowned experts in accelerator physics, laser physics, nanophotonics and nanofabrication. SLAC and two other national laboratories, Deutsches Elektronen-Synchrotron (DESY) in Germany and Paul Scherrer Institute in Switzerland, will contribute expertise and make their facilities available for experiments. In addition to FAU, five other universities are involved in the effort: University of California, Los Angeles, Purdue University, University of Hamburg, the Swiss Federal Institute of Technology in Lausanne (EPFL) and Technical University of Darmstadt.

“The accelerator-on-a-chip project has terrific scientists pursuing a great idea. We’ll know they’ve succeeded when they advance from the proof of concept to a working prototype,” says Robert Kirshner, chief program officer of science at the Gordon and Betty Moore Foundation. “This research is risky, but the Moore Foundation is not afraid of risk when a novel approach holds the potential for a big advance in science. Making things small to produce immense returns is what Gordon Moore did for microelectronics.”

November 19, 2015 02:00 PM

arXiv blog

The Machine-Vision Algorithm for Analyzing Children’s Drawings

Psychologists believe that drawing is an important part of children’s cognitive development. So an objective analysis of these drawings could provide an important insight into this process, a task that machine vision is ideally suited to.

Studying the psychology of children and the way it changes as they develop is a difficult business. One idea is that drawing plays a crucial role in children’s cognitive development, so drawings ought to provide a window into these changes.

November 19, 2015 01:05 PM

Jester - Resonaances

Leptoquarks strike back
Leptoquarks are hypothetical scalar particles that carry both color and electroweak charges. Nothing like that exists in the Standard Model, where the only scalar is the Higgs who is a color singlet. In the particle community, leptoquarks enjoy the similar status as Nickelback in music: everybody's heard of them, but no one likes them.  It is not completely clear why... maybe they are confused with leprechauns, maybe  because they sometimes lead to proton decay, or maybe because they rarely arise in cherished models of new physics.  However,  recently there has been some renewed interest in leptoquarks.  The reason is that these particles seem well equipped to address the hottest topic of this year - the B meson anomalies.

There are at least 3 distinct B-meson anomalies that are currently intriguing:
  1.  A few sigma (2 to 4, depending who you ask) deviation in differential distribution of B → K*μμ decays, 
  2.  2.6 sigma violation of  lepton flavor universality in  B → Kμμ vs B → Kee decays, 
  3.  3.5 sigma violation of lepton flavor universality, but this time in  B → Dτν vs B → Dμν decays. 
Now, leptoquarks with masses in the TeV ballpark can explain either of these anomalies.  How? In analogy to the Higgs, leptoquarks may interact with the Standard Model fermions via Yukawa couplings. Which interactions  are possible is determined by  its color and electroweak charges. For example, this paper proposed a leptoquark transforming as (3,2,1/6) under the Standard Model gauge symmetry (color SU(3) triplet like quarks, weak SU(2) doublet like Higgs,  hypercharge 1/6).  Such particle can have the following Yukawa couplings with b- and s-quarks and muons:
 If both  λb and λs  are non-zero then a tree-level leptoquark exchange can mediate the b-quark decay  b → s μ μ.  This contribution  adds up to the Standard Model amplitudes  mediated by loops of W bosons, and thus affects the B-meson observables. It turns out that the first two anomalies listed above can be fit if the leptoquark mass is in the 1-50 TeV range, depending on the magnitude of λb and λs.

Also the 3rd anomaly above can be easily  explained by leptoquarks. One example from this paper is a leptoquark transforming as (3,1,-1/3) and coupling to matter as

This particle contributes to  b → c τ ν, adding up to the tree-level W boson contribution, and is capable of explaining the apparent excess of semi-leptonic B meson decays into D mesons and tau leptons observed by the BaBar, Belle, and LHCb experiments. The difference to the previous case is that this leptoquark has to be less massive, closer to the TeV scale, because it has to compete with the tree-level contribution in the Standard Model.

There are more kinds of leptoquarks with different charges that allow for Yukawa couplings to matter. Some of them could also explain the 3 sigma discrepancy of the experimentally measured muon anomalous magnetic moment with the Standard Model prediction. Actually, a recent paper says that the (3,1,-1/3) leptoquark discussed above can explain all B-meson and muon g-2 anomalies simultaneously, through a combination of tree-level and loop effects.  In any case, this is something to look out for in this and next year's data.  If a leptoquark is indeed the culprit for the B → Dτν excess, it should be within reach of the 13 TeV run (for the 1st two anomalies it may well be too heavy to produce at the LHC).   The current reach for leptoquarks is up to 1 TeV mass (strongly depending on model details),  see e.g. the recent ATLAS and CMS analyses. So far these searches have provoked little public interest, but that may change soon...

by Jester ( at November 19, 2015 12:58 AM

November 18, 2015

Lubos Motl - string vacua and pheno

First-quantized formulation of string theory is healthy
...and enough to see strings' superiority...

As Kirill reminded us, two weeks ago, a notorious website attracting unpleasant and uintelligent people who just never want to learn string theory published an incoherent rant supplemented by misguided comments assaulting Witten's essay
What every physicist should know about string theory
in Physics Today. Witten presented the approach to string theory that is common in the contemporary textbooks on the subject, the first-quantized approach, and showed why strings eliminate the short-distance (ultraviolet) problems, automatically lead to the gravity in spacetime, and other virtues.

Witten's office as seen under the influence of drugs

This introduction is simple enough and I certainly agree that every physicist should know at least these basic things about string theory but at the end, I think that it isn't the case, anyway. Here I want to clarify certain misunderstandings about the basics of string theory as sketched by Witten; and their relationships, similarities, and differences from quantum mechanics of point-like particles and quantum field theory.

First, let's begin by some light statements that everyone will understand.
These are elephants in the room which are not being addressed.
This latest version takes ignoring the elephants in the room to an extreme, saying absolutely nothing about the problems...
Another huge elephant in the room ignored by Witten’s story motivating string theory as a natural two-dimensional generalization of one-dimensional theories is that the one-dimensional theories he discusses are known to be a bad starting point...
Given the thirty years of heavily oversold publicity for string theory, it is this and the other elephants in the room that every physicist should know about.
So, it can’t have much to do with the real world that we actually live in. These are elephants in the room which are not being addressed.
This makes almost the same argument as the new one, but does also explain one of the elephants in the room (lack of a non-perturbative string theory).
Warren, I think there’s a difference between elephants in the room (we don’t know how to connect string theory to known 4d physics, with or without going to a string field theory) and something much smaller (mice? cockroaches?)...
I kid you not: there are at least 7 colorful sentences in that rant that claim the existence of the elephants in the room. And I didn't count the title and its copies. He must believe in a reduced version of the slogan of his grandfather's close friend (both are co-responsible for the Holocaust), "A lie repeated 7 times becomes the truth."

Sorry but there are no elephants in the room – in Witten's office, in this case. I've seen the office and I know many people who have spent quite some time there and all of them have observed the number of elephants in that room to be zero. It's enough for me to say this fact once.

If that annoying blogger sees the elephants everywhere, he should either reduce the consumption of drugs, increase the dosage of anti-hallucination pills, or both. If his employees had some decency and compassion, they would have already insisted that this particular stuttering computer assistant deals with his psychiatric problems in some way before he can continue with his work.

Fine. Now we can get to the physical issues – to make everyone sure that every single "elephant" is a hallucination.

Before strings were born but after the birth of quantum mechanics, people described the reality by theories that are basically derived from point-like particles and "their" fields. We're talking about the normal models of
  1. quantum mechanics of one or many non-relativistic point-like particles
  2. attempts to make these theories compatible with special relativity
  3. quantum field theory.
You may say that these three classes of theories are increasingly new and increasing more correct and universal. String theory is even newer and more complete so you could think that "it should start" from the category (3) of theories, quantum field theories in the spacetime.

But that's not how it works, at least not in the most natural beginner's way to learn string theory. String theory is not a special kind of the quantum field theories. That's why the construction of string theory has to branch off the hierarchy above earlier than that. String theory already starts with replacing particles with strings in the steps (1) and (2) above and it develops itself analogously to the steps from (1) to (2) to (3) above – but not "quite" identical steps – into a theory that looks like a quantum field theory at long distances but is fundamentally richer and more consistent than that, especially if you care about the behavior at very short distances.

For point-like particles, the Hamiltonians like\[

H = \sum_k \frac{p_k^2}{2m_k} + \sum_{k\lt \ell} V(r_k-r_\ell)

\] work nicely to describe physics of the atoms but they are not compatible with special relativity. The simplest and nice enough generalizations that is relativistic looks like the Klein-Gordon equation\[

(-\square-m^2) \Phi = 0

\] where we imagine that \(\Phi\) is a "wave function of one particle". In quantum mechanics, the wave function ultimately has to be complex because the energy \(E\) pure vector must depend on time as \(\exp(-iEt)\). We may consider \(\Phi\) above to be complex and rewrite the second derivatives with respect to time as a set of equations for \(\Phi\) and \(\partial\Phi/ \partial t\).

When we do so, we find out that the probability density – the time component of the natural Klein-Gordon current \(j^\mu\) – isn't positive definite. It would be a catastrophe if \(j^0\) could have both signs: probabilities would sometimes be positive and sometimes negative. But probabilities cannot be negative. (The "wrong" states have both a negative norm and negative energy so that the ratio is positive but the negative sign of the energy and especially the probability is a problem, anyway.) That's why the "class" of point-like theories (2) is inconsistent.

The disease is automatically solved once we second-quantize \(\Phi\) and interpret it as a quantum field – a set of operators (operator distributions if you are picky) – that act on the Hilbert space (of wave functions) and that are able to create and annihilate particles. We get\[

\Phi(x,y,z,t) = \sum_{k} \zav{ c_k\cdot e^{ik\cdot x} + c_k^\dagger \cdot e^{-ik\cdot x} }

\] Add the vector sign for \(\vec k\), hats everywhere, correct the signs, and add the normalization factors or integrals instead of sums. None of those issues matter in our conceptual discussion. What matters is that \(c_k,c^\dagger_k\) are operators and so is \(\Phi\). Therefore, \(\Phi\) no longer has to be "complex". It may be real – because it's an operator, we actually require it is Hermitian. And I have assumed that \(\Phi\) is Hermitian in the expansion above.

There must exist the ground state – by which we always mean the energy eigenstate \(\ket 0\) corresponding to the lowest eigenvalue of the Hamiltonian \(H\) – and one may prove that\[

c_k \ket 0 = 0.

\] The annihilation operators annihilate the vacuum completely. For this reason, the only one-particle states are the linear superpositions of various vectors \(c^\dagger_k \ket 0\). This is a linear subspace of the full, multi-particle Fock space produced by the quantum field theory. But both the Fock space and this one-particle subspace are positively definite Hilbert spaces. The probabilities are never zero.

You may say that the "dangerous" states that have led to the negative probabilities in the "bad theories of the type (2)" are the states of the type \(c_k\ket 0\) which may have been naively expected to be nonzero vectors in the theories (2) but they are automatically seen to be zero in quantum field theory. Quantum field theories pretty much erases the negative-probability states by hand, automatically.

Now, if you take bosonic strings in \(D=26\), for the sake of simplicity, and ban any internal excitations of the string, the physics of this string will reduce to that of the tachyonic particle. The tachyonic mass is a problem (tachyons disappear once you study the \(D=10\) superstring instead of bosonic string theory; but since 1999, we know that tachyons are not "quite" a hopelessly incurable inconsistency, just signs of some different, unstable physical evolution).

But otherwise the string's physics becomes identical to that of the spinless Klein-Gordon particle. In quantum field theory, the negative-probability polarizations of the spinless particle "disappear" from the spectrum because \(c_k \ket 0=0\) and be sure that exactly the same elimination takes place in string theory.

The correctly treated string theory, much like quantum field theory, simply picks the positive-definite part of the one-particle Hilbert space only. At the end, much like quantum field theory, string theory allows the multi-particle Fock space with arbitrary combinations of arbitrary numbers of arbitrarily excited strings in the spacetime. And this Hilbert space is positive definite.

First-quantized approach to QFT is just fine

Many unpleasant people at that blog believe that for the negative-probability states to disappear, we must mindlessly write down the exact rules and slogans that are taught in quantum field theory courses and no other treatment is possible. They believe that quantum field theory is the only framework that eliminates the wrong states.

But that's obviously wrong. We don't need to talk about quantum fields at all. At the end, we are doing science – at least string theorists are doing science – so what matters are the physical predictions such as cross sections or, let's say, scattering amplitudes.

Even in quantum field theory, we may avoid lots of the talking and mindless formalism if we just want the results – the physical predictions. We may write down the Feynman rules and draw the Feynman diagrams needed for a given process or question directly. We don't need to repeat all the history clarifying how Feynman derived the Feynman rules for QED; we can consider these rules as "the candidate laws of physics". When we calculate all these amplitudes, we may check that they obey all the consistency rules. In fact, they match the observations, too. And that's enough for science to be victorious.

The fun is that even in the ordinary physics of point-like particles, the Feynman diagrams – which may be derived from "quantum fields" – may be interpreted in the first-quantized language, too. The propagators represent the path integral over all histories i.e. trajectories of one point-like particle from one spacetime point to another. The particle literally tries all histories – paths – and we sum over them. When we do so, the relevant amplitude is \(G(x^\mu,y^\mu)\).

However, the Feynman diagrams have many propagators that are meeting at the vertices – singular places of the diagrams. These may be interpreted as "special moments of the histories". The point-like particles are literally allowed to split and join. The prefactors that the Feynman rules force you to add for every vertex represent the probability amplitudes for the splitting/joining event, something that may depend on the internal spin/color or other quantum numbers of all the particles at the vertex.

The stringy Feynman diagrams may be interpreted totally analogously, in the one-string or first-quantized way. Strings may propagate from one place or another – this propagation of one string also includes the general evolution of its internal shape (a history is an embedding of the world sheet into the spacetime) – and they may split and join, too (the world sheet may have branches and loops etc.). In this way, we may imagine that we're Feynman summing over possible histories of oscillating, splitting, and joining strings. The sum may be converted to a formula according to Feynman-like rules relevant for string theory. And the result may be checked to obey all the consistency rules and agree with an effective quantum field theory at long distances.

And because the effective quantum field theories that string theory agrees with may be (for some solutions/compactifications of string theory) those extending the Standard Model (by the addition of SUSY and/or some extra nice new physics) and this is known to be compatible with all the observations, string theory is as compatible with all the observations as quantum field theory. You don't really need anything else in science.

Strings' superiority in comparison with particles

So all the calculations of the scattering amplitudes etc. may be interpreted in the first-quantized language, both in the case of point-like particles and strings. For strings, however, the whole formalism automatically brings us several amazing surprises, by which I mean advantages over the case of point-like particles, including
  1. the automatic appearance of "spin" of the particles from the internal motion of the strings
  2. unification of all particle species into different vibrations of the string
  3. the automatic inclusion of interactions; no special rules for "Feynman vertices" need to be supplemented
  4. automatic removal of short-distance (ultraviolet) divergences
  5. unavoidable inclusion of strings with oscillation eigenstates that are able to perturb the spacetime geometry: Einstein's gravity inevitably follows from string theory
It's a matter of pedagogy that I have identified five advantages. Some people could include others, perhaps more technical ones, or unify some of the entries above into bigger entries, and so on. But I think that this "list of five advantages" is rather wisely chosen.

I am going to discuss the advantages one-by-one. Before I do so, however, I want to emphasize that too many people are obsessed with a particular formalism but that's not what the scientific method – and string theorists are those who most staunchly insist on this method – demands. The scientific method is about the predictions, like the calculation of the amplitudes for all the scattering processes. And string theory has well-defined rules for those. Once you have these universal rules, you don't need to repeat all the details "how you found them" – this justification or motivation or history may be said to be "unnecessary excess baggage" or "knowledge to be studied by historians and social pseudoscientists".

Someone could protest that this method only generalizes the Feynman's approach to quantum field theory. However, this protest is silly for two reasons: it isn't true; and even if it were true, it would be irrelevant. It isn't true because the dynamics of string theory may be described in various types of the operator formalism (quantum field theory on the world sheet with different approaches to the gauge symmetries; string field theory; AdS/CFT; matrix theory, and so on). It's simply not true that the "integrals over histories" become "absolutely" unavoidable in string theory. Second, even if the path integrals were the only way to make physical predictions, there would be nothing wrong about it.

Fine. Let me now discuss the advantages of the strings relatively to particles, one by one.

The spin is included

One of the objection by the "Not Even Wrong" community, if I use a euphemism for that dirty scum, is:
Another huge elephant in the room ignored by Witten’s story motivating string theory as a natural two-dimensional generalization of one-dimensional theories is that the one-dimensional theories he discusses are known to be a bad starting point, for reasons that go far beyond UV problems. A much better starting point is provided by quantized gauge fields and spinor fields coupled to them, which have a very different fundamental structure than that of the terms of a perturbation series of a scalar field theory.
It's probably the elephant with the blue hat. Needless to say, these comments are totally wrong. It is simply not true that the point-like particles and their trajectories described in the quantum formalism – with the Feynman sum or the operators \(\hat x,\hat p\) – are a "bad starting point". They're a perfectly fine starting point. They're how quantum mechanics of electrons and other particles actually started in 1925.

Quantum field theory is one of the later points of the evolution of these theories, not a starting point, and it is not the final word in physics, either.

For point-like particles, the first-quantized approach building on the motion of one particle may look like a formalism restricted to the spinless, scalar, Klein-Gordon particles. But again, this objection is no good because of two reasons. It is false; and even if it were true, it is completely irrelevant for the status of string theory.

The comment that one may only get spinless particles in the first-quantized treatment of point-like particles is wrong e.g. because one can study the propagation of point-like particles in the superspace, a spacetime with additional fermionic spinorial coordinates. And the dynamics of particles in such spaces is equivalent to the dynamics of a superfield which is a conglomerate of fields with different spins. One gets the whole multiplet. More generally and less prettily, one could describe particles with arbitrary spins by adding discrete quantum numbers to the world lines of the Klein-Gordon particles.

But the second problem with the objection is that it is irrelevant because
the stringy generalization of the Klein-Gordon particle is actually enough to describe elementary particles of all allowed values of the spin.
Why? You know why, right? It's because the string has internal dynamics. It may be decomposed to creation and annihilation operators of waves along the string, \(\alpha_{\pm k}^\mu\). The spectrum of the operator \(m^2\) is discrete and the convention is that negative subscripts are creation operators; positive ones are annihilation operators. The ground state of the bosonic string \(\ket 0\) is a tachyon that carries center-of-mass degrees of freedom remembering the location or the total momentum of the string behaving as a particle. And the internal degrees of freedom, thanks to the \(\mu\) superscript (which tells you which scalar field on the string was excited), add spin to the string.

Bosonic strings only carry the spin with \(j\in \ZZ^{0,+}\). If you study the superstring, basically the physics of strings propagating in a superspace, you will find out that all \(j\in \ZZ^{0,+}/2\) appear in the spectrum.

It should be obvious but once again, the conclusion is that
the first-quantized Klein-Gordon particle physics is actually a totally sufficient starting point because once we replace the particles with strings propagating in a superspace, we get particles (and corresponding fields) of all the required spins in the spectrum.
The claim that there's something wrong with this "starting point" or strategy is just wrong. It's pure crackpottery. If you asked the author of that incorrect statement what's wrong about this starting point, he could only mumble some incoherent rubbish that would ultimately reduce to the fact that the first lecture in a low-brow quantum field theory course is the only thing he's been capable of learning and he just doesn't want to learn anything beyond that because his brain is too small and fucked-up for that.

But that doesn't mean that there's something wrong with other ways to construct physical theories. Some of the other ways actually get us much further.

Unification of all particle species into one string

One string can vibrate in different ways. Different energy or mass eigenstates of the internal string oscillations correspond to different particle species such as the graviton, photon, electron, muon, \(u\)-quark, and so on. This is of course a characteristic example of the string theory's unification power.

This idea wasn't quite new in string theory, however. Already in the early 1960s, people managed to realize that the proton and the neutron (or the \(u\)-quark and the \(d\)-quark; or the left-handed electron and the electron neutrino) naturally combine into a doublet. They may be considered a single particle species, the nucleon (if I continue with the proton-neutron case only), which may be found in two quantum states. These two states are analogous to the spin-up and spin-down states. The \(SU(2)\) mathematics is really isomorphic which is why the quantum number distinguishing the proton and the neutron was called the "isospin".

What distinguishes the proton and the neutron is some "detailed information (a qubit) inside this nucleon". It's still the same nucleon that can be switched to be a proton, or a neutron. And the same is true for string theory. What is switched are the vibrations of the internal waves propagating along the string. And there are many more ways to switch them. In fact, we can get all the known particle species – plus infinitely many new, too heavy particle species – by playing with these switches, with the stringy oscillations.

Interactions are automatically included

In the Feynman diagrams for point-like particles, you have to define the rules for the internal lines, the propagators, plus the rules for the vertices where the propagators meet. These are choices that are "almost" independent from each other.

Recall that from a quantum field theory Lagrangian, the propagators are derived from the "free", quadratic terms in the Lagrangian. The vertices are derived from the cubic and higher-order, "interaction" terms in the Lagrangian. Even when the free theory is simple or unique, there may be many choices and parameters to be made when you decide what the allowed vertices should be and do.

The situation is different in string theory. When you replace the interaction vertex by the splitting string, by the pants diagram, it becomes much smoother, as we discuss later. But one huge advantage of the smoother shape is that locally, the pants always look the same. Every square micron (I mean square micro-Planck-length) of the clothes looks the same.

So once you decide what is the "field theory Lagrangian" per area of the pants – the world sheet – you will have rules for the interactions, too. There is no special "interaction vertex" where the rules for the single free string break down. Once you allow the topology of the world sheet to be arbitrary, interactions of the strings are automatically allowed. You produce the stringy counterparts of all Feynman diagrams you get in quantum field theory.

In the simplest cases, one finds out that there is still a topological invariant, the genus \(h\) of the world sheet, and the amplitude from a given diagram may be weighted by \(g_s^{2h}\), a power of the string coupling. But it may be seen that \(g_s\) isn't really a parameter labeling different theories. Instead, its value is related to the expectation value of a string field that results from a particular vibration of the closed string, the dilaton (greetings to Dilaton). This "modulus" may get stabilized – a dynamically generated potential will pick the right minimum, the dilaton vev, and therefore the "relevant value" of the coupling constant, too.

So the choice of the precise "free dynamics of a string" already determines all the interactions in a unique way. This is a way to see why string theory ends up being much more robust and undeformable than quantum field theories.

Ultraviolet divergences are always gone

One old well-known big reason why string theory is so attractive is that the ultraviolet i.e. short-distance divergences in the spacetime don't arise, not even at intermediate stages of the calculation. That's why we don't even need any renormalization that is otherwise a part of the calculations in quantum field theory.

I must point out that it doesn't mean that there's never any renormalization in string theory. If we describe strings using a (conformal) quantum field theory on the two-dimensional world sheet, this quantum field theory requires analogous steps to the quantum field theories that old particle physicists used for the spacetime. There are UV divergences and renormalization etc.

But in string theory, no such divergences may be tied to short distances in the spacetime. And the renormalization on the world sheet works smoothly – the conformal field theory on the world sheet is consistent and, whenever the calculational procedure makes this adjective meaningful, renormalizable. (Conformal theories are scale-invariant so they obviously can't have short-distance problems; the scale invariance means that a problem at one length scale is the same problem at any other length scale.)

This UV health of string theory may be seen in many ways. For example, if you compactify string theory on a circle of radius \(R\), too short a value of \(R\) doesn't produce a potentially problematic dynamics with short-distance problems because the compactification on radius \(R\) is exactly equivalent to a compactification on the radius \(\alpha' / R\), basically \(1/R\) in the "string units", because of the so-called T-duality.

Also, if you consider one-loop diagrams, the simplest diagrams where UV divergences normally occur in quantum field theories, you will find out that the relevant integral in string theory is over a complex \(\tau\) whose region is\[

{\rm Im}(\tau)\gt 0, \,\, |\tau| \gt 1, \,\, |{\rm Re}(\tau)| \lt \frac 12.

\] The most stringy condition defining this "fundamental domain" is \(|\tau|\gt 1\) which eliminates the region of a very small \({\rm Im}(\tau)\). But this is precisely the region where ultraviolet divergences would arise if we integrated over it. In quantum field theory, we would have to integrate over a corresponding region. In string theory, however, we don't because these small values of \({\rm Im}(\tau)\) correspond to "copies" of the same torus that we already described by a much higher value of \(\tau\).

In the Feynman sum over histories, we only sum each shape of the torus once so including the "small \(\tau\) points aside from the fundamental region" would mean to double-count (or to count infinitely many times) and that's not what Feynman tells you to do.

For this reason, if there are some divergences, they may always be interpreted as infrared divergences. It is always possible for every similar divergence in string theory to be interpreted as "the same" divergence that would arise in the effective field theory approximating your string theory vacuum as an "infrared divergence", and no additional divergences occur. In this sense, any kind of string theory – even bosonic string theory – explicitly removes all potential ultraviolet divergences. And it does so without breaking the gauge symmetries or doing similar nasty things that would be unavoidable if you imposed a similar cutoff in quantum field theory.

String theory is extremely clever in the way how it eliminates the UV divergences.

The crackpot-in-chief on the anti-string blog wrote:
From the talks of his I’ve seen, Witten likes to claim that in string perturbation theory the only problems are infrared problems, not UV problems. That’s never seemed completely convincing, since conformal invariance can swap UV and IR. My attempts to understand exactly what the situation is by asking experts have just left me thinking, “it’s complicated”.
I am pretty sure that they meant "it's complicated for an imbecile like you". There is nothing complicated about it from the viewpoint of an intelligent person and string theory grad students understand these things when they study the first or second chapter of the string textbooks. Indeed, modular transformations swap the UV and IR regions and that's exactly why the would-be UV divergences may always be seen as something that we have already counted as IR divergences and we shouldn't count them again.

Grad students understand why there are no UV divergences in string theory but smart 9-year-old girls may already explain to their fellow kids why string theory is right and how compactifications work. According to soon-to-be Prof Peo Webster, who's "personally on Team String Theory", the case of \({\mathcal T}^* S^3\) requires some extra work relatively to an \(S^5\). She explains non-renormalizability and other basic issues that Dr Tim Blais has only sketched.

If you first identify all divergences that may be attributed to long-distance dynamics, i.e. identified as IR divergences, there will be no other divergences left in the string-theoretical integral. Isn't this statement really simple? It's surely too complicated for the crackpot but I hope it won't be too complicated for the readers of this blog.

Now, you may ask about the IR divergences. Aren't they a problem?

Well, infrared divergences are a problem but they are never an inconsistency of the theory. Instead, they may always be eliminated if you ask a more physically meaningful question. When you ask about a scattering process, you may get an IR-divergent cross section. But that's because you neglected the fact that experimentally, you won't be able to distinguish a given idealized process from the processes where some super-low-energy photons or gravitons were emitted along with the final particles you did notice. If you compute the inclusive cross section where the soft particles under a detection threshold \(E_{\rm min}\) – which may be as low as you want but nonzero – are allowed and included, the infrared divergences in the simple processes (without soft photons) exactly cancel against the cross section coming from the more complicated processes with the extra soft photons.

This wisdom isn't tied to quantum field theory per se. The same wisdom operates in any theory that agrees with quantum field theory at long distances – and string theory does. So even in string theory, it's true that IR divergences are not an inconsistency. If you ask a better, more realistically "inclusive" question, the divergences cancel.

In practice, bosonic string theory has infrared divergences that are exponentially harsh and connected with the instability of the vacuum – any vacuum – that allows tachyonic excitations. Tachyons are filtered out of the spectrum in superstring theory but massless particles such as the dilaton may be – and generically, are – sources of "power law" IR divergences, too. However, in type II string theory etc., all the infrared divergences that arise from the massless excitations cancel due to supersymmetry. So ten-dimensional superstring theories avoid both UV (string theory always does) and IR (thanks to SUSY) divergences.

But one must emphasize that in some more complicated compactifications, some IR divergences will refuse to cancel – we know that they don't cancel in the Standard Model and string theory will produce an identical structure of IR divergences because it agrees with a QFT at long distances – but that isn't an inconsistency. It isn't an inconsistency in QFT; and it isn't an inconsistency in string theory – for the same reason. It is a subtlety forcing you to ask questions and calculate answers more carefully. When you do everything carefully, you get perfectly consistent and finite answers to all questions that are actually experimentally testable.

Again, let me emphasize that while the interpretation of infrared divergences is the same in QFT and ST, because those agree at long distances, it isn't the case for UV divergences. At very short (stringy and substringy) distances, string theory is inequivalent to a quantum field theory – any quantum field theory – which is why it is capable of "eliminating the divergences altogether", even without any renormalization, which wouldn't be possible in any QFT.

Also, I want to point out that this ability of string theory to remove the ultraviolet divergences is special for the \(D=2\) world sheets. Higher-dimensional elementary objects could also unify "different particle species" and automatically "produce interactions from the free particles" because the world volume would be locally the same everywhere.

However, membranes and other higher-dimensional fundamental branes (beyond strings) would generate new fatal UV divergences in the world volume. The 2D world sheet is a theory of quantum gravity because the parameterization of the world sheet embedded in the spacetime mustn't matter. A funny thing is that the three components of the 2D metric tensor on the world sheet,\[


\] may be totally eliminated – set to some standard value such as \(h_{\alpha\beta}=\delta_{\alpha\beta}\) – by gauge transformations that are given by three parameters at each point,\[

\delta \sigma^1, \,\, \delta \sigma^2,\,\, \eta,

\] which parameterize the 2D diffeomorphisms and the Weyl rescaling of the metric. So the world sheet gravity may be locally eliminated away totally. That's why no sick "nonrenormalizable gravity" problems arise on the 2D world sheet. But they would arise on a 3D world volume of a membrane where the metric tensor has 6 components but you would have at most 3+1=4 parameters labeling the world volume diffeomorphisms plus the Weyl symmetry. So some polarizations of the graviton would survive, along with the nonrenormalizable UV divergences in the world volume.

In effect, if you tried to cure the quantized Einstein gravity's problems in the spacetime by using membranes, you would solve them indeed but equally serious problems and inconsistencies would re-emerge in the world volume of the membranes. The situation gets even more hopeless if you increase the dimension of the objects; \(h_{\alpha\beta}\) has about \(D^2/2\) components while the diffeomorphisms plus Weyl only depend on \(D+1\) parameters and the growth of the latter expression is slower.

Strings are the only simple fundamental finite-dimensional objects for which both the spacetime and world volume (world sheet) problems are eliminated. That doesn't mean that higher-dimensional objects never occur in physics – they do in string theory (D-branes and other branes) – but what it does mean is that you can't expect as simple and as consistent description of the higher-dimensional objects' internal dynamics as we know in the case of the strings. For example, the dynamics of D-branes may be described by fundamental open strings attached to these D-branes by both end points; you need some "new string theory" even for the world volume that naive old physicists would describe by an effective quantum field theory.

Gravity (dynamical spacetime geometry) is automatically implied by string theory

You may consider the first-quantized equations for a single particle propagating on a curved spacetime. However, the spacetime arena is fixed. The particle is affected by it but cannot dynamically curve it and play with the spacetime around it.

It's very different in string theory. String theory predicts gravity. Gravity was observed by the monkeys (and bacteria) well before they understood string theory which is a pity and a historical accident that psychologically prevents some people from realizing how incredible this prediction made by string theory has been. But logically, string theory is certainly making this prediction – or post-diction, if you wish – and it surely increases the probability that it's right in the eyes of an intelligent beholder.

Why does string theory automatically allow the spacetime to dynamically twist and oscillate and wiggle? Why is the spacetime gravity an unavoidable consequence of string theory?
It may look technical but it's not so bad. The reason is that any infinitesimal change of the spacetime geometry on which a string propagates is physically indistinguishable from the addition of coherent states of closed strings in certain particular vibration patterns – strings oscillating as gravitons – to all processes you may compute.
To sketch how it works in the case of the \(D=26\) bosonic string – the case of the superstring has many more indices and technical complications that don't change anything about the main message – try to realize that when you integrate over all the histories of oscillating, splitting, and joining strings, via Feynman's path integral, you are basically using some world sheet action that looks something like\[

S_{2D} = \int d^2 \sigma \, \sqrt{h}h^{\alpha\beta} \partial_\alpha X^\mu \partial_\beta X^\nu \cdot g_{\mu\nu}^{\rm spacetime}(X^\kappa).

\] Here, \(X^\mu\) and \(h_{\alpha\beta}\) are world sheet fields i.e. functions of two coordinates \(\sigma^\alpha\). I suppressed the dependence (and the overall coefficient) to make the equation more readable. At any rate, \(h\) is the world sheet metric and \(g\) is the spacetime metric which is a predetermined function of \(X^\mu\), the spacetime coordinates. But to calculate the world sheet action in string theory, you substitute the value of the world sheet field \(X^\kappa(\sigma^\alpha)\) as arguments into the function \(g_{\mu\nu}(X^\kappa)\).

What happens if you infinitesimally change the spacetime metric \(g\)? Differentiate with respect to this \(g\). You will effectively produce a term in the scattering amplitude that contains the extra prefactor of the type \(h^{\alpha\beta}\partial_\alpha X^\mu \partial_\beta X^\nu\) with some coefficients \(\delta g_{\mu\nu}\) to contract the indices \(\mu,\nu\).

But the addition of similar prefactors inside the path integral is exactly the string-theoretical rule to add external particles to a process. If you allow me some jargon, it's because external particles attached as "cylinders" to a world sheet may be conformally mapped to local operators (while the infinitely long thin cylinder is amputated) and there's a one-to-one map between the states of an oscillating closed string and the operators you may insert in the bulk of the world sheet. This map is the so-called "state-operator correspondence", a technical insight in any conformal field theory that you probably need to grasp before you fully comprehend why string theory predicts gravity.

And the structure of this prefactor, the so-called "vertex operator", in this case \(\partial X\cdot \partial X\) (the product of two world sheet derivatives of the scalar fields representing the spacetime coordinates), is exactly the vertex operator for a "graviton", a particular massless excitation of a closed string. It's a marginal operator – one whose addition keeps the world sheet action scale-invariant – and this "marginality" is demonstrably the right condition on both sides (consistent deformation of the spacetime background; or the vertex operator of an allowed string excitation in the spectrum).

We proved it for the graviton but it holds in complete generality:
Any consistent/allowed infinitesimal deformation of "the string theory" – the background and properties of the theory governing the propagation of a string – is in one-to-one correspondence with the addition of a string in an oscillation state that is predicted by the unperturbed background.
So the spectrum unavoidably includes the closed string states (graviton states) that exactly correspond to the infinitesimal deformation of the spacetime geometry, and so on. (Only the deformations of the spacetime geometry that obey the relevant equations – basically Einstein's equations – lead to a precisely conformal and therefore consistent string theory so only the deformations and gravitons obeying the Einstein's equations are allowed.) Similarly, if you want to change the string coupling or the dilaton, as we discussed previously, you will find a string state that is predicted in the spectrum whose effect is exactly like that. Gauge fields, Wilson lines, other scalar fields etc. work in the same way. All of them may be viewed either as "allowed deformations of the pre-existing background" or "excitations predicted by the original background".

That's why the undeformed and (infinitesimally but consistently) deformed string theory are always exactly physically equivalent. That is why there don't exist any inequivalent deformations of string theory. String theory is completely unique and because you may define consistent dynamics of a string on an arbitrary Ricci-flat etc. background, such string theory always predicts dynamical gravity, too.

Witten has tried to explain the same point so if I failed to convey this important observation, you should try to get the same message from his review.


To summarize, the most obvious first pedagogic approach to learn how to define string theory and do calculations in string theory deals with the first-quantized formalism, a generalization of the "one-dimensional world lines of particles" to the case of "two dimensions of the stringy world sheet". It's easy to see that analogous rules produce physical predictions that not only share the same qualitative virtues with those in the point-like-particle case and are equally consistent. Instead, the stringy version of this formalism is more consistent and has other advantages.

The stringy version of this computational framework is demonstrably superior from all known viewpoints. And the evidence is overwhelming that there exist particular non-perturbatively exact answers to all the physically meaningful questions and at least in many backgrounds (e.g. those envisioned in matrix models as well as the AdS/CFT correspondence, any version of them), we actually know how to compute them in principle and in the case of a large number of interesting observables, in practice.

Everyone who tries to dispute these claims is either incompetent or a liar or – and it's the case of the system manager – both.

by Luboš Motl ( at November 18, 2015 04:55 PM

Lubos Motl - string vacua and pheno

FQ Hall effect: has Vafa superseded Laughlin?
A stringy description of a flagship condensed matter effect could be superior

Harvard's top string theorist Cumrun Vafa has proposed a new treatment of the fractional quantum Hall effect that is – if correct – more stringy and therefore potentially more unifying and meaningful than the descriptions used by condensed matter physicists, including the famous Laughlin wave function:
Fractional Quantum Hall Effect and M-Theory (cond-mat.mes-hall, arXiv)
Laughlin's theoretical contributions to the understanding of this effect are admired by string theorists and physicists who like "simply clever" ideas. But the classification of the effects and possibilities seemed to be a bit contrived and people could have thought that a more conceptual description requiring fewer parameters could exist.

Let me start at the very beginning of Hall-related physics, with the classical Hall effect. In the 19th century, they failed to learn quantum mechanics (the excuse was that it didn't exist yet) so they called it simply "the Hall effect".

At any rate, Edwin Hall took a conductor in 1879, a "wire" going in the \(z\) direction, and applied a transverse magnetic field (orthogonal to the wire) pointing in the \(x\) direction. What he was able to measure was a voltage not just in the \(z\) direction, as dictated by Ohm's law, but also in the third \(y\) direction (the cross product of the current and the magnetic field – a direction perpendicular both to the current as well as the magnetic field).

This Hall voltage was proportional to the magnetic field and its origin is easy to understand. The charge carriers that are parts of the electric current are subject to the Lorentz force \[

\vec F = q\cdot \vec v \times \vec B

\] and because the electrons in my conventions are pushed in the \(y\) direction by the Lorentz force, there will be a voltage in that direction, too.

That was simple. At some moment, quantum mechanics was born. Many quantities in quantum mechanics have a discrete spectrum. It turned out that given a fixed current, the Hall voltage has a discrete spectrum, too. It only looks continuous for "regular" magnetic fields that were used in the classical Hall effect. But if you apply really strong magnetic fields, you start to observe that the "Hall conductance" (which is a conductance only by the units; the current and the voltage in the ratio are going in different directions)\[

\sigma = \frac{I_{\rm channel}}{V_{\rm Hall}} = \nu \frac{e^2}{h}

\] only allows discrete values. \(e\) and \(h\) are the elementary charge and Planck's constant (perhaps surprisingly, \(h/e^2\) has units of ohms, it's called the von Klitzing constant \(25815.8\) ohms) but the truly funny thing is that the allowed values of \(\nu\) (pronounce: "nu"), the so-called filling factor, has to be a rather simple rational number. (The classical Hall effect is the usual classical limit of the quantum Hall effect prescribed by Bohr's 1920 "correspondence principle"; the integers specifying the eigenstate are so large that they look continuous.)

Experimentally, \(\nu\) is either an integer or a non-integral rational number. In the latter case, we may call \(\nu\) the "filling fraction" instead. The case of \(\nu\in\ZZ\), the "integer quantum Hall effect", is easy to explain. You must know the mathematics of "Landau levels". The Hamiltonian (=energy) of a free charged particle in the magnetic field is given by\[

\hat H = \frac{m|\hat{\vec v}|^2}{2} = \frac{1}{2m} \zav{ \hat{\vec p} - q\hat{\vec A}/c }^2

\] Note that in the presence of a magnetic field, it's still true that the kinetic energy is \(mv^2/2\). However, \(mv\) is no longer \(p\). Instead, they differ by \(qA/c\), the vector potential. This shift is needed for the \(U(1)\) gauge symmetry. If you locally change the phase of the charged particle's wave function by a gauge transformation, the kinetic energy or speed can't change, and that's why the kinetic energy has to subtract the vector potential which also changes under the gauge transformation.

At any rate, for a uniform magnetic field, \(\hat{\vec A}\) is a linear function of \(\hat{\vec r}\) and you may check that the Hamiltonian above is still a bilinear function in \(\hat{\vec x}\) and \(\hat{\vec p}\). For this reason, it's a Hamiltonian fully analogous to that of a quantum harmonic oscillator. It has an equally-spaced discrete spectrum, too (aside from some continuous momenta that decouple). The excitation (Landau) level ends up being correlated with the filling factor. The mathematics needed to explain it is as simple as the mathematics of a harmonic oscillator applied to each charge carrier separately.

However, experimentally, \(\nu\) may be a non-integer rational number, too. It's the truly nontrivial case of the fraction quantum Hall effect (FQHE).\[

\nu = \frac 13, \frac 25, \frac 37, \frac 23, \frac 35, \frac 15, \frac 29, \frac{3}{13}, \frac{5}{2}, \frac{12}{5}, \dots

\] How is it possible? The harmonic oscillators clearly don't allow levels "in between the normal ones". It seems almost obvious that the interactions between the electrons are actually critical and they conspire to produce a result that looks simple and theoretical condensed matter physics is full of cute phenomenological explanations for such a conspiracy – and various quasiparticles etc.

In condensed matter physics – with many interacting charged particles – it's possible for the electrons to seemingly get fractional (FQHE); for the charge and spin to separate much like when your soul escapes from your body (spinon, chargons), and so on. These phenomena may be viewed as clever phenomenological ideas designed to describe some confusing observations by experimenters. However, closely related and sometimes the same phenomena appear in string theory when string theorists are explaining various transitions and dualities in their much more mathematically well-defined theory of fundamental physics (I am talking about fractional D-branes, Myers effect, and tons of other things).

The importance of thinking in terms of quasiparticles for string theory – and the post-duality-revolution string theory's ability to blur the difference between fundamental and "composite" particles – is another reason to say that string theory is actually much closer to fields like condensed matter physics – disciplines where the theorists and experimenters interact on a daily basis – than "older" parts of the fundamental high-energy physics.

At any rate, I've mentioned the Landau-level-based explanation of the integer quantum Hall effect. Because we're dealing with harmonic oscillators of a sort, there are some Gaussian-based wave functions for the electrons. Robert Laughlin decided to find a dirty yet clever explanation for the fractional quantum Hall effect, too. His wave function contains the "Landau" Gaussian factor as well, aside from a polynomial prefactor:\[

\psi(z_i,\zeta_a) &= \prod_{i,a} (z_i-\zeta_a)\prod_{i\lt j}(z_i-z_j)^{\frac{1}{\nu}}\times\\
&\times \exp\left(-B\sum_i |z_i|^2\right)

\] Here, \(z_i\) and \(\zeta_a\) are complex positions of electrons and quasi-holes, respectively. This basic wave function explained the FQHE with the filling fraction \(\nu = 1/m\) and Laughlin could have shared the Nobel prize. Note that for \(\nu=1/m\), the exponent \(1/\nu\) is actually integer which makes the wave function single-valued.

We're dealing with wave functions in 1 complex dimension i.e. 2 real dimensions so it looks like the setup is similar to the research of conformal field theories in 2 real dimensions (or 1 complex dimension: we want the Euclidean spacetime signature), the kind of mathematics that is omnipresent in the research of perturbative string theory (and its compactifications on string-scale manifolds, e.g. in Gepner models). Indeed, the classification of similar wave functions and dynamical behaviors has been mapped to RCFTs (rational conformal field theories), basically things like the "minimal models".

Also, the polynomial prefactors may remind you of the prefactors (especially the Vandermonde determinant) that convert the integration over matrices in old matrix models to the "fermionic statistics of the eigenvalues".

Cumrun Vafa enters the scene

Cumrun is the father of F-theory (in the same sense in which Witten is the mother of M-theory). He's written lots of impressive papers about topological string theory; and cooperated with Strominger in the first microscopic calculation of the Bekenstein-Hawking (black hole) entropy using D-branes in string theory. Also, he's behind the swampland paradigm (including our Weak Gravity Conjecture) etc.

In this new condensed-matter paper, Cumrun has shown evidence that a more unified description of the FQHE may exist, one that may be called the "holographic description". First of all, he has employed his knowledge of 2D CFTs to describe the dynamics of the FQHE by the minimal models. The minimal models that are needed are representations of either the Virasoro algebra that we already know in bosonic string theory; or the super-Virasoro algebra that only becomes essential in the case of the supersymmetric string theory.

If that part of Vafa's paper is right, it's already amusing because I believe that Robert Laughlin, as a hater of reductionism and a critic of string theory, must dislike supersymmetry as well. If super-Virasoro minimal models are needed for the conceptually unified description of the effect he is famous for, that's pretty juicy. If I roughly get it, Cumrun may de facto unify several classes of "variations of FQHE" into a broader family with one parameter only.

Laughlin with a king in 1998. Should Cumrun have been there instead?

But Cumrun goes further, to three dimensions. 2D CFTs are generally dual to quantum gravity in 3 dimensions. Why it shouldn't be true in this case? The question is what is the right 3D theory of gravity. He identifies the Chern-Simons theory with the \(SL(2,\CC)\) gauge group as a viable candidate.

Note that Chern-Simons theory, while a theory with a gauge field and no dynamical metric, seems at least approximately equivalent to gravity in 3 dimensions. One may perform a field redefinition – that was sometimes called a 3D toy model of Ashtekar's "new variables" field redefinition in 4D. In 3 dimensions, things are less inconsistent because pure 3D gravity has no local excitations. The Ricci tensor and the Einstein tensor have 6 independent components each; but so does the Riemann tensor. So Einstein's equations in the vacuum – Ricci-flatness – are actually equivalent to the complete (Riemann) flatness. No gravitational waves can propagate in the 3D vacuum.

And the gravitational sources in 3D only create "deficit angles". The space around them is basically a cone, something that you may create by cutting a wedge from a flat sheet of paper with the scissors and by gluing. Not much is happening "locally" in the 3D spacetime of quantum gravity which is also why the room for inconsistencies is reduced.

The gauge group \(SL(2,\CC)\) basically corresponds to a negative value of the cosmological constant. In this sense, the 3D gravitating spacetime may be an anti de Sitter space and Cumrun's proposed duality is a low-dimensional example of AdS/CFT. This is a somewhat more vacuous statement because there are no local bulk excitations in Chern-Simons theory – you can't determine the precise geometry – so I think that the assignment "AdS" is only topological in character. Moreover, the big strength of Chern-Simons and topological field theories is that you may put them on spaces of diverse topologies so even the weaker topological claim about the AdS space can't be considered an absolute constraint for the theory.

Also, Chern-Simons theory may actually deviate from a consistent theory of quantum gravity once you go beyond the effective field-theory description – e.g. to black hole microstates. But maybe you should go beyond Chern-Simons theory in that case. Cumrun proposes that the black holes that should exist in the 3D theory of quantum gravity should be identified with Laughlin's quasi-holes. If true, it's funny. Couldn't have the people checked that the two kinds of holes may actually be physically equivalent in the given context?

At any rate, if the 3D theory has boundaries, there is FQHE-like dynamics on the boundary and he may make some predictions about this dynamics. In particular, he claims that some excitations exist and obey exotic statistics. In 4 dimensions and higher, we can have bosons and fermions. In 2+1 dimensions, the trip of one particle around another is topologically different from no trip at all (the world lines may get braided which is why knot theory exists in 3D), so there may be the generic interpolations between the bosons and fermions, the anyons (with a phase). And you may also think about non-Abelian statistics and Cumrun actually claims that it has to be true if his model is correct.

Many non-string theorists have played with topological field (not string) theory and related things and you could think that Vafa's paper is just a "paper by a string theorist", not a "paper using string theory per se". (Strominger semi-jokingly said that he defined string theory as anything studied by his friends. I am sort of annoyed by that semi-joke because that's how you would describe an ill-defined business ruled by nepotism which string theory is certainly not.) But you would be wrong. At the end, Vafa constructs his full candidate description of the FQHE in terms of a compactification of M-theory. Well, he picks the dynamics of M5-branes in M-theory, the \((2,0)\) superconformal field theory in \(d=6\). Many of us have played with this exotic beast in many papers.

These six-dimensional theories with a self-dual three-form field strength at low energies are classified by the ADE classification. Cumrun compactifies such theories on the genus \(g\) Riemann surfaces \(\Sigma\) and claims that the possibilities correspond different forms of the FQHE. Lots of very particular technical constructions in the research of string/M-theory are actually used. Many facts are known about the compactifications which is why Cumrun can make numerous predictions for the actual lab experiments.

I can't reliably verify that Vafa's claims are right. It looks OK according to the resolution I have but to be sure about the final verdict, one has to be familiar with lots of details about the known theoretical and experimental knowledge of the FQHE as well, not to mention theoretical knowledge about the compactifications of the \((2,0)\) theory etc., and I am not fully familiar with everything that is needed.

However, I am certain that if the paper is right and the observed FQHE behavior may be mapped to compactifications of an important limit of string/M-theory, then condensed matter theoretical physicists at good schools should be trained in string/M-theory. A string course – perhaps a special "Strings for CMT" optimized course – should be mandatory. Subir Sachdev-like AdS/viscosity duality has been important for quite some time but in some sense, this kind of Vafa's description of FQHE – and perhaps related compactifications that describe other quantized, classifiable behaviors in condensed matter physics – could make string/M-theory even more fundamental for a sensible understanding of experiments that condensed matter physicists actually love to study in their labs.

It seems that we may be getting closer to the "full knowledge" of all similar interesting emergent behaviors exactly because the ideas we encounter start to repeat themselves – and sometimes in contexts so seemingly distant as Laughlin's labs and calculations of (for Laughlin esoteric) compactifications of limits of M-theory.


Aside from a nicety, Cumrun added in an e-mail:
I will only add that the experiments can settle this one perhaps even in a year, as I am told: Anomalies in neutral upstream currents that have already been observed, if confirmed for \(\nu=n/(2n+1)\) filling fraction, will be against the current paradigm and in line with my model.
Good luck to science and Cumrun in particular. ;-)

by Luboš Motl ( at November 18, 2015 08:26 AM

November 17, 2015

Symmetrybreaking - Fermilab/SLAC

Cleanroom is a verb

It’s not easy being clean.

Although they might be invisible to the naked eye, contaminants less than a micron in size can ruin very sensitive experiments in particle physics.

Flakes of skin, insect parts and other air-surfing particles—collectively known as dust—force scientists to construct or conduct certain experiments in cleanrooms, special places with regulated contaminant levels. There, scientists use a variety of tactics to keep their experiments dust-free.

The enemy within

Cleanrooms are classified by how many particles are found in a cubic foot of space. The fewer the particles, the cleaner the cleanroom.

To prevent contaminating particles from getting in, everything that enters cleanrooms must be covered or cleaned, including the people. Scratch that: especially the people.

“People are the dirtiest things in a cleanroom,” says Lisa Kaufman, assistant professor of nuclear physics at Indiana University. “We have to protect experiment detectors from ourselves.”

Humans are walking landfills as far as a cleanroom is concerned. We shed hair and skin incessantly, both of which form dust. Our body and clothes also carry dust and dirt. Even our fingerprints can be adversaries.

“Your fingers are terrible. They’re oily and filled with contaminants,” says Aaron Roodman, professor of particle physics and astrophysics at SLAC National Accelerator Laboratory.

In an experiment detector susceptible to radioactivity, the potassium in one fingerprint can create a flurry of false signals, which could cloud the real signals the experiment seeks.

As a cleanroom’s greatest enemy, humans must cover up completely to go inside: A zip-up coverall, known as a bunny suit, sequesters shed skin. (Although its name alludes otherwise, the bunny suit lacks floppy ears and a fluffy tail.) Shower-cap-like headgear holds in hair. Booties cover soiled shoes. Gloves are a must-have. In particularly clean cleanrooms, or for scientists sporting burly beards, facemasks may be necessary as well.

These items keep the number of particles brought into a cleanroom at a minimum.

“In a normal place, if you have some surface that’s unattended, that you don’t dust, after a few days you’ll see lots and lots of stuff,” Roodman says. “In a cleanroom, you don’t see anything.”

Getting to nothing, however, can take a lot more work than just covering up.

Washing up at SNOLAB

“This one undergrad who worked here put it, ‘Cleanroom is a verb, not a noun.’ Because the way you get a cleanroom clean is by constantly cleaning,” says research scientist Chris Jillings.

Jillings works at SNOLAB, an underground laboratory studying neutrinos and dark matter. The lab is housed in an active Canadian mine.

It seems an unlikely place for a cleanroom. And yet the entire 50,000-square-foot lab is considered a class-2000 cleanroom, meaning there are fewer than 2000 particles per cubic foot. Your average indoor space may have as many as 1 million particles per cubic foot.

SNOLAB’s biggest concern is mine dust, because it contains uranium and thorium. These radioactive elements can upset sensitive detectors in SNOLAB experiments, such as DEAP-3600, which is searching for the faint whisper of dark matter. Uranium and thorium could leave signals in its detector that look like evidence of dark matter.

“People are the dirtiest things in a cleanroom... We have to protect experiment detectors from ourselves.”

Most workplaces can’t guarantee that all of their employees shower before work, but SNOLAB can. Everyone entering SNOLAB must shower on their way in and re-dress in a set of freshly laundered clothes.

“We’ve sort of made it normal. It doesn’t seem strange to us,” says Jillings, who works on DEAP-3600. “It saves you a few minutes in the morning because you don’t have to shower at home.” More importantly, showering removes mine dust.

SNOLAB also regularly wipes down every surface and constantly filters the air.

Clearing the air for EXO

Endless air filtration is a mainstay of all modern cleanrooms. Willis Whitfield, former physicist at Sandia National Laboratories, invented the modern cleanroom in 1962 by introducing this continuous filtered airflow to flush out particles.

The filtered, pressurized, dehumidified air can make those who work in cleanrooms thirsty and contact-wearers uncomfortable.

“You get used to it over time,” says Kaufman, who works in a cleanroom for the SLAC-headed Enriched Xenon Observatory experiment, EXO-200.

EXO-200 is another testament to particle physicists’ affinity for mines. The experiment hunts for extremely rare double beta decay events at WIPP, a salt mine in New Mexico, in its own class-1000 cleanroom—even cleaner than SNOLAB.

As with SNOLAB experiments, anything emitting even the faintest amount of radiation is foe to EXO-200. Though those entering EXO-200’s cleanroom don’t have to shower, they do have to wash their arms, ears, face, neck and hands before covering up.

Ditching the dust for LSST

SLAC laboratory recently finished building another class-1000 cleanroom, specifically for assembly of the Large Synoptic Survey Telescope. LSST, an astronomical camera, will take over four years to build and will be the largest camera ever.

While SNOLAB and the EXO-200 cleanroom are mostly concerned with the radioactivity in particles containing uranium, thorium or potassium, LSST is wary of even the physical presence of particles.

“If you’ve got parts that have to fit together really precisely, even a little dust particle can cause problems,” Roodman says. Dust can block or absorb light in various parts of the LSST camera.

LSST’s parts are also vulnerable to static electricity. Built-up static electricity can wreck camera parts in a sudden zap known as an electrostatic discharge event.

To reduce the chance of a zap, the LSST cleanroom features static-dissipating floors and all of its benches and tables are grounded. Once again, humans prove to be the worst offenders.

“Most electrostatic discharge events are generated from humans,” says Jeff Tice, LSST cleanroom manager. “Your body is a capacitor and able to store a charge.”

Scientists assembling the camera will wear static-reducing garments as well as antistatic wrist straps that ground them to the floor and prevent the buildup of static electricity.

From static electricity to mine dust to fingerprints, every cleanroom is threatened by its own set of unseen enemies. But they all have one visible enemy in common: us.

by Chris Patrick at November 17, 2015 02:00 PM

November 16, 2015

ZapperZ - Physics and Physicists

Symmetry And Higgs Physics Via Economic Analogy?
Juan Maldacena is trying to do the impossible: explain the symmetry principles and the Higgs mechanism using analogies that one would find in economics.

I'm not making this up! :)

If you follow the link above, you will get the actual paper, which is an Open Access article. Read for yourself! :)

I am not sure if non-physicists will be able to understand it. If you are a non-physicist, and you went through the entire paper, let me know! I'm curious.


by ZapperZ ( at November 16, 2015 07:19 PM

Tommaso Dorigo - Scientificblogging

The Mysterious Z Boson Width Measurement - CDF 1989

As I am revising the book I am writing on the history of the CDF experiment, I have bits and pieces of text that I decided to remove, but which retain some interest for some reason. Below I offer a clip which discusses the measurement of the natural width of the Z boson produced by CDF with Run 0 data in 1989. The natural width of a particle is a measurement of how undetermined is its rest mass, due to the very fast decay. The Z boson is in fact the shortest lived particle we know, and its width is of 2.5 GeV.

read more

by Tommaso Dorigo at November 16, 2015 03:29 PM

November 15, 2015

November 13, 2015

Clifford V. Johnson - Asymptotia

One Hundred Years of Certitude

Einstein_CentennialSince the early Summer I've been working (with the help of several people at USC*) toward a big event next Friday: A celebration of 100 years since Einstein formulated the field equations of General Relativity, a theory which is one of the top one or few (depending upon who you argue with over beers about this) scientific achievements in the history of human thought. The event is a collaboration between the USC Harman Academy of Polymathic Study and the LAIH, which I co-direct. I chose the title of this post since (putting aside the obvious desire to resonate with a certain great work of literature) this remarkable scientific framework has proven to be a remarkably robust and accurate model of how our universe's gravity actually works in every area it has been tested with experiment and observation**. Despite being all about bizarre things like warped spacetime, slowing down time, and so forth, which most people think is to do only with science fiction. (And yes, you probably test it every day through your [...] Click to continue reading this post

The post One Hundred Years of Certitude appeared first on Asymptotia.

by Clifford at November 13, 2015 03:09 AM

November 12, 2015

Symmetrybreaking - Fermilab/SLAC

Giving physics students options

Many physics degree programs excel at preparing students for an academic career, but more than half of those who complete the programs head to industry instead.

“I was drawn to physics because I thought it was amazing,” says Crystal Bailey, recalling the beginnings of her graduate work in the early 2000s. “There’s a sense of wonder that we’re really understanding something fundamental and elegant about the universe.”

But when she decided that an academic career path wasn’t right for her, she left her degree program. Bailey assumed, like many physics students, that the purpose of earning a physics degree is to remain in academia. In fact, statistics describe a different reality.

The American Institute of Physics states that roughly half of those who enter the workforce with a degree in physics—either a bachelor’s, master’s or doctorate—work in the private sector. 

In an AIP survey of PhD recipients who had earned their degrees the previous year, 64 percent of respondents who identified their jobs as potentially permanent positions were working in industry.

Institutions in the United States currently grant around 1700 physics PhDs each year, though only about 350 academic faculty positions become available in that time, according to the AIP.

Most university physics programs are rooted in academic tradition, and some members of the physics community have expressed concern that not enough emphasis is placed on preparing students for potential jobs in industry. Among these members are the professors and students in three physics programs that are bucking this trend, taking a decidedly different approach to prepare aspiring physicists for what awaits beyond graduation.

Scientists at work

By the time Nicholas Sovis graduates in 2016 with his bachelor’s degree in applied physics, he’ll have two and a half years of work experience analyzing fuel injector movements at Argonne National Laboratory. Like the rest of his colleagues at Kettering University in Flint, Michigan, Sovis arranged an industry co-op through his school starting his freshman year. He alternates every three months between full-time school and full-time work. He’ll graduate in four years by taking an accelerated schedule of courses during academic terms.

“There are a lot of people who work in industry who are at—and really need to be at—the PhD level.”

Co-ops and internships are certainly not unique to this program, but the unparalleled emphasis on co-op experience is part of what Kathryn Svinarich, department head of physics at Kettering, calls their “industry-friendly” culture. 

The university operated for many years training automotive engineers under the name General Motors Institute. Although the school is now an independent institution, Kettering still produces some of the most industry-oriented physicists in the country.

“We’re really turning heads in the [American Physical Society],” says Svinarich. “We’re the only fully co-op school with strong programs in physics.”

The program’s basic purpose is to provide students marketable skills while offering participating companies access to a talent pipeline. The tandem training at both Kettering and a private company or government institution lets students experience academic and industry life and connect with mentors in each realm. 

Sovis says the combination of mentors has broadened his perspective. “I have learned how incredibly diverse the field of physics truly is,” he says. 

As he weighs his options for the future, he adds that he is considering working for an agency such as NASA while remaining open to opportunities to do research at academic institutions.

Fueling innovation

In the 1990s, the graduate program in physics at Case Western Reserve University in Cleveland, Ohio, was getting mixed reviews from its alumni. On one hand, many former students were finding success leading innovative start-ups. On the other, they were struggling, finding themselves unprepared to handle the logistics of running a business.

In response, the university formed the Physics Entrepreneurship Program. This terminal masters degree program aims at providing its students skills in market analysis, financing strategies and leadership, while also connecting students to mentors, funding and talent. Students couple courses in physics with courses in business and law.

“Innovation is not speculative,” says Ed Caner, director of science and technology entrepreneurship programs. “You cannot simply write a business plan and get investors on board.”

Nathan Swift, a second-year student in the program, found this lesson valuable. For his thesis, he’s starting his own company. “We're developing a biomimetic [nature-imitating] impact material that could be integrated into helmets in place of conventional foam,” he says.

His business partners are biologists—PhD candidates at the University of Akron. Without Swift, the students didn’t have the business savvy or mechanical background to develop the idea, which they originally sketched out for a class. The team is currently fundraising and testing early prototypes. 

Swift says that, though he is excited by the opportunity, participating in the Case Western program isn’t about definitively choosing one career path over another. “I'm doing it to gain the necessary skills so that I can be dangerous with both a technical and business fluency—in whatever I choose to pursue.”

Lessons in leadership

James Freericks, professor and director of graduate studies in physics at Georgetown University, says that 20 years ago, professional organizations were telling him that the universities were overproducing PhDs.

Freericks looked deeper and found that an imbalance had existed for decades. The supply of physics doctorates has far outpaced their academic demand as far back as the 1960s.

“To say the only reason you’re producing PhDs is for academics is a very narrow point of view,” Freericks says. “There are a lot of people who work in industry who are at—and really need to be at—the PhD level.”

Freericks now directs Georgetown’s Industrial Leadership in Physics program, organized in 2001. The program is expressly designed to train physics students to secure advanced positions in industry.

“You have to do a certain amount of problem-solving, a certain amount of head-scratching—and hitting your head against the wall...”

As with a traditional program, students engage in rigorous coursework and original research. But the program also blends in elements similar to those at Case Western and Kettering, such as courses in business and patent law and a yearlong apprenticeship in industry. An advisory committee of scientific leaders representing Lockheed Martin, IBM, BASF Corporation and other companies guides the program and provides mentorship.

The lengthy internships give students time to become fully immersed in the research and methods of a company. Ultimately, such apprenticeships prepare students to manage sophisticated scientific projects—including their significant budgets and groups of other scientists.

Academically industrial or industrially academic

What, then, is the right balance in physics education? Barbara Jones, a theoretical physicist at IBM and an ILP advisory committee member, advocates broad training that prepares students for work in industry as well as at a college or a national lab. She points out that the traditional classroom is not a complete failure in this regard.

“To get a PhD in physics, you have to do a certain amount of problem-solving, a certain amount of head-scratching—and hitting your head against the wall—that’s independent of any job,” Jones says. These skills translate, which is why classically trained physicists have been successfully obtaining and thriving in industry jobs for a long time, she says. 

But “students even at more traditional programs can take a pointer from these programs and see about arranging for industrial internships for themselves.”

Perhaps the greatest value of programs such as these, Jones suggests, is giving students options.

Bailey agrees. She eventually completed a PhD in nuclear physics and now serves as the careers program manager at the American Physical Society. She organizes resources to help students navigate the many paths of being a physicist, including a new program now in its pilot stage, APS Industry Mentoring for Physicists (IMPact). The program connects early-career physicists with other physicists working in industry. 

Bailey’s job also frequently involves giving talks on pursuing physics as a career. She tells students, “Your career will not always take you where you expect. But you can always find a way to do the things you love.”

by Troy Rummler at November 12, 2015 03:36 PM

Jester - Resonaances

A year at 13 TeV
A week ago the LHC finished the 2015 run of 13 TeV proton collisions.  The counter in ATLAS stopped exactly at 4 inverse femtobarns. CMS reports just 10% less, however it is not clear what fraction of these data is collected with their magnet on (probably about a half). Anyway, it should have been better, it could have been worse...   4 fb-1 is one fifth of what ATLAS and CMS collected in the glorious year 2012.  On the other hand, the higher collision energy in 2015 translates to larger production cross sections, even for particles within the kinematic reach of the 8 TeV collisions.  How this trade off work in practice depends on the studied process.  A few examples are shown in the plot below
We see that, for processes initiated by collisions of a quark inside one proton with an antiquark inside the other proton, the cross section gain is the least favorable. Still, for hypothetical resonances heavier than ~1.7 TeV, more signal events were produced in the 2015 run than in the previous one. For example, for a 2 TeV W-prime resonance, possibly observed by ATLAS in the 8 TeV data, the net gain is 50%, corresponding to roughly 15 events predicted in the 13 TeV data. However, the plot does not tell the whole story, because the backgrounds have increased as well.  Moreover, when the main background originates from gluon-gluon collisions (as is the case for the W-prime search in the hadronic channel),  it grows faster than the signal.  Thus, if the 2 TeV W' is really there, the significance of the signal in the 13 TeV data should be comparable to that in the 8 TeV data in spite of the larger event rate. That will not be enough to fully clarify the situation, but the new data may make the story much more exciting if the excess reappears;  or much less exciting if it does not... When backgrounds are not an issue (for example, for high-mass dilepton resonances) the improvement in this year's data should be more spectacular.

We also see that, for new physics processes initiated by collisions of a gluon in 1 proton with another gluon in the other proton, the 13 TeV run is superior everywhere above the TeV scale, and the signal enhancement is more spectacular. For example, at 2 TeV one gains a factor of 3 in signal rate. Therefore, models where the ATLAS diboson excess is explained via a Higgs-like scalar resonance will be tested very soon. The reach will also be extended for other hypothetical particles pair-produced in gluon collisions, such as  gluinos in the minimal supersymmetric model. The current lower limit on the gluino mass obtained by  the 8 TeV run is m≳1.4 TeV  (for decoupled squarks and massless neutralino). For this mass, the signal gain in the 2015 run is roughly a factor of 6. Hence we can expect the gluino mass limits will be pushed upwards soon, by about 200 GeV or so.  

Summarizing,  we have a right to expect some interesting results during this winter break. The chances for a discovery  in this year's data are non-zero,  and chances for a tantalizing hints of new physics (whether a real thing or a background fluctuation) are considerable. Limits on certain imaginary particles will be somewhat improved. However, contrary to my hopes/fears, this year is not yet the decisive one for particle physics.  The next one will be.

by Jester ( at November 12, 2015 03:05 PM

November 11, 2015

The n-Category Cafe

Burritos for Category Theorists

You’ve probably heard of Lawvere’s Hegelian taco. Now here is a paper that introduces the burrito to category theorists:

The source of its versatility and popularity is revealed:

To wit, a burrito is just a strong monad in the symmetric monoidal category of food.

Frankly, having seen plenty of attempts to explain monads to computer scientists, I thought this should have been marketed as ‘monads for chefs’. But Mike Stay, who pointed me to this article, explained its subtext:

Haskell uses monads all over the place, and programmers who are not used to functional programming often find them confusing. This is a quote from a widely-shared article on the proliferation of “monad tutorials”:

After struggling to understand them for a week, looking at examples, writing code, reading things other people have written, he finally has an “aha!” moment: everything is suddenly clear, and Joe Understands Monads! What has really happened, of course, is that Joe’s brain has fit all the details together into a higher-level abstraction, a metaphor which Joe can use to get an intuitive grasp of monads; let us suppose that Joe’s metaphor is that Monads are Like Burritos. Here is where Joe badly misinterprets his own thought process: “Of course!” Joe thinks. “It’s all so simple now. The key to understanding monads is that they are Like Burritos. If only I had thought of this before!” The problem, of course, is that if Joe HAD thought of this before, it wouldn’t have helped: the week of struggling through details was a necessary and integral part of forming Joe’s Burrito intuition, not a sad consequence of his failure to hit upon the idea sooner.

The article is this:

by john ( at November 11, 2015 11:41 PM

ZapperZ - Physics and Physicists

What Is Computational Physics?
Rhett Allain has published his take on what "computational physics" is.

Many of us practicing physicists do work in computational physics. Some very often, some now and then. At some point, many of us have to either analyze data, do numerical modeling, or solve intractable equations. We either use pre-made codes, modify some other computer codes, write our own code, or use commercial software.

But certainly, this is less involved than someone who specializes in computational physics. But many of us do have the need to know how to do some of these things as part of our job. People who have to simulate particle beam dynamics, and those design accelerating structures are often accelerator physicists rather than computational physicists.

Hum... now I seem to be rambling on and can't remember the point I was trying to make. Ugh! Old age sucks!


by ZapperZ ( at November 11, 2015 05:49 PM

Symmetrybreaking - Fermilab/SLAC

Dark matter’s newest pursuer

Scientists have inaugurated the new XENON1T experiment at Gran Sasso National Laboratory in Italy.

Researchers at a laboratory deep underneath the tallest mountain in central Italy have inaugurated XENON1T, the world’s largest and most sensitive device designed to detect a popular dark matter candidate.

“We will be the biggest game in town,” says Columbia University physicist Elena Aprile, spokesperson for the XENON collaboration, which has over the past decade designed, built and operated a succession of ever-larger experiments that use liquid xenon to look for evidence of weakly interacting massive dark matter particles, or WIMPs, at the Gran Sasso National Laboratory.

Interactions with these dark matter particles are expected to be rare: Just one a year for every 1000 kilograms of xenon. As a result, larger experiments have a better chance of intercepting a WIMP as it passes through the Earth.

XENON1T’s predecessors—XENON 10 (2006 to 2009) and XENON 100 (2010 to the present)—held 25 and 160 kilograms of xenon, respectively. The new XENON11 experiment’s detector measures 1 meter high and 1 meter in diameter and contains 3500 kilograms of liquid xenon, nearly 10 times as much as the next-biggest xenon-filled dark matter experiment, the Large Underground Xenon experiment.

Looking for WIMPs

Should a WIMP collide with a xenon atom, kicking its nucleus or knocking out one of its electrons, the result is a burst of fast ultraviolet light and a bunch of free electrons. Scientists built a strong electric field in the XENON1T detector to direct these freed particles to the top of the chamber, where they will create a second burst of light. The relative timing and brightness of the two flashes will help the scientists determine the type of particle that created them.

“Since our detectors can detect even a single electron or photon, XENON1T will be sensitive to even the most feeble particle interactions,” says Rafael Lang, a Purdue University physicist on the XENON collaboration.

Scientists cool the xenon to minus 163 degrees Fahrenheit to turn it into a liquid three times denser than water. One oddity of xenon is that its boiling temperature is only 7 degrees Fahrenheit above its melting temperature. So “we have to control our temperature and pressure precisely,” Aprile says.

The experiment is shielded from other particles such as cosmic rays by separate layers of water, lead, polyethylene and copper—not to mention 1400 meters of Apennine rock that lie above the Gran Sasso lab’s underground tunnels.

Keeping the xenon free of contaminants is essential to the detector’s sensitivity. Oxygen, for example, can trap electrons. And the decay of some radioactive krypton isotopes, which are difficult to separate from xenon, can obscure a WIMP signal. The XENON collaboration’s solution is to continuously circulate and filter 100 liters of xenon gas every minute from the top of the detector through a filtering system before chilling it and returning it to service.

A matter of scale

XENON researchers hope that their new experiment will finally be the one to see definitive evidence of WIMPs. But just in case, XENON1T was designed to accommodate a swift upgrade to 7000 kilograms of xenon in its next iteration. (At the same time, the LUX and UK-based Zeplin groups joined forces to design a similar-scale xenon detector, LZ.)

“If we see nothing with XENON1T, it will still be worth it to move up to the 7000-kilogram device, since it will be relatively easy to do that,” Aprile says. “If we do see a few events with XENON1T—and we’re sure they are from the dark matter particle—then the best way to prove that it’s real is to confirm that result with a larger, more sensitive experiment.

“In any case,” Aprile says, “we should know whether the WIMP is real or not before 2020.”

by Mike Ross at November 11, 2015 04:31 PM

The n-Category Cafe

Weil, Venting

From the introduction to André Weil’s Basic Number Theory:

It will be pointed out to me that many important facts and valuable results about local fields can be proved in a fully algebraic context, without any use being made of local compacity, and can thus be shown to preserve their validity under far more general conditions. May I be allowed to suggest that I am not unaware of this circumstance, nor of the possibility of similarly extending the scope of even such global results as the theorem of Riemann–Roch? We are dealing here with mathematics, not theology. Some mathematicians may think they can gain full insight into God’s own way of viewing their favorite topic; to me, this has always seemed a fruitless and a frivolous approach. My intentions in this book are more modest. I have tried to show that, from the point of view which I have adopted, one could give a coherent treatment, logically and aesthetically satisfying, of the topics I was dealing with. I shall be amply rewarded if I am found to have been even moderately successful in this attempt.

I was young when I discovered by harsh experience that even mathematicians with crashingly comprehensive establishment credentials can be as defensive and prickly as anyone. I was older when (and I only speak of my personal tastes) I got bored of tales of Grothendieck-era mathematical Paris.

Nonetheless, I find the second half of Weil’s paragraph challenging. Is there a tendency, in category theory, to imagine that there’s such a thing as “God’s own way of viewing” a topic? I don’t think that approach is fruitless. Is it frivolous?

by leinster ( at November 11, 2015 12:33 AM

November 09, 2015

ZapperZ - Physics and Physicists

100 Years Of General Relativity
General Relativity turns 100 years this month. The Universe was never the same again after that! :)

APS Physics has a collection of articles related to various papers published in their family of journals related to GR. Check them out. Many are fairly understandable to non-experts.


by ZapperZ ( at November 09, 2015 07:03 PM

Symmetrybreaking - Fermilab/SLAC

Neutrino experiments win big again

The Fundamental Physics Prize recognized five collaborations studying neutrino oscillations.

Hot on the heels of their Nobel Prize recognition, neutrino oscillations have another accolade to add to their list. On November 8, representatives from five different neutrino experiments accepted a joint award for the 2016 Breakthrough Prize in Fundamental Physics.

The Breakthrough Prizes, also given for life sciences and mathematics, celebrate both science itself and the work of scientists. The award was founded by Sergey Brin, Anne Wojcicki, Jack Ma, Cathy Zhang, Yuri and Julia Milner, Mark Zuckerberg and Priscilla Chan with the goal of inspiring more people to pursue scientific endeavors.

This year’s $3 million prize for physics will be shared evenly among five teams: the Daya Bay Reactor Neutrino Experiment based in China, the KamLAND collaboration in Japan, the K2K (KEK to Kamioka) and T2K (Tokai to Kamioka) long-baseline neutrino oscillation experiments in Japan, Sudbury Neutrino Observatory (SNO) in Canada, and the Super-Kamiokande collaboration in Japan. These experiments explored the nature of the ghostly particles that are the most abundant massive particle in the universe, and how they change among three types as they travel.

Almost 1400 people contributed to these experiments that discovered and unraveled neutrino oscillations, “revealing a new frontier beyond, and possibly far beyond, the standard model of particle physics,” according to the Breakthrough Prize’s press release.

This year’s physics Nobel laureates Takaaki Kajita (Super-K) and Arthur B. McDonald (SNO) appeared onstage to accept to the Breakthrough Prize along with Yifang Wang, Kam-Biu Luk, Atsuto Suzuki, Koichiro Nishikawa and Yoichiro Suzuki.

“The quest for the secrets of neutrinos is not finished yet, and many more mysteries are yet to be discovered,” Wang said during the ceremony at Mountain View, California. There are many questions left to answer about neutrinos, including how much mass they have, whether there are more than three types, and whether neutrinos and antineutrinos behave differently.

A broad slate of oscillation experiments are currently studying neutrinos or planned for the future. Daya Bay, Super-K, T2K and KamLAND continue to research the particles, as does an upgraded version of SNO, SNO+. The US-based MINOS+ and NOvA are currently taking long-baseline neutrino oscillation data. The Jiangmen Underground Neutrino Observatory is under construction in China, and the international Deep Underground Neutrino Experiment is progressing quickly through the planning phase. Many others dot the neutrino experiment landscape, using everything from nuclear reactors to giant chunks of Antarctic ice to learn more about the hard-to-catch particles. With so much left to discover, it seems like there are plenty of prizes left in neutrino research.

by Lauren Biron at November 09, 2015 06:40 PM

Symmetrybreaking - Fermilab/SLAC

Physics Photowalk voting begins

Pick your favorites from among 24 photos taken during the Global Physics Photowalk.

Twenty-four top photos have been selected to enter the next stage of the Global Physics Photowalk competition.

In September, eight world-leading research laboratories invited photographers to take a look behind the scenes at their facilities­ to share the beauty behind physics. More than 200 photographers collectively participated in the international photowalk, submitting thousands of photos into local competitions. After careful deliberation, each laboratory selected their three winning photos from their local submissions to enter into the global competition.

In the next stage of the global competition, the top 24 photos will be judged in two categories: a jury competition, facilitated through a panel of international judges, and a people’s choice competition, conducted via an online popular vote. Starting today, the public is invited to view and choose their favorite photos on the Interactions Collaboration website. Voting closes November 30.

While voting for the people’s choice selection is underway, an international jury comprising artists, photographers and scientists will convene to scrutinize the photos and crown the global winners.

Those winners will be announced in December and will have the opportunity to be featured in Symmetry magazine, the CERN Courier, and as part of a traveling exhibit across laboratories in Australia, Asia, Europe and North America.

Visit to view additional photographs from each laboratory’s local event.

by Interactions at November 09, 2015 02:00 PM

November 08, 2015

John Baez - Azimuth

Tale of a Doomed Galaxy

Part 1

About 3 billion years ago, if there was intelligent life on the galaxy we call PG 1302-102, it should have known it was in serious trouble.

Our galaxy has a supermassive black hole in the middle. But that galaxy had two. One was about ten times as big as the other. Taken together, they weighed a billion times as much as our Sun.

They gradually spiraled in towards each other… and then, suddenly, one fine morning, they collided. The resulting explosion was 10 million times more powerful than a supernova—more powerful than anything astronomers here on Earth have ever seen! It was probably enough to wipe out all life in that galaxy.

We haven’t actually seen this yet. The light and gravitational waves from the disaster are still speeding towards us. They should reach us in roughly 100,000 years. We’re not sure when.

Right now, we see the smaller black hole still orbiting the big one, once every 5 years. In fact it’s orbiting once every 4 years! But thanks to the expansion of the universe, PG 1302-102 is moving away from us so fast that time on that distant galaxy looks significantly slowed down to us.

Orbiting once every 4 years: that doesn’t sound so fast. But the smaller black hole is about 2000 times more distant from its more massive companion than Pluto is from our Sun! So in fact it’s moving at very high speed – about 1% of the speed of light. We can actually see it getting redshifted and then blueshifted as it zips around. And it will continue to speed up as it spirals in.

What exactly will happen when these black holes collide? It’s too bad we won’t live to see it. We’re far enough that it will be perfectly safe to watch from here! But the human race knows enough about physics to say quite a lot about what it will be like. And we’ve built some amazing machines to detect the gravitational waves created by collisions like this—so as time goes on, we’ll know even more.

Part 2

Even before the black holes at the heart of PG 1302-102 collided, life in that galaxy would have had a quasar to contend with!

This is a picture of Centaurus A, a much closer galaxy with a quasar in it. A quasar is huge black hole in the middle of a galaxy—a black hole that’s eating lots of stars, which rip apart and form a disk of hot gas as they spiral in. ‘Hot’ is an understatement, since this gas moves near the speed of light. It gets so hot that it pumps out intense jets of particles – from its north and south poles. Some of these particles even make it to Earth.

Any solar system in Centaurus A that gets in the way of those jets is toast.

And these jets create lots of radiation, from radio waves to X-rays. That’s how we can see quasars from billions of light years away. Quasars are the brightest objects in the universe, except for short-lived catastrophic events like the black hole collisions and gamma-ray bursts from huge dying stars.

It’s hard to grasp the size and power of such things, but let’s try. You can’t see the black hole in the middle of this picture, but it weighs 55 million times as much as our Sun. The blue glow of the jets in this picture is actually X rays. The jet at upper left is 13,000 light years long, made of particles moving at half the speed of light.

A typical quasar puts out a power of roughly 1040 watts. They vary a lot, but let’s pick this number as our ‘standard quasar’.

But what does 1040 watts actually mean? For comparison, the Sun puts out 4 x 1026 watts. So, we’re talking 30 trillion Suns. But even that’s too big a number to comprehend!

Maybe it would help to say that the whole Milky Way puts out 5 x 1036 watts. So a single quasar, at the center of one galaxy, can have the power of 2000 galaxies like ours.

Or, we can work out how much energy would be produced if the entire mass of the Moon were converted into energy. I’m getting 6 x 1039 joules. That’s a lot! But our standard quasar is putting out a bit more power than if it were converting one Moon into energy each second.

But you can’t just turn matter completely into energy: you need an equal amount of antimatter, and there’s not that much around. A quasar gets its power the old-fashioned way: by letting things fall down. In this case, fall down into a black hole.

To power our standard quasar, 10 stars need to fall into the black hole every year. The biggest quasars eat 1000 stars a year. The black hole in our galaxy gets very little to eat, so we don’t have a quasar.

There are short-lived events much more powerful than a quasar. For example, a gamma-ray burst, formed as a hypergiant star collapses into a black hole. A powerful gamma-ray burst can put out 10^44 watts for a few seconds. That’s equal to 10,000 quasars! But quasars last a long, long time.

So this was life in PG 1302-102 before things got really intense – before its two black holes spiraled into each other and collided. What was that collision like? I’ll talk about that next time.

The above picture of Centaurus A was actually made from images taken by three separate telescopes. The orange glow is submillimeter radiation – between infrared and microwaves—detected by the Atacama Pathfinder Experiment (APEX) telescope in Chile. The blue glow is X-rays seen by the Chandra X-ray Observatory. The rest is a photo taken in visible light by the Wide Field Imager on the Max-Planck/ESO 2.2 meter telescope, also located in Chile. This shows the dust lanes in the galaxy and background stars.

Part 3

What happened at the instant the supermassive black holes in the galaxy PG 1302-102 finally collided?

We’re not sure yet, because the light and gravitational waves will take time to get here. But physicists are using computers to figure out what happens when black hole collide!

Here you see some results. The red blobs are the event horizons of two black holes.

First the black holes orbit each other, closer and closer, as they lose energy by emitting gravitational radiation. This is called the ‘inspiral’ phase.

Then comes the ‘plunge’ and ‘merger’. They plunge towards each other. A thin bridge forms between them, which you see here. Then they completely merge.

Finally you get a single black hole, which oscillates and then calms down. This is called the ‘ringdown’, because it’s like a bell ringing, loudly at first and then more quietly. But instead of emitting sound, it’s emitting gravitational waves—ripples in the shape of space!

In the top picture, the black holes have the same mass: one looks smaller, but that’s because it’s farther away. In the bottom picture, the black hole at left is twice as massive.

Here’s one cool discovery. An earlier paper had argued there could be two bridges, except in very symmetrical situations. If that were true, a black hole could have the topology of a torus for a little while. But these calculations showed that – at least in the cases they looked at—there’s just one bridge.

So, you can’t have black hole doughnuts. At least not yet.

These calculations were done using free software called SpEC. But before you try to run it at home: the team that puts out this software says:

Because of the steep learning curve and complexity of SpEC, new users are typically introduced to SpEC through a collaboration with experienced SpEC users.

It probably requires a lot of computer power, too. These calculations are very hard. We know the equations; they’re just tough to solve. The first complete simulation of an inspiral, merger and ringdown was done in 2005.

The reason people want to simulate colliding black holes is not mainly to create pretty pictures, or even understand what happens to the event horizon. It’s to understand the gravitational waves they will produce! People are building better and better gravitational wave detectors—more on that later—but we still haven’t seen gravitational waves. This is not surprising: they’re very weak. To find them, we need to filter out noise. So, we need to know what to look for.

The pictures are from here:

• Michael I. Cohen and Jeffrey D. Kaplan and Mark A. Scheel, On toroidal horizons in binary black hole inspirals, Phys. Rev. D 85 (2012), 024031.

Part 4

Let’s imagine an old, advanced civilization in the doomed galaxy PG 1302-102.

Long ago they had mastered space travel. Thus, they were able to survive when their galaxy collided with another—just as ours will collide with Andromeda four billion years from now. They had a lot of warning—and so do we. The picture here shows what Andromeda will look like 250 million years before it hits.

They knew everything we do about astronomy—and more. So they knew that when galaxies collide, almost all stars sail past each other unharmed. A few planets get knocked out of orbit. Colliding clouds of gas and dust form new stars, often blue giants that live short, dramatic lives, going supernova after just 10 million years.

All this could be handled by not being in the wrong place at the wrong time. They knew the real danger came from the sleeping monsters at the heart of the colliding galaxies.

Namely, the supermassive black holes!

Almost every galaxy has a huge black hole at its center. This black hole is quiet when not being fed. But when galaxies collide, lots of gas and dust and even stars get caught by the gravity and pulled in. This material form a huge flat disk as it spirals down and heats up. The result is an active galactic nucleus.

In the worst case, the central black holes can eat thousands of stars a year. Then we get a quasar, which easily pumps out the power of 2000 ordinary galaxies.

Much of this power comes out in huge jets of X-rays. These jets keep growing, eventually stretching for hundreds of thousands of light years. The whole galaxy becomes bathed in X-rays—killing all life that’s not prepared.

Let’s imagine a civilization that was prepared. Natural selection has ways of weeding out civilizations that are bad at long-term planning. If you’re prepared, and you have the right technology, a quasar could actually be a good source of power.

But the quasar was just the start of the problem. The combined galaxy had two black holes at its center. The big one was at least 400 million times the mass of our Sun. The smaller one was about a tenth as big—but still huge.

They eventually met and started to orbit each other. By flinging stars out the way, they gradually came closer. It was slow at first, but the closer they got, the faster they circled each other, and the more gravitational waves they pumped out. This carried away more energy—so they moved closer, and circled even faster, in a dance with an insane, deadly climax.

Right now—here on Earth, where it takes a long time for the news to reach us—we see that in 100,000 years the two black holes will spiral down completely, collide and merge. When this happens, a huge pulse of gravitational waves, electromagnetic radiation, matter and even antimatter will blast through the galaxy called PG 1302-102.

I don’t know exactly what this will be like. I haven’t found papers describing this kind of event in detail.

One expert told the New York Times that the energy of this explosion will equal 100 million supernovae. I don’t think he was exaggerating. A supernova is a giant star whose core collapses as it runs out of fuel, easily turning several Earth masses of hydrogen into iron before you can say “Jack Robinson”. When it does this, it can easily pump out 1044 joules of energy. So, 100 millon supernovae is 1052 joules. By contrast, if we could convert all the mass of the black holes in PG 1302-102. into energy, we’d get about 1056 joules. So, our expert was just saying that their merger will turns 0.01% of their combined mass into energy. That seems quite reasonable to me.

But I want to know what happens then! What will the explosion do to the galaxy? Most of the energy comes out as gravitational radiation. Gravitational waves don’t interact very strongly with matter. But when they’re this strong, who knows? And of course there will be plenty of ordinary radiation, as the accretion disk gets shredded and sucked into the new combined black hole.

The civilization I’m imagining was smart enough not to stick around. They decided to simply leave the galaxy.

After all, they could tell the disaster was coming, at least a million years in advance. Some may have decided to stay and rough it out, or die a noble death. But most left.

And then what?

It takes a long time to reach another galaxy. Right now, travelling at 1% the speed of light, it would take 250 million years to reach Andromeda from here.

But they wouldn’t have to go to another galaxy. They could just back off, wait for the fireworks to die down, and move back in.

So don’t feel bad for them. I imagine they’re doing fine.

By the way, the expert I mentioned is S. George Djorgovski of Caltech, mentioned here:

• Dennis Overbye, Black holes inch ahead to violent cosmic union, New York Times, 7 January 2015.

Part 5

When distant black holes collide, they emit a burst of gravitational radiation: a ripple in the shape of space, spreading out at the speed of light. Can we detect that here on Earth? We haven’t yet. But with luck we will soon, thanks to LIGO.

LIGO stands for Laser Interferometer Gravitational Wave Observatory. The idea is simple. You shine a laser beam down two very long tubes and let it bounce back and forth between mirrors at the ends. You use this compare the length of these tubes. When a gravitational wave comes by, it stretches space in one direction and squashes it in another direction. So, we can detect it.

Sounds easy, eh? Not when you run the numbers! We’re trying to see gravitational waves that stretch space just a tiny bit: about one part in 1023. At LIGO, the tubes are 4 kilometers long. So, we need to see their length change by an absurdly small amount: one-thousandth the diameter of a proton!

It’s amazing to me that people can even contemplate doing this, much less succeed. They use lots of tricks:

• They bounce the light back and forth many times, effectively increasing the length of the tubes to 1800 kilometers.

• There’s no air in the tubes—just a very good vacuum.

• They hang the mirrors on quartz fibers, making each mirror part of a pendulum with very little friction. This means it vibrates very well at one particular frequency, and very badly at frequencies far from that. This damps out the shaking of the ground, which is a real problem.

• This pendulum is hung on another pendulum.

• That pendulum is hung on a third pendulum.

• That pendulum is hung on a fourth pendulum.

• The whole chain of pendulums is sitting on a device that detects vibrations and moves in a way to counteract them, sort of like noise-cancelling headphones.

• There are 2 of these facilities, one in Livingston, Louisiana and another in Hanford, Washington. Only if both detect a gravitational wave do we get excited.

I visited the LIGO facility in Louisiana in 2006. It was really cool! Back then, the sensitivity was good enough to see collisions of black holes and neutron stars up to 50 million light years away.

Here I’m not talking about supermassive black holes like the ones in the doomed galaxy of my story here! I’m talking about the much more common black holes and neutron stars that form when stars go supernova. Sometimes a pair of stars orbiting each other will both blow up, and form two black holes—or two neutron stars, or a black hole and neutron star. And eventually these will spiral into each other and emit lots of gravitational waves right before they collide.

50 million light years is big enough that LIGO could see about half the galaxies in the Virgo Cluster. Unfortunately, with that many galaxies, we only expect to see one neutron star collision every 50 years or so.

They never saw anything. So they kept improving the machines, and now we’ve got Advanced LIGO! This should now be able to see collisions up to 225 million light years away… and after a while, three times further.

They turned it on September 18th. Soon we should see more than one gravitational wave burst each year.

In fact, there’s a rumor that they’ve already seen one! But they’re still testing the device, and there’s a team whose job is to inject fake signals, just to see if they’re detected. Davide Castelvecchi writes:

LIGO is almost unique among physics experiments in practising ‘blind injection’. A team of three collaboration members has the ability to simulate a detection by using actuators to move the mirrors. “Only they know if, and when, a certain type of signal has been injected,” says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who leads the Advanced LIGO’s data-analysis team.

Two such exercises took place during earlier science runs of LIGO, one in 2007 and one in 2010. Harry Collins, a sociologist of science at Cardiff University, UK, was there to document them (and has written books about it). He says that the exercises can be valuable for rehearsing the analysis techniques that will be needed when a real event occurs. But the practice can also be a drain on the team’s energies. “Analysing one of these events can be enormously time consuming,” he says. “At some point, it damages their home life.”

The original blind-injection exercises took 18 months and 6 months respectively. The first one was discarded, but in the second case, the collaboration wrote a paper and held a vote to decide whether they would make an announcement. Only then did the blind-injection team ‘open the envelope’ and reveal that the events had been staged.

Aargh! The disappointment would be crushing.

But with luck, Advanced LIGO will soon detect real gravitational waves. And I hope life here in the Milky Way thrives for a long time – so that when the gravitational waves from the doomed galaxy PG 1302-102 reach us, hundreds of thousands of years in the future, we can study them in exquisite detail.

For Castelvecchi’s whole story, see:

• Davide Castelvecchi Has giant LIGO experiment seen gravitational waves?, Nature, 30 September 2015.

For pictures of my visit to LIGO, see:

• John Baez, This week’s finds in mathematical physics (week 241), 20 November 2006.

For how Advanced LIGO works, see:

• The LIGO Scientific Collaboration Advanced LIGO, 17 November 2014.


To see where the pictures are from, click on them. For more, try this:

• Ravi Mandalia, Black hole binary entangled by gravity progressing towards deadly merge.

The picture of Andromeda in the nighttime sky 3.75 billion years from now was made by NASA. You can see a whole series of these pictures here:

• NASA, NASA’s Hubble shows Milky Way is destined for head-on collision, 31 March 2012.

Let’s get ready! For starters, let’s deal with global warming.

by John Baez at November 08, 2015 01:13 AM

November 06, 2015

Clifford V. Johnson - Asymptotia

Anthropological Friends

From_museo_nacional_antropologia_mexicoAs promised, on the right is the companion figure to the one I shared earlier (on the left). Click for a larger view. These were two jolly fellows I found in glass cases at Mexico City's Museo Nacional de Antropologia, and sort of had to sketch them.

-cvj Click to continue reading this post

The post Anthropological Friends appeared first on Asymptotia.

by Clifford at November 06, 2015 05:40 PM



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