Particle Physics Planet


February 09, 2010

Phil Plait - Bad Astronomy

More good science in HuffPo

netherlands_meteorSteve Newton of the wonderful National Center for Science Education has written another article promoting science in the Huffington Post, this time about asteroid impacts. And special bonus; he gives your loyal host here a shout-out.

Specifically, he mentions that I have said that the Hale-Bopp comet was larger than what wiped out the dinosaurs. It’s true: the object that created the Chicxulub crater off the coast of the Yucatan was something like 10 km (6 miles) across. The nucleus of Hale-Bopp was roughly 60 km (36 miles) across, meaning it would have had something like 100 times the mass of the dinosaur killer. I have vivid nightmares about asteroid impacts, and one 100x the size of the K-T extinction event is beyond scary.

Right now we lack the capability to stop such a comet impact; Hale-Bopp was discovered less than two years before it sailed by the Earth. It missed us by a huge margin, but had it been aimed at us things would look a lot different around here right now. We may be years away from being able to stop such an event, but as I’ve written before, people like Rusty Schweikart and Dan Durda are seriously considering what we can do, and have even started the B612 Foundation to look into it.

If we’re serious about such threats, were just a few years away from being able to prevent them. Given that statistically big impacts are very rare and only happen every few hundred thousand years or so, I’m rather liking where we stand right now. But that’s if we actually do something now. We need to start working on mitigation techniques, and rockets to carry them. I’m glad the B612 Foundation is working on it.

Related articles: A Pro-science article on HuffPo?

by Phil Plait at February 09, 2010 09:14 PM


Utah to be destroyed by a comet! Or not!

Utah is only one state over, so when I see a website that tells me a fragment of a comet will hit it on March 1 of this year, I sit up and take notice.

Then I see the flashing text. The multiple colors. The GIANT FONT. The URL: satansrapture.com. Well, still. It can’t be all wrong can it? And then I see the title: "BIBLE CODE PREDICTIONS 2010".

Oh. I guess it can be all wrong.

OK, Utah, you can rest easy. I’m guessing March 1 will come and go with no comet impact, fragment or otherwise. The Bible code is a long debunked piece of antiscience garbage, basically just people looking at random patterns until they find one that kinda sorta if you squint your eyes and plug up your ears and yell LALALALALALA looks like it might say something sorta correct.

Maybe.

Anyway, I wouldn’t normally link to such low-level and obvious nonsense, but no matter how silly a doomsday claim is, there will always be people out there who take it seriously. So just in case, here you go: there are no scientific predictions that a comet piece will hit Utah, and the Bible Code is total 100% fictitious nonsense.

Unless… hmmm. The Earth is hit by about 100 tons of cosmic debris every night. A lot of that is from comets, small (and I mean small) bits of fluff shed off of previous comet passes. And if you live in Utah and go out March 1, you’re sure to see at least one or two shooting stars…

So maybe that website is right!

Or not. I’m guessing not.

by Phil Plait at February 09, 2010 07:00 PM


Cormac O’Raifeartaigh - Antimatter (Life in a puzzling universe)

q15


One the great surprises about heat energy is that the transfer of heat can occur by any or all of three very different mechanisms.

In conduction, heat transfer occurs by a process of molecular collision. If you heat up one end of a bar of iron, the energy is transferred from the hot end to the cold by atoms or molecules bumping into one another i.e. while there is a net drift of energy from hot to cold, the molecules do not change their respective positions. This is the primary method of heat transfer in solids and it works best of all in metals (because loosely bound electrons play a role).  It is also an efficient method of conduction in liquids, but occurs hardly at all in gases. In gases, a low density of atoms or molecules inhibits conduction very effectively – hence air is an excellent insulator.

This fact is used to good effect in double glazing; a layer of air between two panes of glass allows one to have good light and views in a house without too much heat loss. Similarily, modern mountaineers keep warm by wearing many thin layers as the air trapped between each set of layers acts as an effective insulator.

Conduction in a solid: the molecules do not change position much

Heat transfer can also occur by the process of convection. In this case, heat energy is transferred by a movement of molecules. The classic example is hot air rising: on a hot day, air close to ground absorbs heat from the earth’s  surface, expands, and rises because it has become less dense than other air. Cooler and denser air then rushes down from above to fill its place, only to be heated in turn and a cycle is set up. As you might expect, this an important method of heat transfer in gases, and convection currents are responsible for everything from sea breezes at shore to major wind patterns around the globe.

Sea breeze close to shore on a hot day

Convection also occurs in liquids: indeed, convection currents are of great importance in the oceans of the world. For example, the seas around Ireland are warmer than might be expected for our latitude. This warming is a result of the famous North Atlantic Drift, a huge ocean current that is part of a giant conveyor belt that delivers heat from the seas off South America all the way up to the seas near Greenland. One of the concerns of global warming is that as the ice caps melt, this current may weaken or even shut down: in which case Ireland and Britain could become very cold indeed!

The North Atlanic Drift keeps Ireland’s climate mild

What is the third process of heat transfer? Well, that is a different story altogether…

by cormac at February 09, 2010 06:30 PM


Higgs - Theater Adhoc

Publish or Perish - Eerste resultaten CMS

Bij CMS zit de stemming er in. Na allerlei tegenslagen hebben wij ons eerste resultaat gepubliceerd. De analyse van de botsingen die de Large Hadron Collider eind 2009 heeft geproduceerd heeft enkele maanden geduurd, maar nu is iedereen binnen de collaboratie (dus meer dan 2500 natuurkundigen) ervan overtuigd dat de resultaten solide genoeg zijn om met de rest van de Deeltjesfysica-gemeenschap te delen. Maar wat hebben we nou precies gemeten?
(more…)

by Freya Blekman at February 09, 2010 04:17 PM


Cosmic Variance

From Eternity to Book Club: Chapters Four and Five

Welcome to this week’s installment of the From Eternity to Here book club. This week we’re tackling two chapters at once: Chapter Four, “Time is Personal,” and Chapter Five, “Time is Flexible.” That’s just because these chapters are relatively short; next time we’ll return to one chapter per week.

Excerpt:

Starting from a single event in Newtonian spacetime, we were able to define a surface of constant time that spread uniquely throughout the universe, splitting the set of all events into the past and the future (plus “simultaneous” events precisely on the surface). In relativity we can’t do that. Instead, the light cone associated with an event divides spacetime into the past of that event (events inside the past light cone), the future of that event (inside the future light cone), the light cone itself, and a bunch of points outside the light cone that are neither in the past nor the future.

It’s that last bit that really gets people. In our reflexively Newtonian way of thinking about the world, we insist that some far away event either happened in the past, the future, or at the same time as some event on our own world line. In relativity, for spacelike separated events (outside one another’s light cones), the answer is “none of the above.” We could choose to draw some surfaces that sliced through spacetime, and label them “surfaces of constant time,” if we really wanted to. That would be taking advantage of the definition of time as a coordinate on spacetime, as discussed in Chapter One. But the result reflects our personal choice, not a real feature of the universe. In relativity, the concept of “simultaneous faraway events” does not make sense.

These two chapters take on a task that is part of the responsibility of any good book on modern cosmology or gravity: explaining Einstein’s theory of relativity. Both special relativity and general relativity, hence two chapters. In retrospect they are pretty short, so an argument could be made that I should have just combined them into a single chapter.

The special challenge of these chapters is precisely that many readers — but not all — will already have read numerous other popular-level expositions of relativity. But you have to do it. Fortunately, my favorite way of talking about relativity is a little bit different from the standard one, and lines up well with the overarching goal of understanding the meaning of “time.” In particular, I try to make the point that the secret to relativity is to think locally — to compare things happening right next to each other in spacetime, not events that are widely separated. You’re allowed to compare separated events, of course, but the answers are necessarily dependent on arbitrary choices of coordinates, and that leads to endless confusion. So you won’t read a lot about “length contraction” or “time dilation,” but you will read a lot about the actual amount of time measured along a trajectory.

Unfortunately, a search for vivid examples of the maxim “freely-falling paths through spacetime experience the longest amount of proper time” led me directly to the most embarrassing mistake in the book. (At least, “most embarrassing mistake so far uncovered.”) Sordid details below the fold!

The mistake is the claim that a clock that sits stationary on a tower will experience less proper time than a clock that orbits the Earth at the same height above ground. That’s wrong: the orbiting clock will measure less time. This appears in the paragraph at the bottom of page 85 and top of 86, and is elevated from “unfortunate” to “a real doozy” by being illustrated in graphic detail by the figure on page 86. Not really any way I can claim it was just a typo.

sphere.two.geodesics The subtle issue underlying the mistake is illustrated in this figure, which shows two paths connecting two points on a sphere. Both paths are great circles. The shortest distance between two points on a sphere is a great circle; but it certainly doesn’t follow that any path following a great circle gives us the shortest distance between two points. If you go more than half the way around the sphere, you end up with a pretty long path!

The same kind of thing happens in spacetime. The trajectory of longest proper time between two events will always be a freely-falling trajectory (a geodesic). But not every freely-falling path gives us the longest time, and that’s exactly the case in this example. Given two events at the same position above the Earth, the actual path of longest time is a radial freely-falling orbit. If you want your clock to experience the longest time it can, you throw it straight up in the air to where the gravitational field is weaker (and clocks run more quickly with respect to time measured at infinity) and let it fall back down. A circular orbit actually loses time by staying at the same altitude but zipping around the Earth. I relied on my affection for the general underlying principle, and didn’t bother to sit down and work out the actual numbers in this case, so I never found the mistake. Pretty sure my membership in the general relativists’ guild is going to be permanently revoked for this one.

If you’re still not convinced of the wrongness of my example, here’s an equation, the line element along a circular trajectory in the equatorial plane in the Schwarzschild metric:

d\tau^2 = \left(1-\frac{2GM}{r}\right) dt^2 - r^2 d\phi^2\,.
On the left we have a small interval (squared) of the proper time τ, what a clock would measure along some path. The first term on the right is the contribution from our motion with respect to t, the time measured at infinity; for any given amount of t, we experience less proper time τ as our height r decreases and the coefficient (1-2GM/r) becomes smaller. The second term on the right is the contribution from our angular motion φ. Taking the square root of the whole thing and integrating along a path gives you the proper time.

We don’t have to go through the entire calculation to convince ourselves that staying stationary on the tower has a longer proper time than the circular orbit does. Both trajectories get the same contribution from the first term on the right side, while the second term is zero for the clock on the tower (it’s not moving, so =0), but it’s negative for the orbit. So the orbit is definitely less time. To be corrected in the next printing.

The deep point, of course, remains true: the time measured by clocks in general relativity depends on their path through spacetime, and the way to maximize that time is to take a freely-falling path. Just not that one.


by Sean at February 09, 2010 04:05 PM


Emily Lakdawalla - The Planetary Society Blog

Endeavour Launch Ignites Night Sky

Planetary Society volunteer Ken Kremer is reporting for us from the Kennedy Space Center, where he is covering the launch of Space Shuttle Endeavour, set to launch this weekend. Kremer is a research scientist and freelance journalist who spends his spare time giving public outreach presentations on behalf of The Planetary Society as a volunteer and NASA's Jet Propulsion Laboratory as a Solar System Ambassador. Thanks Ken! by Ken Kremer Space ....

February 09, 2010 04:03 PM


Phil Plait - Bad Astronomy

Opportunity for anaglyphs

Oh, I have a very cool anaglyph (red-green 3D images) for you! Stuart Atkinson from the Cumbrian Sky blog has created some fantastic anaglyphs of images from the Mars rover Opportunity as it investigates Concepcion crater. Here are some blocks that look like ejecta from the impact itself:

opportunity_anaglyph

[Click to embiggen.]

These are beautiful! They almost look sedimentary, which at least makes some sense given that the region Opportunity is roving, Meridiani Planum, was once under water. Closeups of those rocks show they have the famous "blueberries", concretions of jarosite formed by mineral-laden water.

Stuart has lots more pictures he’s fiddled with, too, and it’s well worth your looking around his site. You should also read Emily Lakdawalla’s great description of Concepcion, talking about how we know it’s a fresh crater about 1000 years old. It’s a fascinating read.

by Phil Plait at February 09, 2010 03:00 PM


Emily Lakdawalla - The Planetary Society Blog

A Space Carnival (#140) and some new names for Enceladus

This week the Carnival of Space is over at one of my favorite new blogs, Lights in the Dark. Actually it's not so new -- evidently this week marks its first anniversary! Also, yesterday there were a pile of new names announced for features on Enceladus: Al-Medinah Sulci, Al-Yaman Sulci, Andalús Sulci, Bulak Sulcus, Makran Sulci, Misr Sulci, Shiraz Sulcus, Sind Sulci, Bishangarh Fossae, and Kaukabán Fossae. This is great, because ....

February 09, 2010 02:39 PM


Pretty picture: Opportunity around Concepcion

Here's a neat picture from Opportunity, a panorama composed of its wide-angle, mast-mounted Navcam cameras, showing the crater Concepcion. In this one I particularly like the parallel slices cut through a low dune by Oppy's wheels as she cut over to begin her clockwise circumnavigation of the crater. In front of her are the three cubic ejecta blocks named "Chocolate Hills."Click to enlarge >Concepcion crater, Opportunity sol 2147On sol 2,147 ....

February 09, 2010 02:32 PM


ZapperZ - Physics and Physicists

Science of the Winter Olympics

With the Vancouver Winter Olympics looming, NBC Learn has launch an educational website on the Science of the Winter Olympics. It has a collection of videos discussing the various physics (mechanics) aspects of the various sports in the Winter Olympics.

The site also has a bunch of other educational items and videos suitable at almost any level.

Zz.

by ZapperZ (noreply@blogger.com) at February 09, 2010 05:09 AM


Clifford V. Johnson - Asymptotia

Cassandra Wilson

cassandra_wilsonTime for a little music with my nostalgia. I remember my days in Princeton (where I was a postdoc at the Institute for Advanced Study and, later, at the University) particularly well when it comes to certain special things, and one of them was the music I was discovering, and venturing up to New York or down to Philadelphia to see live. The wonderful Cassandra Wilson had just firmly settled into her astonishingly good Blue Note phase at that time, and the (then) newly released album "Blue Light 'Til Dawn" was pure magic to me (and remains so), and was considerably inspiring to me during that time of intense work and during a key period of career and personal development. I went to see her sing at the Theatre of the Living Arts in Philly one wonderful evening. Here she is, (from around that time, I think, or at least it has the right feel), singing the opening song from the album in a slightly shaky live recording. It is a bit [...]

by Clifford at February 09, 2010 04:22 AM


Sabine Hossenfelder - Backreaction

Why, oh why, is the Psi called Psi?

I'm currently reading Sean Carroll's book "From Eternity to Here" and stumbled over this remark
In Newtonian mechanics, the space of states is called "phase space" for reasons that are pretty mysterious.

A mystery that hadn't occurred to me before, probably because the German word "Zustandsraum" means literally "state space," so no mystery there. Stefan and I were guessing Gibbs, who introduced the word, might have generalized the terminology from the harmonic oscillator where the location in phase space does indeed tell you the phase of the oscillation. (You find a nice applet depicting the phase-space diagram of the damped and undamped oscillator here).

In any case, this caused me to ponder what other words with funny origin physicists like to use. (Both funny ha-ha, and funny peculiar.) Why, for example, is the recombination in the early universe called recombination if there was no prior combination? Not that I was the first to ask that question. Sean offered the explanation that the word is borrowed from nuclear physics. But then why don't nuclear physicists call the fragmentation refragmentation?

There are more interesting nomenclatures though than presence or absence of prefixes.

A particularly well known oddity is the name "quarks" introduced by Gell-Mann, who couldn't decide how to spell the sound ducks make:
In 1963, when I assigned the name "quark" to the fundamental constituents of the nucleon, I had the sound first, without the spelling, which could have been "kwork". Then, in one of my occasional perusals of Finnegans Wake, by James Joyce, I came across the word "quark" in the phrase "Three quarks for Muster Mark". Since "quark" (meaning, for one thing, the cry of the gull) was clearly intended to rhyme with "Mark", as well as "bark" and other such words, I had to find an excuse to pronounce it as "kwork".
~M. Gell-Mann, The Quark and the Jaguar, via Wikipedia

Had Gell-Mann read a German dictionary instead of Joyce, he'd have noticed "Quark" is the German word for a milk product (often mistakenly translated as "cottage cheese" which is something entirely different). Besides this, "Quark" is a frequently used colloquial expression for nonsense.

But at least we know how that word came along. A mystery remained to me why the English adaption of the German word "Eigenvektor" came out to be "eigenvector." The German word "eigen" simply means "innate," and could easily have been translated.

A better example fo imaginative nomenclature is the Psi-particle (now known as J/Psi) whose cloud-chamber pictures frequently have the shape of a Psi (see picture above).

Then there is the "Penguin diagram", which owes its name to a lost bet and some illegal substances, and the "tadpole diagram" which once run risk of turning into a "spermion." Probably a good thing the tadpoles kept their name - just imagine what issues the anti-abortionists would have had with spermion cancellation.

In General Relativity, we have the conjecture of "cosmic censorship" to prevent us from seeing "naked singularities," and "wormholes" are already a classic. Cosmologists have further blessed us with MACHOs and WIMPs, acronyms for MAssive Compact Halo Object and Weakly Interacting Massive Particles respectively. Loop Quantum Gravity features a LOST theorem, after the last names of its authors. The large gap between the energy scale of currently known physics and the scale where grand unification is thought to occur is also known as "desert." We have a seesaw mechanism, play with Mexican hat potentials, have ghosts and talk about stop particles. There's a swiss cheese universe and neutron stars have pasta-antipasta layers with a spaghetti-phase. The most stupid nomenclature I so far have come up with is a "pullover". Yes, I know, not terribly original, but then I didn't expect a Nobelprize for it ;-)

Did I miss something? Leave it in the comments!

by Bee (noreply@blogger.com) at February 09, 2010 02:14 AM


February 08, 2010

Emily Lakdawalla - The Planetary Society Blog

WISE has found its first comet, P/2010 B2 (WISE)

Having discovered its first asteroid on January 12, Wide-field Infrared Survey Explorer (WISE) has now officially discovered its first comet, P/2010 B2 (WISE). The comet was first observed by WISE on January 22, and has since been followed up on by Mauna Kea, Spacewatch, and by Robert Holmes, a two-time winner of a Planetary Society Shoemaker NEO Grant. Here's the Minor Planet Electronic Circular officially naming P/2010 B2 (WISE). ....

February 08, 2010 08:50 PM


Manic Monday: Chocolate Hills, Io, and NASA's budget

Although I am not suffering under the "snowpocalypse" on the East Coast, I woke up to Monday absolutely buried under a massive pile of things to do for both home and work, and it looks like it's going to take me a few days to dig out. So, with apologies, I'm going to make today's post a linky one. Firstly, today's guest on Planetary Radio is my boss Lou Friedman, giving his perspective on the Administration's plans for NASA's future. Over the ....

February 08, 2010 06:21 PM


Quantum Diaries

Strange goings on in Brazil

Around a week ago, I submitted the first paper to have me as the sole author. For someone working in such a large collaboration this is a pretty exciting moment, even if it is just proceedings :-)

Last September, I was given the incredible opportunity to attend one of the most prestigious conferences in the world of quark-related research. The Strangeness in Quark Matter conference, held every few years, gathers physicists from around the world to an exotic location to discuss our current understanding of the strange quark, and the unusual behavior of the particles it creates. In September last year it was held in Buzios, a tiny fishing village on the coast north of Rio de Janeiro.  I was invited to give a talk at the conference, and I was lucky enough to get funding for the trip as I was also giving a talk on diffraction the week before in Rio (See Strong couplings: Tales from Brazil).

photoSQM7

This was truly the most beautiful place I have ever seen (even compared to the stunning French snowy mountains I was falling down just a few weeks ago). It was also one of the strangest experiences of my life, and I am not attempting a pun. International conferences are a world unto themselves – indulgent in every sense. You feast frequently on a variety of delicious foods. You mingle with minds that are expertly extreme, taking various representations and interpretations of experimental analysis, sampling ideas and concepts from theorists from around the globe and across the field. Having never been to South America (or anywhere near as far as that) before in my life, the setting, for me, was entrancing and alien. Everywhere you looked there was a mango tree or a parasitic orchid hanging from a palm. Our buffets and breakfasts were adorned with Papaya and Guava. We were even treated to an exciting boat trip to a nearby island (nicknamed “ugly island”), and got to dive into the salty waters and snorkel!

photoSQM3

photoSQM6

Outside scheduled talk time we were constantly supplied with Caipirinas – cocktails with ice, sugar, lime and Cachaca (a spirit made from sugar-cane). In fact, after one long day, during a lively and late discussion that united the attendees with outstanding questions, drinks were brought round to encourage us to stay!

photoSQM5

The topics under discussion, (and to some extent, debate), were just as unusual. At the start of my PhD, I had only known my own limitations in understanding data, theoretical concepts or predictions. Before the conference, discussion with many theorists to help me to understand the expectations for the LHC only served to confuse and excite me more. However, as well as answering a lot of questions for me, this conference demonstrated the true nature of being at the very front end of science – right now, we know very little for certain. Ask any scientist about what the LHC and RHIC heavy ion experiments are all about, and they will very quickly start to tell you about exciting things such as the “Quark Gluon Plasma”, and evidence to suggest its properties, like “strangeness enhancement”. Try saying either one of these phrases too loudly at a conference like this, however, and expect some funny looks. The fact is, there isn’t much you can say without a little skepticism (or careful rewording) right now.

photoSQM4

One thing I know for sure is that my analysis area is not lacking in interest. Strange particle production in heavy ion collisions at RHIC, compared to pp collisions, can be explained quite powerfully by theory, but the phi resonance, which is not technically strange (made up of an s and anti-s quark) is somewhat more confusing. Asking what might happen to phi production in Pb-Pb collisions at the LHC is a tough enough question. However, begin to postulate what might occur in pp collisions with such high energy density that they become (in some ways) comparable to heavy ions, and you start to get some of those funny looks I mentioned. This was exactly what I did, and it sparked an argument between theorists of two extreme viewpoints, who eventually were asked to leave the room whilst the poor speaker continued. Of course, myself and another (very brilliant) ALICE physicist, Federico Antinori, who was keen to understand this issue, followed them out to take notes. :-)

photoSQM

The conference was full of moments like this, and I am sure many of them are. Unusual data presented by experimentalists struggling to interpret it, theorists arguing passionately about the consequences. I’d like to make a rather controversial statement that there is probably an equivalent to the “Phlogiston” phenomenon at work in much of front-line science. (If you don’t know what I am talking about, don’t just Wikipedia it, you should also watch “Chemistry: A Volatile History”, presented by Prof. Jim Al-Khalili on BBC4 Catch up TV, and hurry as you only have a few days left!) What I mean is, wherever we are dealing with the unknown, there are many contradicting ideas and some of them have to be nonsense. Unfortunately what seems like nonsense can be exactly what we are looking for. You only have to look at the history and evolution of science to see how these red herrings can take a long time to unveil, and how what looks like a ridiculous mistake (parity violation, for example!) could turn out to be a curiously perfect answer.

photoSQM2

by Zoe Louise Matthews at February 08, 2010 06:02 PM


Phil Plait - Bad Astronomy

Looks like the Sun is in its teens again

I’ve been posting sporadically on how sunspots are starting to come back to the Sun, and I’m glad to see a new group sprouted up recently… and it’s a monster:

soho_sun_feb2010

These images are from SOHO, the Solar and Heliospheric Observatory. The orange one is in visible light, and the sunspots are pretty obvious. The green one shows the Sun in the far ultraviolet, and you can see the sunspots are pretty intense, blasting out high-energy light. Sunspots are indicators of magnetic activity, and the intense magnetic field can accelerate plasma (ionized gas) to high energies.

Just so’s you know, a hundred Earths could fit across this image, so that oughta give you an idea of just how big these blemishes are.

What this means is that the Sun is becoming active again. You can see it better in this video I put together using SOHO animations. These are real SOHO observations. Note that some of the data are missing so the Sun’s rotation is a bit jerky, and that you can see that data dropouts and other problems plague these sort of observations. Oh– actually, another group popped up on the Sun earlier, too, and you can see those in the visible light data.


You can actually see the plasma flowing along the magnetic field lines in the latter part of the video.

Right now, the Sun is struggling to climb back up to the peak of its magnetic cycle, which will probably occur in 2013 or later, given how slow this has been — which you might want to keep in mind if some crackpot or scammer is trying to sell you on the idea that solar activity will destroy the Earth in 2012. When the Sun is at its peak, the magnetic field is at its strongest, and we see the most sunspots. However, the strongest solar flares and other explosive events tend not to happen until well after the cycle peaks, so it’ll be late 2013 or 2014 before we see the most vigorous activity, if the Sun holds to its previous behavior.

Again, people selling you on 2012 disasters generally have a very tenuous grasp on science. The less you know the better for them.

I expect we’ll be seeing more and more sunspots now as time goes by. It’s nice to see this happening, as it adds to the activity seen in December, and ends a long period of minimal sunspots — heck, for a long time, there were none at all. Boring. Now we can look forward to some exciting action again… just in time for SDO to launch, too!

[P.S. If anyone can tell me why the first few frames of my uploaded videos turn gray sometimes, that would be nice. I don't know whether to curse iMovie, Flash, YouTube, or all three.]

Image credit: SOHO (ESA and NASA)

by Phil Plait at February 08, 2010 03:00 PM


Higgs - Theater Adhoc

Watch ‘HIGGS’ online

The documentary ‘Higgs - Into the heart of imagination’ was broadcasted the 4th of February 2010 bij HUMAN on Dutch television. From today of on, the Dutch version of the documentary can also be watched online.

Get Microsoft Silverlight

by Redactie HUMAN at February 08, 2010 01:40 PM


ZapperZ - Physics and Physicists

The First Lasers

In light of the 50th anniversary of the laser (pun intended), the AIP has a wonderful historical account of the birth and development of the laser. This is an opportunity not only to read about how the laser came to be what it is today, but also be aware of all the important figures in its history, and not just those who got the Nobel prizes for it.

Zz.

by ZapperZ (noreply@blogger.com) at February 08, 2010 11:42 AM


New Revolutions In Particle Physics

I thought I should put this up now that we truly have a series going with the 3rd lecture now online. This series is given by Lenoard Susskind, and being produced by Stanford University. This is the brief synopsis of the lecture:

Leonard Susskind gives the first lecture of a three-quarter sequence of courses that will explore the new revolutions in particle physics. In this lecture he explores light, particles and quantum field theory.










Zz.

by ZapperZ (noreply@blogger.com) at February 08, 2010 11:34 AM


Job Outlook for Theoretical High Energy Physicists Sucks

In case you missed Peter Woit's blog entry, or Erich Poppitz latest data, you might want to read a summary of it here.

I've always stressed the issue of "employability" when students asked me about majoring in various areas of physics. While this is not an issue when students first get into grad school, it will be a huge issue when they graduate and looking for jobs. This doesn't mean that one shouldn't go into areas such as theoretical high energy physics. It means that one needs to be aware of the challenges when doing such a thing and be prepared for not finding the job that one wants. There has to be a backup plan in anything, and more so when one is specializing in an area with very limited employability.

Zz.

by ZapperZ (noreply@blogger.com) at February 08, 2010 11:20 AM


Quantum Diaries

L’invasion des réseaux de neurones

Si je vous dis réseau de neurones, vous pensez certainement au cerveau, ou même si vous avez suivi des cours de biologie vous pensez aux synapses, dendrites etc… Mais ce n’est pas là où je veux vous amener. Pour le moment.
Vous êtes vous déjà demandé comment était lu le code postal sur les enveloppes, ou encore comment le filtre anti-spam de votre messagerie préférée faisait pour stopper les mails indésirables ? Tout ceci demande une capacité à effectuer une décision reliée à un processus statistique. En effet, 2 personnes n’écriront jamais le même chiffre de la même manière et deux spams ne contiendront pas exactement les mêmes mots. Nous nous retrouvons face un ensemble d’éléments potentiellement infini tous différents les uns des autres et qui pourtant peuvent se regrouper en un nombre restreint de groupes de même caractéristique (ce caractère est un 3 ou encore ce mail est un spam…).

C’est dans cet objectif de tri que sont utilisés ce qu’on appelle des algorithmes d’apprentissage, dont font partie les réseaux de neurones artificiels. Ceux-ci vont être capable d’apprendre à identifier une certaine caractéristique dans un échantillon qui lui est soumis.

Architecture d'un réseau de neurones

Architecture d'un réseau de neurones

Les réseaux de neurones sont basés sur un modèle simplifié du neurone biologique, ils se composent généralement de neurones d’entrée, puis une couche dite cachée enfin une couche de sortie (voir schéma). Le tout reliés par des synapses. En entrée sont donnés les différents critères utiles au tri (par exemple l’occurrence de certains mots pour l’identification de spams), la sortie est la réponse du réseau (c’est plutôt un spam ou non).
Mathématiquement le principe repose sur le fait que n’importe quelle fonction peut être approximée par une combinaison linéaire de fonctions d’activation ( sigmoïde, tangente hyperbolique ou fonction de Heaviside ). Ainsi chaque neurone se trouve doté de cette fonction et chaque lien entre les neurones (synapse) est pondéré suivant le problème à résoudre.

Un tel réseau est à la base parfaitement stupide, il ne sait rien faire à part un traitement purement aléatoire de l’information. Comme quand vous voulez apprendre à faire quelque chose, il va falloir s’entraîner!
Durant cette étape nous allons soumettre à notre algorithme un échantillon de caractéristiques connues à trier. On pourra ainsi comparer la réponse du réseau à la réponse correcte. Sachant cela, nous pourrons améliorer le résultat en modifiant les poids synaptiques. Après plusieurs essais, le réseau de neurones aura une sortie proche de celle attendue et sera désormais prêt à utiliser ses capacités sur un échantillon quelconque.
L’analogie avec l’apprentissage humain est très fort : imaginez que je doive apprendre à quelqu’un à reconnaître une souris d’ordinateur. Je vais lui présenter plusieurs objets en lui disant à chaque fois si c’est une souris. Si je lui montre un nombre important de souris (diverses et variées), il va au final réussir à repérer les caractéristiques pertinentes et va pouvoir en extrapoler un «concept souris». Après la phase d’apprentissage, la comparaison à ce concept général sera utilise à chaque fois qu’il devra reconnaître une souris :
«Ah d’accord… Une souris est plus ou moins ovale, possède deux boutons et parfois un bouton au milieu, et elle est souvent raccordée par un fil etc…  Donc si je vois toutes ces caractéristiques sur un objet, j’aurai de bonnes chances de présumer que c’est une souris d’ordinateur».

Très bien, mais je suis un peu loin de la physique des particules ici n’est-ce pas? Alors revenons-y.
En physique des particules, le principe critique est de pouvoir discerner un phénomène bien particulier (le signal) au milieu des millions de collisions amenant à des phénomènes qui ne nous intéresse pas (le bruit de fond). Autrement dit, trouver l’aiguille dans la botte de foin… La théorie physique sous-jacente aux phénomènes observés dans les collisionneurs de particules étant la mécanique quantique, nous ne pouvons jamais avec certitude connaître l’issue d’une collision en particulier. Nous ne pouvons donner que les probabilités.
La méthode première pour augmenter nos chances est d’effectuer des «coupures» : je ne regarde que ce qui a une énergie supérieure à un tel seuil ou encore je ne prends que ce qui a été détecté dans une certaine partie du détecteur etc… Car je sais que c’est dans ces cas que j’ai le plus de probabilités de trouver mon bonheur.
C’est exactement ce que va faire un réseau de neurone, mais de manière optimisée, il va, de part son entraînement, apprendre à ne sélectionner que les évènements possédant les caractéristiques qui ont le plus de chance d’être du signal et rejeter tout ce qui a de fortes chances d’être du bruit de fond.
Le sujet de ma thèse est justement de mettre en évidence un phénomène particulier qui fait intervenir le boson de Higgs et de par la même découvrir (ou exclure) son existence. Il faut savoir que ce phénomène a une probabilité extrêmement faible de survenir, il est donc crucial de pouvoir trier ces évènements. C’est pour cela que je travail à l’aide de réseaux de neurones adaptés à la reconnaissance de ce phénomène.

Akinator, une application internet capable de deviner à quoi vous pensez grâce a un algorithme d'apprentissage.

Akinator, une application internet capable de deviner à quoi vous pensez grâce a un algorithme d'apprentissage.

L’intérêt pour les réseaux de neurones et les algorithmes d’apprentissage en général n’a cessé de croître ces 20 dernières années et sont couramment utilisés dans des domaines aussi variés que les milieux financiers (prédiction des fluctuations de marches), dans le domaine bancaire (pour déceler les fraudes aux cartes de crédit), en aéronautique (pilotes automatiques), en intelligence artificielle etc… Même certaines applications internet se vantant de pouvoir lire dans vos pensées ont vu le jour sur la toile comme 20q ou encore Akinator et utilisent ces algorithmes.

Nous pouvons voir que ces nouvelles techniques d’analyse ont un bel avenir devant eux. Au delà des applications sans cesse plus nombreuses, celles-ci s’améliorent de jour en jour grâce au travail des chercheurs et deviennent ainsi plus puissantes, plus rapides et plus précises. Mais comme nous l’avons vu, malgré le mot neurone, nous sommes encore bien loin d’un cerveau humain. Alors avant d’imaginer une invasion de robot tueurs, sachez bien que Terminator sait pour le moment à peine lire et que c’est déjà pas mal!

by Florian at February 08, 2010 10:29 AM


arXiv blog

The Future of Gaming: The Hot Potato Experience

The next generation of pervasive games are beginning to appear.

It's no secret that the Nintendo Wii has changed the landscape for gaming. The question is what comes next.

Sony has been developing camera-based games for some time and Microsoft has a similar system in the works. But there are plenty of people who think that neither of these approaches will be game-changers (ahem).

Instead, the most exciting developments are coming from the world of mobile phones or other sensor networks where engineers are testing a new generation of games that can be played anywhere there is a mobile phone or wireless network. These games are location aware, involve multiple players, rapid physical activity and Wii-like gesturing.

So-called pervasive games generate an entirely new set of challenges--and not just for the people who play them. They must work with multiple types of input-an iPhone must be able to play against a Nexus One. They involve many players communicating rapidly, so these devices need to synchronise with each other.

To test how such games might work and the problems they generate, Ioannis Chatzigiannakis and a few mates at the University of Patras in Greece created one called Hot Potato. The game works like this:

A hot potato is a kind of virtual timer that is passed between mobile devices that players hold. During the game, the hot potato counts down to zero when it explodes. When this happens, the person holding it is out. The game repeats until only one player is left. Players can "throw" the hot potato to another player by moving close and making a throwing action with their arm (while holding their device).

Moving too far from the other players increases the chances that a new hot potato will be generated on your device. That keeps the players together. Until one of them receives a hot potato, in which case it pays to move away from them so they can't throw it to you.

Chatzigiannakis and company created the game using Sun's Spot sensor network device, which has an 180MHz ARM 9 processor with 512KB of RAM and 4MB Flash. It is IEEE 802.15.4 compliant and relies on a CC2420 Chipcon transceiver for communication. The device has two buttons, eight LEDs and a number of sensors such as an accelerometer, a thermistor, and a light sensor.

Today, the team reveal the results of tests with the game including user surveys. And it looks as if it works well. They say that players reacted very positively to the game.

As for the limits of the gaming experience, they say: "Up to 14 players can participate in a game session simultaneously in a completely distributed environment; above this limit, inherent technology factors come into play and prevent a seamless gaming experience."

Hot Potato isn't the first pervasive game, by any means. But others such as CatchBob!, SupaFly, and Human Pacman have had limited impact. This is an area that is waiting for its breakthrough game.

Hot Potato sounds like fun and a potential money spinner too. But now comes the difficult part: the final beta testing to take any rough edges off the product and then the sales and marketing.

iPhone app, anybody?

Ref: arxiv.org/abs/1002.1099: The "Hot Potato" Case: Challenges in Multiplayer Pervasive Games Based on Ad Hoc Mobile Sensor Networks and the Experimental Evaluation of a Prototype Game



February 08, 2010 05:00 AM


US/LHC Blogs

How much data, how soon?

First off, we should mention here that CMS’s first paper from collision data has now been accepted for publication by the Journal of High Energy Physics. It’s a measurement of the angular distribution and momentum spectrum of charged particles produced in proton collisions at 0.9 and 2.36 TeV, using about 50,000 collision events recorded in December. It is really wonderful that this result could be turned around so quickly! The first of many papers to come, we hope.

Meanwhile, as already mentioned here, we now have the news of the run plan for the LHC. CERN is preparing for the longest continuous accelerator run of its history, 18 to 24 months. The inverse femtobarn of data to be recorded in that time is a lot, and will give us an opportunity to make many interesting measurements. Whether any of them will be evidence of new physics, I for one am not going to speculate! But if nothing else, this plan sets out what our LHC life for the next ~three years is going to look like.

But a shorter-term question comes to mind — 1 fb-1 over 18 to 24 months is one thing. But what about just the next few months? There is a major international conference coming up in July. What sort of LHC results might be ready by then? That will depend in part on how many collisions are delivered. I’ve seen various estimates for that, but they vary by an order of magnitude depending on the level of optimism, so I’d rather not guess. It will also depend on the experiments’ performance. How efficiently can we record those collisions? How quickly can we process them? How soon will we understand various parts of the detectors well enough to make quality measurements? How smart and clever can we be throughout the entire process? How much sleep is everyone going to get?

Ask me again in July. Meanwhile, game on.

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by Ken Bloom at February 08, 2010 03:38 AM


February 07, 2010

Quantum Diaries

Heat to kill the pain

A sliver of sunlight on the next mountain, amindst clouds and snowfall.

A sliver of sunlight on the next mountain, amidst clouds and snowfall.

I’ve been a bit slow with blogging lately… And the reason is not a lack of things that are going on, far from that. Things got even more busy because of a long-planned week of skiing, and all the things I had to finish before then. Now, teaching is over for this semester, and since yesterday around noon, we are in a small mountain village in south western Austria.

Over the last few days there has been quite a bit of fresh snow, good for the slopes, but bad for visibility, especially since the clouds this morning were right at the altitude of the ski resort. After lunch, we saw a first sliver of sunlight, and the day ended with sunshine. The snow was great to ski on, but of course in the middle of a cloud I went into a little depression a bit to fast without seeing it, and jolted my back. But the sauna in our hotel hopefully helped to loosen the muscles again… Nothing like baking for a while to kill the pain of a long day of skiing.

I hope that over the next few days I’ll also have the time to write a bit about things that have been going on lately: A submitted paper, meeting in Paris, maybe more…. But no promises, skiing comes first!

The tool to soothe sore muscles: The sauna in our hotel in the ski resort.

The tool to soothe sore muscles: The sauna in our hotel in the ski resort.

by Frank Simon at February 07, 2010 09:40 PM


Clifford V. Johnson - Asymptotia

Bad, but ever so Good

bakes_squidThe other day I had a moment of nostalgia and made some of what we called bakes when I was a child, growing up (for some years) in the Caribbean. Bakes are known as Johnny cakes in the US, as far as I understand, and used in much the same ways that we used them. This is certainly not something you should have every day, since they involve fat (vegetable shortening, or lard as we called it, although elsewhere the term is used for a kind of pig fat), flour, salt, and a pan half full of oil to deep fry it all in. Definitely sinful. I have very happy memories of having bakes with tasty oily fishy goodness of some sort. Salt fish (salt dried cod) would be a typical thing (bacalao as the Portuguese and [...]

by Clifford at February 07, 2010 06:23 PM


Tommaso Dorigo - Scientificblogging

And CMS, In The Meantime...

Earlier today I reported about the publication of a paper by a non-professional physicist, Carl Brannen. Now I have to do the same for a paper -the first one in a long and groundbreaking series, you can bet- from the CMS collaboration, one of the two main experiments at the CERN Large Hadron Collider.


read more

by dorigo at February 07, 2010 04:23 PM


Phil Plait - Bad Astronomy

SDO launches on February 9

sdoThe Solar Dynamics Observatory, due for launch on February 9 at 10:30 Eastern time (15:30 GMT), is a revolution in solar observing: equipped with state-of-the art detectors, it’ll stare at the Sun and teach us far more about our closest star than we’ve ever had a chance to before. It’s like SOHO on steroids.

I was going to write up a lengthy post about it, but then I found out my friend Nicole Gravitationaliotta, aka The Noisy Astronomer, already put together a great post about it. That saves me time.

Something I want to point out: SDO will have a continuous science data streamrate of a whopping 16 megabytes per second. You might want to read that again. That’s 1.4 terabytes per day, or half a petabyte per year. Given that a Blu-Ray disk holds 50 gigabytes at most, that means SDO would fill 28 disks a day just to store that data. Cripes. That’s a vast amount of data to sift through. If the Sun is hiding anything, it has about a week to figure out what to do. After that we’ll be watching everything it does.

barbara_thompsonAlso, a fun thing about this for me is that the project scientist for SDO is Barbara Thompson, a woman I’ve known a long, long time: her office was across from mine when I was working on Hubble, and I would often drop by to swap stories with her and generally mix it up. It’s very cool to know that an old friend will be helping run such a fantastic astronomical instrument.

by Phil Plait at February 07, 2010 04:17 PM


Tommaso Dorigo - Scientificblogging

When Amateurs Get Published

This just in: Carl Brannen (here his blog) got a paper on gravitation published in a scientific magazine. Carl, who is the typical amateur who many "established scientists" in the blogosphere have labeled a crackpot in the last few years, does not actually fit the bill very well: he is a deep thinker who knows the literature of what he studies, and the fact that he is not salaried by a research institute means as little as this: he does it for Science, and not for a pay.


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by dorigo at February 07, 2010 08:47 AM


Sabine Hossenfelder - Backreaction

Black Holes and Information Loss

Here is - finally! - the continuation of my previous posts on Causal Diagrams and The Causal Diagram of the Black Hole. Due to popular demand, this time we will discuss the black hole information loss paradox. I previously wrote about this topic here, where I also listed the most common solution attempts. I am not going to repeat this list of solution attempts, so please refer to the older post for that. I want to focus here instead on the causal diagram.

Preliminaries

Last time, we finally arrived at the diagram of the evaporating black hole:

More precisely, it's a non-rotating uncharged black hole.

The most important features of this spacetime are that it has a (spacelike) singularity and an event horizon. The blue line indicates the surface of some collapsing matter configuration [1]. Let me remind you that since we've chosen radial coordinates, curves that pass through r=0 (where it is non-singular) look like they are reflected back. These segments of curves are also referred to as in- and outgoing in an obvious terminology.

Shown in the figure is v0, the last ray of light that passes through the collapsing matter and still manages to escape [2]. In the background depicted in the diagram, particle creation takes place at the horizon, which causes the black hole to lose mass. It then shrinks until it has finally completely evaporated, leaving behind nothing but thermal Hawking radiation [3].

Another important fact is that this spacetime is "asymptotically flat" or "asymptotically Minkowski," which means that at an infinite distance from the black hole spacetime is flat (flat as in "the curvature tensor vanishes"). This doesn't necessarily have to be the case (i.e. it could be asymptotically AdS instead), but it will make our discussion leaner. The reason for this asymptotic flatness is simply that in the beginning as well as in the end the matter is arbitrarily thinly dispersed.

To wrap up the summary, note that this diagram depicts a highly idealized situation. It's an evaporating black hole in an otherwise entirely empty spacetime. Realistic black holes are surrounded by matter and accrete mass, and occasionally Bob sends one of his Alices behind the horizon. But, as so often in physics, the uncluttered idealized version will help us understand the situation better without spoiling the conclusions.

Evolution

To understand the black hole information loss problem you need one further ingredient, that's what physicists mean with time-evolution. Intuitively, it means that one specifies a system at one moment in time, known as "initial conditions" and from this determines the status of that system at any other time by the help of a differential equation [4]. The most basic example is throwing a ball. The initial conditions needed are the location and velocity at one moment. The equation you use is Newton's law (or something equivalent).

In General Relativity the situation is more complicated but conceptually similar. You specify the initial conditions of your matter configuration at one moment in time and use Einstein's field equations to determine what space-time and matter are doing at any other time [5]. The attentive reader might remark that already in Special Relativity "one moment in time" is ambiguous. Indeed, and this is also the case in General Relativity. Point is, you can use any "moment in time" for you initial conditions, as long as it's at one moment, but everywhere in space (this is not the only option, but the most commonly used one). We call that a "complete spacelike hypersurface." Complete means basically it doesn't have holes and no expandable boundaries.

Almost there now. In the below picture I've added two complete spacelike hypersurfaces denoted Σ1 and Σ2


Information Loss

The evolution of a quantum mechanical state is unitary. That means in particular it is time-reversible [6]. You can evolve the status of your system back and forth how you like. There are many ways to think about information, and when talking about the black hole evolution some people like to hang themselves up on the exact meaning of information. That's a very interesting topic, but we'll cut this discussion short because it's irrelevant to understand the problem. Consider you have an initial state and you evolve it into a final state. If your final state does not uniquely specify the initial state we'll consider this loss of information. It means you can't tell what happened.

Black hole evaporation causes a loss of information because the outgoing radiation depends only on the total mass. Once the black hole is evaporated, all states with the same initial mass are converted into the same endstate. There are many ways a system can be composed if you only know the total mass [7]. There's only one way it will look after evaporation. This process is thus not reversible: it is not possible to reconstruct the initial state from the final state. But if it's not reversible, it can't be unitary. And for beginners that's the problem: The formation and complete evaporation of the black hole seems to be incompatible with quantum mechanics. On the advanced level it's more complicated since we know the computation leading to Hawking radiation breaks down when quantum gravity becomes important. In this case the problem is that this quantum gravitational contribution doesn't help you to get enough information out.

There are several points that people tend to misunderstand about the problem already on the beginner's level, so let me mention some pitfalls. First, note that the problem is not that the information is inaccessible behind a horizon. There is no horizon in the endstate, look at the diagram. It's flat Minkowski space with infinitely thinly dispersed thermal radiation. Think of the black hole as a black box. You start with flat Minkowski space, something happens in between, you end with flat Minkowski space. Yet, this evolution cannot be described by quantum mechanics as we know it. Second, to lay out the problem I didn't have to refer to measurement at all. It's a fundamental incompatibility in the evolution, you don't solve that incompatibility by waving your hands and yelling "measurement problem." Third, we are talking about the microscopic laws. Yes, on macroscopic scales we do have an arrow of time and entropy tends to increase anyway, but the problem is to accommodate the black hole evolution with the fundamentals of quantum mechanics prior to coarse graining. Fourth, yes, it is possible to cover the the Schwarzschild geometry by what is known as "nice slices," hypersurfaces that avoid the singularity for any finite time. (You find some very good graphics for that here, on slide 10). That doesn't solve the problem either because no matter how you turn it, your black hole evaporates away and you'll finally have to face that all you have left at scri minus is thermal radiation.

If you want to argue that the problem is a thought-experiment and unobservable, please read my earlier post on Thoughts and Experiments. We have to pay attention to inconsistencies even if they are not observable since they document a gap in our knowledge. While troubelsome, they also offer us opportunities to improve our understanding of Nature, which is why physicists turn problems like this upside-down and inside-out.

The value of the causal diagram once again is that it captures a lot of physics in one simple picture. If you look at it one more time you can see the problem. At the singularity matter gets crushed to infinite density and absent non-local effects everything that crossed the horizon has to fall into the singularity. Recall that curves on 45° angles depict the trajectories light travels on. You'd have to be faster than light to avoid the singularity once you've passed the horizon. All information about the initial state that evolves into the singularity is thus not available on the final slice. And that's exactly what happens in the calculation. You have to finally let go of the part of the initial wave-function that vanished behind the horizon, because it cannot avoid the singularity.

Now what

This then opens the playground for solutions to the problem. You either have to get the information out before it hits the singularity or avoid that it crosses the horizon at all. Lee and I argued in our last year's paper (see previous post for details) that the easiest way to avoid hitting the singularity is if there is no singularity. This by itself doesn't mean information behind the horizon becomes accessible again for the observer outside the horizon. But if you recall, this wasn't the problem to begin with. The problem was to achieve compatibility with unitary evolution, and this doesn't require information to be accessible to everybody as long as it exists.

In any case, since the black evaporation is and will likely remain elusive to experiment, everybody has their favorite solution. String theorists like the idea that information never gets lost because the evolution of the black hole is equivalently described by a dual, unitary, theory formulated on the boundary of the space-time which has been shown to encode regions of the bulk both inside and outside the horizon. People working on other approaches to quantum gravity seem to favor the idea that the singularity is avoided and the information somehow makes it out of the horizon, though at least to me it's remained unclear how so. (I sometimes suspect they'll finally reinvent and adopt the string theory solution.) Scenarios with stable or quasi-stable remnants that keep information or slowly release it also occasionally reoccur, and then there's parallel- and baby universes and a long list of miscellaneous other. The idea that black holes can't be formed to begin with lies in a shadowy fringe-area and is not considered plausible by the vast majority of researchers in the field.

I personally am somewhat agnostic on the how of information release, but am certain it can eventually only be achieved if the singularity is avoided (in the sense explained in mentioned paper.)

So. *wiping sweat off forehead* If you still haven't enough let me know.



[1] Modulo the question where it hits the singularity, see comments to previous post, but that's not relevant for our purposes.
[2] To be more precise, since we have assumed spherical symmetry to be able to draw a 4 dimensional manifold, a point in the figure is actually a sphere, but this distinction isn't so relevant. One can decompose the solutions to the wave-equation in spherical harmonics as usual. We are then talking here only about the s-wave state. States with higher angular momentum have a more complicated behavior.
[3] In the upheaval around the alleged risk of black holes at the LHC, some people ridiculed the fact that Hawking's calculation does not "automatically" decrease the mass of the black hole but that energy conservation is "put in by hand." That is in fact true. But that in this calculation the radiation does not "automatically" carry away the mass of the black hole is an artifact of doing the analysis in a fixed background, which "by hand" prohibits the mass from changing. There is absolutely nothing wrong with the argument that taking into account the energy loss through radiation the mass is not in fact constant. This in turn does not render the calculation false, it merely sets limits to its accuracy, and Hawking's calculation can be shown to be an excellent approximation as long as the ratio of mass loss is small. It is only in the end stage of evaporation when quantum gravity is important that the mass loss becomes relevant for the properties of the emitted radiation. This phase is thus still a matter of discussion.
[4] Note that it is entirely irrelevant the "initial" conditions are indeed the beginning of the evolution from which you determine the past. You could equally well specify the state of your system in the future and evolve it into the past.
[5] Note that this means once you've specified an equation of state for the matter, General Relativity does not allow you to specify what you want the matter to do over the course of time.
[6] The reverse is not true. A reversible evolution is in general not also unitary.
[7] Even if it's spherically symmetric. You lose all information in the radial direction.


by Bee (noreply@blogger.com) at February 07, 2010 07:22 AM


ZapperZ - Physics and Physicists

Steven Chu Plays "Not My Job"

If you haven't listened to this, you ought to, because it is hysterical. Steven Chu plays "Not My Job" on NPR's "Wait, wait, don't tell me".

You also get to learn a little bit about Steven Chu, but more than anything, this is rather entertaining and utterly funny. Too bad he didn't do as well as George Smoot on "Are You Smarter Than a 5th Grader?" game show, but this is to be expected since the questions were "not his job"! :)

Zz.

by ZapperZ (noreply@blogger.com) at February 07, 2010 06:05 AM


arXiv blog

The Mobile Phone Conundrum: If I Call You, Will You Call Back?

The study of reciprocity between mobile phone users reveals surprising insights about the flow of information in society.

What do your mobile phone habits say about you? Probably more than you might imagine.

At least, that's the suggestion from Lauri Kovanen and pals at the Aalto University School of Science and Technology, Finland. These guys have studied the 350 million calls made by 5.3 million customers over an unnamed mobile phone network during a period of 18 weeks. The primary question they ask is whether mobile phone calls are mutually reciprocated: in other words, does somebody who calls another individual receive in return as many calls as he or she makes, a phenomenon known as reciprocity.

Mobile phone calls are a particularly good way to study reciprocity because they are directed in a way that sms messages and email are not. In a mobile phone call, the caller initiates the conversation and then both parties invest a certain amount of time in the event. But afterwards there is usually no immediate reason for the recipient to call back. So it's clear who initiated the event.

But SMS messages or e-mails are entirely different: here a conversation usually means sending a sequence of reciprocated messages and this makes it much more difficult to study reciprocity by simply counting the number of messages.

This has allowed Kovanen and company to unearth a number of interesting phenomena. For a start, the calling patterns of prepaid users is very different from those with a contract who pay later. Postpaid users tend to be more prolific, having on average 5.41 people they call.

Prepaid users, by contrast, have only 3.41 contacts on average (although the notion of "average" is a little strange here since there is a very long tail on these distributions).

Not only that but postpaid users make 10 times as many calls as prepaid users. "We can also see that prepaid users receive more calls than they make, while the most active postpaid users make more calls than they receive," says Kovanen and company.

Prepaid users are also have more skewed relationships. Among prepaid users, the relationships where one participant makes more than 80 percent of all calls make up over 25 percent of the total.

The figures for postpaid users are far less skewed but they are greater than you'd expect from an ordinary probabilistic distribution in which each party in a relationship was just as likely to call the other.

So what's the difference between prepaid and postpaid callers? One of the most important is probably that prepaid users are much more likely to be young people. And sociologists already know that relationships between young people tend not to be equally reciprocated.

A few years ago, the National Longitudinal Study of Adolescent Health asked US students to name up to five of their best friends. Between them, the students named 7,000 individuals but only 35 percent of the nominations were reciprocated. So perhaps it's not suprising that a similar picture emerges from the study of mobile phone calls.

More puzzling is the skew in reciprocity in postpaid users which may not be as significant as for prepaid users but is still worthy of note.

What Kovanen and co are uncovering may be some fundamental property of human relationships; only more study will reveal that.

But the work is important for another reason: the skewed reciprocity between mobile phone users may influence other things such as the spread of ideas and information in society or, just as likely, the spread of viruses.

And that could have important implications for the way antivirus efforts are organised and directed.

Ref: arxiv.org/abs/1002.0763: Reciprocity of Mobile Phone Calls




February 07, 2010 05:00 AM


February 06, 2010

Clifford V. Johnson - Asymptotia

Categorically Not! - Grand Challenges!

So yes, the Categorically Not! series was a bit thin on the ground in the last several months. I think KC was a bit busy travelling to tell people about her Frank Oppenheimer book. Well, it is back on the calendar, and I probably should have mentioned it earlier, but the next one is tomorrow, so I thought I'd remind you. Remember that the series of events is held at the Santa Monica Art Studios, (with occasional exceptions). It's a series - started and run by science writer K. C. Cole - of fun and informative conversations deliberately ignoring the traditional boundaries between art, science, humanities, and other subjects. I strongly encourage you to come to them if you're in the area. Here is the website that describes past ones, and upcoming ones. See also the links at the end of the post for some announcements and descriptions (and even video) of previous events. The theme this month is Grand Challenges!. Here's the description from K. C. Cole: [...]

by Clifford at February 06, 2010 07:28 PM


Jon Butterworth - Life and Physics

batman gets it right


Somewhere in HM Treasury or thereabouts, 2007.

Batman makes the right call.

The STFC Proposal

(Thanks to the Batman Comic generator)

by Jon Butterworth at February 06, 2010 05:31 PM


Quantum Diaries

Nishina center.

The theoretical physics laboratory of Riken, to which I belong to, is a part of Nishina accelerator center. The center has a huge accelerator which is specialized for “RIBF”, RI beam factory, with a world-”strongest” superconducting ring cyclotron. You can find movies on how isotopes are accelerated in the beam lines, at the webpage of the Nishina center. My research is on superstring theory but I am currently applying string theory technique to nuclear physics, so this Nishina center is a perfect place for me to get in touch with real nuclear experiments. SRCblack800

Last week, hosted by the director of Nishina center, we had a big party, with alcohol drinks and cakes. This was a get-together party, which the director aimed to have all of the center members to know each other. I eventually enjoyed this party since, as the director aimed, I have met one person who is a visiting experimentalist working in Italy. Her experiments sound very interesting to me, and in fact quite much related to my recent work on strange physics. We talked at the party, and we made a promise that we would get together sometime soon. However, to tell you the truth, I haven’t expected much on this promise, as this was at a party and we have met just for the first time, and I am just a string theorist who should look apparently “different” from nuclear experimentalists. However, on the next day, in the morning, she came to my room! — and we had a good discussion. It was amazing to me that, just at this get-together party, I happend to see an interesting person and could talk really on my project, although she came from the other side of the globe. I thank Nishina center, and the director En’yo.

I hope to report on the progress of my research, on the application of superstring theory to nuclear physics, here. As for my current project, my Mathematica says “I need more memory”…. well, I’ll try to write a new and beautiful code which may cost less memory, hopefully.

by Koji Hashimoto at February 06, 2010 12:36 PM


Emily Lakdawalla - The Planetary Society Blog

That's a lot of motion for a "stuck" rover!

The Jet Propulsion Laboratory posted a video to YouTube today showing what seems to be a remarkable amount of motion out of Spirit lately, especially given that she's supposed to be a "stationary science platform" now. The video consists animations from Spirit's forward and rear Hazard Avoidance Cameras -- the belly-mounted, fish-eye cameras that help Spirit understand the terrain immediately in front of and behind it. Spirit's Last Moves ....

February 06, 2010 12:14 PM


February 05, 2010

Tommaso Dorigo - Scientificblogging

2000 Years Ago Cicero Knew It, Do You ?

"Quidquid oritur, qualecumque est, causam habet a natura. Cum autem res nova et admirabilis fieri videtur, causam invetigato, si poteris, ratione confisus. Si nullam causam reperis, illud tamen certum habeto, nihil fieri potuisse sine causa naturali. Repelle igitur terrorem quem
res nova tibi attulit et semper verbis sapientium confidere aude:
sapiens enim facta, quae prodigiosa videntur , numquam fortuito
evenisse dicet, quod nihil fieri sine causa potest, nec quicquam fit
quod fieri non potest: nulla igitur portenta sunt. Nam si portentum
putare debemus id quod raro fit, sapientem esse portentum est: facilius
esse enim mulam parere arbitror quam sapientem esse."

Marcus Tullius Cicero

Quick and dirty translation:


read more

by dorigo at February 05, 2010 10:46 PM


Cosmic Variance

Quantum Photosynthesis

This is an idea that has been bouncing around for a while, but is now apparently seen in experiments: real-world photosynthesis taking advantage of quantum mechanics. (Story in Wired, via @symmetrymag. Here’s the Nature paper on which it’s all based.)

pc645-view4 The idea is both simple and awesome: you want to transport energy through an “antenna protein” in a plant cell to the “reaction-center proteins” where it is chemically converted into something useful for the rest of the plant. Obviously you’d like to transport that energy in the most efficient way possible, but you’re in a warm and wet environment where losses are to be expected. But the plants somehow manage the nearly impossible, of sending the energy with nearly perfect efficiency through the judicious use of quantum mechanics.

We can think about this in terms of Feynman’s way of talking about quantum mechanics: rather than a particle taking a unique path between two points, as in classical mechanics, a quantum particle takes every possible path, with simple paths getting a bit more weight than complicated ones. In the case of the protein, different paths for the energy might be more or less efficient at any particular moment, but this bit of quantum trickery allows the energy to find the best possible route at any one time. Imagine at rush hour, if your car could take every possible route from your home to the office, and the time it officially took would be whatever turned out to be the shortest path. How awesome would that be?

The reason you can’t do that is that your car is a giant macroscopic object that can’t really be in two places at once, even though the world is governed by quantum mechanics at a deep level. And the reason for that is decoherence — even if you tried to put your car into a superposition of “take the freeway” and “take the local roads,” it is constantly interacting with the outside world, which “collapses the wave function” and keeps your car looking extremely classical.

Proteins in plants aren’t as big as cars, but they’re still made of a very large number of atoms, and they’re constantly bumping into other molecules around them. That’s why it’s amazing that they can actually maintain quantum coherence long enough to pull off this energy-transport trick. Previous studies had hinted at the possibility, but only by cooling the proteins down and shielding them from external jiggling. This new work happens at room temperature in the context of marine algae, so it seems to indicate that it can happen in real environments.

One step closer to building my teleportation machine. Get to work, quantum engineers!


by Sean at February 05, 2010 04:22 PM


Tommaso Dorigo - Scientificblogging

Tevatron Higgs Searches: Past And Future

To see the future, you must know the past: these nine words nicely summarize a syllogism which knows few exceptions. Turning to known data to check the power of one's extrapolations is a quite well-founded scientific approach. So if we are to try and guesstimate how much will the CDF and DZERO experiments manage to deliver in the next few years, we must check how well they delivered this far, by comparing results with early expectations.

But why bother ? Well, of course because there is a real challenge on: bookmakers need to tune the odds they offer!

Fermilab versus CERN


read more

by dorigo at February 05, 2010 12:58 PM


ZapperZ - Physics and Physicists

Teaching and Understanding of Quantum Interpretations in Modern Physics Courses

We all know that teachers and instructors can have a profound influence in the way a student not only understands a subject, but also how he/she thinks about how to view something. This notion is further reinforced by this study on how students interpret the complexity of quantum mechanics. {Note that you can get full access to the paper}

Abstract: Just as expert physicists vary in their personal stances on interpretation in quantum mechanics, instructors vary on whether and how to teach interpretations of quantum phenomena in introductory modern physics courses. In this paper, we document variations in instructional approaches with respect to interpretation in two similar modern physics courses recently taught at the University of Colorado, and examine associated impacts on student perspectives regarding quantum physics. We find students are more likely to prefer realist interpretations of quantum-mechanical systems when instructors are less explicit in addressing student ontologies. We also observe contextual variations in student beliefs about quantum systems, indicating that instructors who choose to address questions of ontology in quantum mechanics should do so explicitly across a range of topics.

It appears that if the students were left to their own devices, i.e. the instructor made no explicit interpretation, they tended to adopt a realist view of QM.

I've yet to read this paper fully and more closely (hey, I have a lot of things to do this week! :)). But you're welcome to it. If you are, or have been, an instructor in a similar class, I would be very interested to hear what you have to say, and especially on your approach for such a class.

Zz.

by ZapperZ (noreply@blogger.com) at February 05, 2010 11:01 AM


AAAS 2010 Annual Meeting

Hey, is anyone reading this will be attending the upcoming AAAS Annual Meeting in San Diego? If you are, I would be so grateful if you could contact me. You may e-mail me, or if you don't have my e-mail address, please leave a comment to this blog with your contact info. Don't worry, this blog is moderated, and your comment will NOT appear in public (I PROMISE!), so your contact info will be kept private and will be deleted.

Thanks a bunch!

Zz.

by ZapperZ (noreply@blogger.com) at February 05, 2010 10:14 AM


arXiv blog

Doped Graphane Should Superconduct at 90K

New calculations reveal that p-doped graphane should superconduct at 90K, making possible an entirely new generation of devices cooled by liquid nitrogen.

There's a problem with high temperature superconductors. It's now more than two decades since the discovery that certain copper oxides can superconduct at temperatures above 30 K.

Those years have been filled with promise, hyperbole and feverish research. Physicists know that copper oxides superconduct in an entirely different way to conventional BCS superconductors (after Bardeen, Cooper and Schrieffer, who worked out the theory behind them). And yet, nobody agrees on precisely what the new mechanism is. Neither has anybody created a superconductor that works at a usable temperature, that is above the temperature of liquid nitrogen.

Even the resurgence of excitement last year over the discovery that magnesium diboride superconducts at high temperatures, probably in the old fashioned BCS way, soon gave way to malaise as physicists found they were unable to build on the breakthrough to make better superconductors. It's tempting to think that superconductors will never pass the liquid nitrogen barrier.

But today hope is restored thanks to a fascinating set of calculations carried out by Gianluca Savini at the University of Cambridge in the UK and a couple of buddies. They calculate the properties of p-doped graphane from first principles and say that it ought to superconduct at a balmy 90K or more, well within the range of liquid nitrogen cooling.

What's more p-doped graphane should superconduct in the same way as the old fashioned BCS superconductors. That's curious because everybody believes that BCS superconductivity cannot work at high temperatures.

The reason is the energy of the interaction between the superconducting electrons and the surrounding material. In ordinary BCS superconductors this is thought to be just a few tens of meVs. In the copper oxides, however, these interactions have an energy of a few hundred meVs. It's this difference, that makes physicists think that BCS superconductors will never work at the temperature of copper oxides.

And yet the discovery that magnesium diboride superconducts challenges that thinking--energy of these interactions in MgB2 is much higher. Three factors seem to come together to make it possible, say Savini and co. First is the characteristic energy of the phonons in MgB2 which is due to bond stretching and plays an important part in helping superconductors through the structure. Second is the electron density of states in the material and finally they point to the balance between the attractive electron-phonon coupling and the repulsive electron-electron interaction in MgB2.

Might it be possible to find materials in which these quantities can be manipulated further? You betcha. Savini and co noticed that p-doped diamond has two of these characteristics but superconducts only at 4K.

However, they calculate that p-doped graphane fits the bill exactly and should superconduct in the old-fashioned BCS way at 90K. What's more they say there are hints that p-doped diamond nanowires might have similar properties.

Various groups are already playing around with doped diamond nanowires.

The implications of all this are astounding. First up is the possibility of useful superconducting devices cooled only by liquid nitrogen. At last!

But there's another, more exotic implication: by creating transistor-like gates out of graphane doped in different ways, it should be possible to create devices in which the superconductivity can be switched on and off. That'll make possible an entirely new class of switch.

Before all of that, however, somebody has to make p-doped graphane. That will be hard. Graphane itself was made for the first time only last year at the University of Manchester. It should be entertaining to follow the race to make and test a p-doped version.

Ref: arxiv.org/abs/1002.0653: Doped Graphane: a Prototype High-Tc Electron-Phonon Superconductor



February 05, 2010 05:10 AM


Sabine Hossenfelder - Backreaction

The LHC Proton Source

Yesterday, we had a very nice colloquium by Jonas Strandberg from the University of Michigan on "The startup of the LHC and the very first collisions in the ATLAS detector" (abstract and video here). If you have an hour time, watching the video is a good way to spend it. Here, I just want to pick out one image he showed because it got stuck in my head.

As you might know the protons the LHC is circulating are accelerated in various stages. From a duoplasmatron, they are first injected into a linear accelerator (up to 50 MeV), then in the first small circular accelerator, the Proton Synchroton Booster (50 MeV -> 1.4 GeV), then in the Proton Synchroton (1.4 GeV -> 26GeV) and then in the Super Proton Synchroton (26 GeV -> 450 GeV). Only after this are they injected into the LHC tunnel for the real kick (450 GeV -> 7 TeV).


[picture source]


But where do the protons come from? Rather banally, out of a bottle of hydrogen:


[picture source]


I find it amazing, the contrast between that hydrogen bottle and the mighty LHC complex necessary to accelerate the protons.

by Bee (noreply@blogger.com) at February 05, 2010 04:56 AM


David Berenstein, Moshe Rozali - Shores of the Dirac Sea

holes


At some point I promised that I was going to write about my  most recent paper. So here is my promotion. In a sense, that paper is an exercise to understand what does it mean to have quantum gravity in a setup of emergent geometry: this is a situation where geometry is not there a priori, but it is extracted from some collective behavior of a system. I don’t want to go into semantics of what emergence means. For our purposes it is something that is extracted from a non-trivial procedure in systems with a lot of degrees of freedom, where we extract stuff that involves all degrees of freedom simultaneously in a non-trivial  way. The system is quantum mechanical, and therefore there are quantum fluctuations and whatnot, and the whole purpose of our study is to measure some property that can be associated with a distance, but taking into account that the measurement will give you some type of probability distribution on some variable that is supposed to be geometric. Instead of getting something where this is all done analytically, we did it by computer simulations and ran it like an experiment. To top it off, the research was done with an undergraduate student, who ran the simulations and did some of the basic data analysis of the numbers we got.

Here is the longer version.

Preliminary remarks:

I’m using  the word quantum in the sense of quantum mechanics. This means that we need a Hilbert space of states, and in this Hilbert space of states we get particular states. In the case of quantum gravity, for each quantum state you get one universe so to speak. The states can be superposed, you can have dead/alive cats, there can be interference and in the end you compute probabilities, or probability distributions.

Most interesting Hilbert spaces are infinite dimensional, and they all look the same to the untrained eye. Heck, even to the trained eye they look the same. Thus, one can not just do random quantum mechanics with a random Hilbert space and see what happens. You need extra structure that lets you make meanigful statement about these states. This usually shows up into having a preferred set of variables (let us call them x for lack of a better name) and then one can write a wavefunction of these variables, usually denoted by

\psi(x)

A wave function defines a state so long as it satisfies some basic properties, and given a state, one can measure various things that depend on x (remember that x is a set of variables).

The second point to make is what does gravity mean above? Well, the modern understanding of gravity is based on the geometry of spacetime being curved. So if you have quantum gravity, you are talking about a system where you have some geometric information, and it is governed by quantum mechanics. This means that the geometry can and should fluctuate.

Once you are here, if you have some quantum system, you might  be able to talk about geometry. If you have a single variable x, it could be the volume of the universe. So if your wave function is peaked at large x, you might say that you predict a large universe.

However, in theories of emergent phenomena, in particular, emergent geometry, the geometry is not there as the natural description of your system. The geometry appears as some organizing principle that correlates your various variables in an interesting way and only in some situations. Geometry is not automatic given a wavefunction. So you need models where this can be done in a reasonable way. You need a lot of variables (geometry can have a lot of positions), and the variables have to more or less look the same (they are not completely random). Afterward, you can ask where in this mess of many variables is the geometry hiding. My favorite model for this is the AdS/CFT correspondence, but this is still too hard to be able to give a full analysis of where the geometry comes from.

Main point

Understanding emergent phenomena is hard. You need toy models. Especially so if the stuff that is emerging is the geometry of spacetime. In such situations, the spacetime is not there at the beggining. It has to be extracted from some other data. Moreover, if you change your wave function, you change your geometry.

What this means is that the lengths of features and such change depending on your wavefunction, so you not only need to be able to say that you have geometry, you need to be able to measure distances on it. And tot op it off, your geometry is fluctuating, meaning that when you measure you might get more than one answer with some probability distribution.

Ok, preamble is done. What did we do with my student?

We took a model  of emergent geometry that I developed way back when (‘05) that goes some way towards understanding the geometry of the AdS/CFT correspondence. The model has some nice wavefunctions that can be argued to have geometric features, and changing your wavefunction lets you change the geometry and topology of these features. This is still too general, so we picked very simple wave functions in the model, and we put them on a computer, so that we could generate probability distributions and see how the geometry dependend on the parameters of the model.

You get pictures like the one shown below:

A geometry with a hole

The wave functions have many variables (6N to be precise), where N varies. All of these variables are pretty similar to each other, and get grouped into N collections of 6 variables. Each member of this collection of N is like any other, so it makes sense to compare them all by plotting the individual data in 6 dimensions. Excuse me? 6 dimensions? I don’t know how to see that. So for the picture above I projected them all into 2 out of those six dimensions.

Well, you can clearly tell that you get a disk-like geometry and that it has a hole, and it looks more or less like like a donut. Now remember that this is only one picture of a typical configuration as described by the wave function \psi, so you can get a lot of pictures like the one above. And then you have to decide what is the radius of the hole.

Well, our procedure was designed before taking the data. We wanted to average over configurations some set of functions that tell how far are the particles from the origin. We wanted these functions to be dominated by the particles that are close to the hole of the donut, rather than the ones that are far. We also wanted these functions to be easily computable and democratic between the degrees of freedom. We decided on

f_k = \sum_i |\Pi_{12}\vec x_i|^{-2k}

So that given a configuration, you weigh the projected position on the 12 plane by how close the different variables are close to the origin. The Pi in the equation above is a projection. You want to average these over the particles, so you need to divide by N, and then you have to choose what to do with all the f_k that you record between configurations. Finally, the idea was to take k to infinity  (meaning large) to define the radius.

The ideal definition is that you take it as above, average, do some simple arithmetic, and voila you get some typical radius with some statistical distribution attached to it. However, we found that if k was large enough, the average of the above expression was infinity and that is bad. Because then it would tell you that the radius of the hole is zero.What we found was a probability distribution with some moments not defined.

So we had to work different averaging procedures that take this feature of the probability distribution into account, and each of these different ways of doing things gives different answers. Moreover, if you go to large N, the result is supposed to converge for all different measurements to the same value no matter what. But how fast does it converge depends on the procedure of getting there and what precisely one is averaging.

Our main conclusion was that how you average matters, and that our N were not really large enough to be very conclusive on some things we wanted to determine. Therefore, we need a bigger simulation. In a certain sense,  this helps to understand the difficulty in various issues related to emergent wobbly geometries.

Finally, I have the Homer Simpson solution to all of this: eat the donut! Duh.

Filed under: computers, quantum fields, Quantum Gravity

by dberenstein at February 05, 2010 01:58 AM


February 04, 2010

Phil Plait - Bad Astronomy

Hubble catches Pluto red-faced

Pity poor Pluto. The debate over its planethood has caused much consternation over the years. Part of the problem is that it’s so dinky and so far away! If it were closer, or bigger, we almost certainly wouldn’t be having this debate.

But whether or not you think Pluto should be part of the gang or not, one thing is certain: it’s a world unto itself. And to bring this point literally home, the Hubble Space Telescope has revealed the changing face of this tiny iceball:

hst_pluto_feb2010

These images, just released today (but taken in 2002), represent the most detailed surface map of Pluto ever taken. Even in Hubble’s Advanced Camera for Surveys Pluto is only a few pixels across, but it’s possible using sophisticated image processing techniques to tease out the detail seen.

Here’s a nifty animation of Pluto rotating using these maps:


Very cool. But these maps are more than just eye candy. They show significant changes on Pluto’s surface since the last maps were made using Hubble 16 years ago. Pluto’s north pole is brighter and the south pole darker, implying that material has migrated from one pole to the other, or at least that the poles are changing in different ways. Pluto orbits the Sun "on its side", dramatically more tilted than Earth’s mere 23.5°. Right now, the north pole of the world is facing the Sun, meaning it’s summer on Pluto’s northern hemisphere (as it’ll remain for a long time, given Pluto’s 248 Earth-year long year).

Not only that, these images show that Pluto has reddened quite a bit in the past few years. This is one reason it took so long to release the images; Marc Buie, the astronomer who took them, saw some things in the data that were difficult to understand, and wanted to make sure they were correct. These images are composites of pictures taken using a blue and a green filter. During the time these observations were made, in 2000 – 2002, Pluto got much darker in blue, which was unexpected. Pluto’s moon, Charon, did not get any bluer, indicating that the cause was something intrinsic to Pluto and not that something weird happened with Hubble.

So why is Pluto redder now? That’s not clear. In general, ultraviolet light from the Sun interacts with the chemicals on Pluto, creating reddish organic molecules; this is seen on lots of distant, icy objects in the Kuiper Belt (the region past Neptune where Pluto orbits). Incredibly, even at the numbing distance of over 4 billion kilometers (3 billion miles) from the Sun, Pluto is still strongly affected by it. But this is happening while overall the northern hemisphere got brighter and the southern darker. You’d expect Pluto to get darker if it gets redder, so clearly there’s more going on here than meets the eye.

hst_pluto_map_feb2010These maps will prove crucial in planning the imaging run of the New Horizons probe, which will scream past Pluto in 2015. Having even a crude map in advance of the encounter will help scientists plan their limited time more carefully.

Plus, these Hubble images may very well be the best view we’ll get until New Horizons gets to Pluto, for that matter. And whether you think Pluto is the littlest planet or one of the biggest of the Kuiper Belt Objects, it’s a fascinating place worthy of a lot more study. And in just a little more than five years we’ll see fantastic images of it, too. I can’t wait!

Video courtesy Emily Lakdawalla (and my thanks to her for a helpful conversation). Image and video credit: NASA, ESA, and M. Buie (SwRI)

by Phil Plait at February 04, 2010 08:07 PM


Emily Lakdawalla - The Planetary Society Blog

New maps of Pluto show pretty amazing amounts of surface change

I just posted my writeup of today's press briefing on a new map of Pluto produced from Hubble images. The main conclusion was that Pluto has shown an astonishing amount of changes across its surface between 1994 and 2002 -- more, in fact, than any other solid surface in the solar system. An interesting perspective on the announcement, which concerned four years of computational work done by Marc Buie, was provided by Mike Brown. Buie said ....

February 04, 2010 06:17 PM


Michael Schmitt - Collider Blog

CMS_rts


Today the first CMS physics paper appeared on the arXiv, 1002.0621,

Transverse momentum and pseudorapidity distributions of charged hadrons in pp collisions at √s = 0.9 and 2.36 TeV.

This paper reports measurements similar to, but going beyond, the ALICE paper,
which I discussed earlier on this blog.

Notice that data from the √s = 2.36 TeV data are included – these are the highest-energy data in the world at the present time, hence a kind of feather in CMS’s cap.

a feather in the cap

More soberly, the actual parton-parton energies are quite low, since the events are non-single-diffractive interactions, basically, glancing blows of the two protons and a far cry from, say, the production of a pair of top quarks or W bosons.

The measurements concern the transverse momentum pT and (pseudo-)rapidity η distributions of charged hadrons. As I discussed earlier, these distributions can be related to scaling arguments started by Feynman, and as such lie in the area of non-perturbative physics of hadron production, for which there are phenomenological models. At a minimum, these models must be tested and constrained so that they can be used for modeling underlying event structure for high-energy collisions. These measurements also serve as a baseline for heavy-ion collisions.

The event selection was as open and simple as can be imagined, demanding little more than signals in beam monitors indicating that bunches as collided at the center of the CMS detector. A very loose cut on the number of pixel detector hits was enough to eliminate beam-gas events entirely. The event had to have a reconstructed vertex, too. Selections efficiencies are high, naturally, around 86% or so.

Three methods were used to measure the rapidity distribution, dN/dη. The most primitive method simple counts reconstructed clusters in the pixel barrel detector, since the shape of a cluster already provides a good indication for a charged hadron track. The second method links such clusters to build short track segments called “tracklets” which do not provide curvature information but clearly allow for a better indication of the origin of the track. Finally, a full-blown track reconstruction, using both the pixel and the silicon strip detector, was used, which provides momentum measurements as well as direction. The point of the three methods is to demonstrate the robustness of the measurements with respect to the methods used and the performance of the detector – which was excellent in any case.

The systematic uncertainties concern the acceptance and efficiency estimates and to what they degree they depend on the phenomenological (Monte Carlo) models. The exclusion of single-diffractive events, in which the hadronic final state is typically very forward and difficult to observe, is only partially successful; the purity of the final samples is roughly 95%. This purity estimate depends again on the models, so there is a systematic uncertainty. The net uncertainty is only 3%. An additional 2-3% comes from reconstruction efficiencies, and 1% for knowledge of the tracker geometry.

Transverse Momentum:

The transverse momentum distribution of primary charged hadrons is shown below, for both 0.9 and 2.36 TeV. The mean pT is measured to be 0.46±0.01±0.01 GeV at 0.9 TeV, increasing slightly to 0.50±0.01±0.01 GeV at 2.36 TeV. This increase is clearly seen in the tail of the pT distribution:

CMS dN/dpT

measured transverse momentum distributions for primary charged hadrons

Pseudorapidity:

The number density as a function of pseudorapidity, dN/dη, is presented for both √s = 0.9 TeV and 2.36 TeV. The results from the three methods (not shown) agree within errors. One might notice the much greater accuracy provided by the CMS measurement as compared to the early one by ALICE.

pseudorapidity distribution, dN/dη

The distribution dN/dY, where Y is the rapidity, is expected to be flat for Y≈0. The pseudorapidity usually coincides with the rapidity except when the rest mass of the particle is not negligible, which is the case with pions and kaons with a transverse momentum of a couple hundred MeV. The subtle wavy effect seen in the figure comes from the numerical differences between η and Y.

It is clear from the figure that more hadrons are produced per unit of rapidity at high energies than at lower energies, which is interesting given that peripheral nature of these collisions. According to the original arguments of Feynman, the variation with center-of-mass energy should go as ln(s), but some years later experiments showed that the rise is quadratic in ln(s). It is also worth noting that the difference between p+p and p+anti-p collisions is less than a couple of percent, again underscoring the soft, peripheral nature of these collisions.

From the CMS data, dN/dη = 3.48±0.02±0.13 at 0.9 TeV and dN/dη = 4.47±0.04±0.15 at 2.36 TeV.

As stated in the paper, the increase of 28.4% is significantly more than the 18.5% predicted by a tuned version of PYTHIA, and the 14.5% predicted by PHOJET models. Interesting.

Here are the summary plots showing the dependence on center-of-mass energy culled from several experiments over the years:
variation with root(s)
The red points are the ones from CMS.

So the first paper from CMS is interesting, with a non-trivial result, and perhaps a good harbinger for the next few months, when we will surely see many measurements of hadronic event properties from the LHC experiments. :)

(Note: I am a member of the CMS Collaboration and my name appears on the author’s list.)

by Michael Schmitt at February 04, 2010 06:06 PM


Symmetrybreaking - Fermilab/SLAC

New facility makes accelerator cavities easy as pie

Brookhaven National Laboratory has invested in a new, private facility to treat the superconducting cavities within a few miles of the site. The new facility is top of the line, located almost next door, and shows the power of joining government and private industry.

by Calla Cofield at February 04, 2010 05:03 PM


Cosmic Variance

Sport Science: Human vs. Bow

Super Bowl Sunday is, of course, the great American holiday. Past years have seen inspirational performances by Joe Namath, Joe Montana, and Janet Jackson. This year pits the New Orleans Saints against the Indianapolis Colts. New Orleans, of course, is known as a city of saintly behavior, while Indianapolis’s claim to fame involves horsepower in some tangential way.

When faced with contests of ritualized violence, we like to look for the science. So check out this video of Saints quarterback Drew Brees participating in a rigorous laboratory experiment by throwing the ol’ pigskin at an archery target. Joking aside, that is some pretty sick accuracy there.

Impressive that a human arm beats a bow and arrow for accuracy (although it’s not completely clear that the distances and conditions were perfectly analogous). All in the wobble, apparently. But if I were defending my castle from the barbarian hordes or something, I’d still prefer archers over some guys throwing footballs.


by Sean at February 04, 2010 05:02 PM


Emily Lakdawalla - The Planetary Society Blog

Way-cool Martian flyovers by Doug Ellison

Doug Ellison has been playing with Martian digital terrain models recently, to great effect. Here is a marvelous animation he produced that flies you over the Pathfinder landing site, out of -- and back into -- its own camera view of the terrain. It's surprising how deep the big crater south of the Pathfinder landing site actually is; it's quite a bit closer to the original bowl shape of the crater than most Martian craters I've seen in ....

February 04, 2010 01:16 PM


Jester - Resonaances

How much is one inverse femtobarn?

Blog readers know this since ages, but today the news was made official.
Last week, the Chamonix workshop once again proved its worth as a place where all the stakeholders in the LHC can come together, take difficult decisions and reach a consensus on important issues for the future of particle physics. The most important decision we reached last week is to run the LHC for 18 to 24 months at a collision energy of 7 TeV (3.5 TeV per beam). After that, we’ll go into a long shutdown in which we’ll do all the necessary work to allow us to reach the LHC’s design collision energy of 14 TeV for the next run. This means that when beams go back into the LHC later this month, we’ll be entering the longest phase of accelerator operation in CERN’s history, scheduled to take us into summer or autumn 2011.

This announcement does not mention the luminosity goal, but both blogs and some Chamonix slides point to 1fb${}^{-1}$. How much is that? The Tevatron by the end of 2011 will have acquired 10-12 inverse femtobarns of luminosity. Using advanced calculus one concludes that 1 inverse femtobarn is less than 10 inverse femtobarns, but at the same time 7 TeV is more than 2 TeV. To unravel this, here is a handful of back-of-a-madgraph estimates of how many interesting events can the colliders get by the end of 2011.

Higgs Boson (120 GeV Higgs produced in gluon fusion)
Tevatron: 10 000 LHC: 11 000

Both experiments will have a similar sensitivity to the Higgs. Although 10k looks like whole lotta events, Higgs signatures are notoriously difficult to search. For example, one promising discovery channel at the LHC is when the Higgs decays into two photons, which happens roughly twice per thousand events for a 120 GeV Higgs. For this and other reasons, neither Tevatron nor the LHC has good prospects of discovering the Higgs, unless in lucky circumstances (e.g. production cross section larger than in the standard model, or Higgs mass sitting close to the sweet spot of 160 GeV).

Top Quark Pairs
Tevatron: 80 000 LHC: 130 000

Similarly as for the Higgs, the Tevatron and the LHC will acquire comparable top samples. There should be some, though not dramatic, improvement in top precision measurements. Who knows, maybe there will emerge some 3-sigmish discrepancies with the standard model. The general lesson is that the LHC will be competitive in measuring the standard model processes, but it cannot beat the Tevatron black and blue. What about beyond the standard model?

500 GeV Quark
Tevatron: 15 LHC: 300

This illustrates the obvious truth: LHC fares much better with particles who sit close to the kinematical limit of the Tevatron. In that case one finds that $7 \gg 2$: the energy advantage trumps the luminosity handicap. However, in that particular case the discovery is not guaranteed because of the large standard model background, for example from the top quark pair production. So let's try something easier.

1 TeV Z' (U(1)' gauge boson coupled to B-L with g'=0.1 and decaying to electrons or muons)
Tevatron: 5 LHC: 25

In this case the standard model background is almost non-existent, so 25 events might be enough to claim a discovery. But there is only a tiny sliver of parameter space which the Tevatron cannot reach but the first LHC run can. Make the Z' mass 1.3 TeV and the number of dilepton events at the LHC drops to 5. The final lesson to take home: the LHC can be lucky if Tevatron is extremely unlucky. Let's then hope for the worst, to some.

by Jester (noreply@blogger.com) at February 04, 2010 01:01 PM


Jon Butterworth - Life and Physics

Jon Butterworth


There has been a great deal of speculation about how a major research funding body – the Science and Technology Funding Council – has lurched from one financial crisis to another.

Yesterday in the evidence session of the Science and Technology Select Committee, we heard Michael Sterling, chair of the STFC, give a crystal clear description of the fundamental problem.

… see the rest of this article I wrote on the New Scientist’s S-Word.

by Jon Butterworth at February 04, 2010 01:00 PM


Clifford V. Johnson - Asymptotia

Essence

Today I've got to give a guest lecture in a class of KC Cole's at USC's Annenberg School. I'm supposed to talk on the theme of Art and Science. I'll cover a number of aspects, I expect (have not written it yet), but it put me in mind of two posts I did a while back on the subject. One was over at Correlations (remember that?) and the other, called Transcendence, was here. I thought I'd reproduce some of the Correlations post, called Essence, here. The back story was that I was working up a contribution to SEED magazine (the December 2007 issue I think) which was doing a cover story on Science and Art, and... well, I'll let the 2007 me tell you the rest:
While working on the contribution, I was hugely conflicted, for many reasons (variety of themes, variety of pieces, art forms, only 100 words, etc...) and another major theme struggled for dominance - "essence". How both science and art strive to identify the essential truth about a subject. My original contribution that I submitted to the editors to get their feedback on whether I was on the right track for what they were looking for therefore had a bit more of this in it, and referred to two pieces of art (I eventually chose one and focussed on developing and rewriting around that, using the "transcendence" theme). The piece I used that did I did not use for the final article is perfect for illustrating the "essence" theme, and so to provoke some thoughts in you [...] I include it here, along with some fragments of the paragraphs I was playing with at the time:
[...]

by Clifford at February 04, 2010 08:02 AM


ZapperZ - Physics and Physicists

Weinberg Says That Obama Gets Space Funding Right

In light of the new proposed 2011 budget and the Obama Administration decision to cut spending on the human spaceflight to the moon and Mars, Steven Weinberg wrote an opinion piece that praised this step.

The manned space flight program masquerades as science, but it actually crowds out real science at NASA, which is all done on unmanned missions. In 2004 President George W. Bush announced a new vision for the space agency: a return of astronauts to the moon followed by a manned expedition to Mars. A few days later NASA's office of Space Science announced major cutbacks in its important Beyond Einstein and Explorer programs of unmanned research in astronomy. The explanation was that they "do not clearly support the goals of the President's vision for space exploration."

Soon after Mr. Bush's announcement I predicted that sending astronauts to the moon and Mars would be so expensive that future administrations would abandon the plan. This prediction seems to have come true.


The cost-to-benefit ratio, at least in terms of science output, is just not there for human spaceflight, and even the ISS for that matter. Decisions to do this have been based predominantly on politics, and funding appropriate home constituents. When one argues that such cutbacks can hurt science, and then to hear very prominent scientists disagree with that assessment, it kinda throw that claim into question.

Zz.

by ZapperZ (noreply@blogger.com) at February 04, 2010 05:17 AM


arXiv blog

The Curious Case of the Evolving Apostrophe

A new technique for analyzing early English texts is gradually revealing the history of the apostrophe.

Last year, grammatical tragedy struck in the heart of England when Birmingham City Council decreed that apostrophes were to be forever banished from public addresses. To the horror of purists and pedants alike, place names such as St Paul's Square were banned and unceremoniously replaced with an apostrophe-free version: St Pauls Square.

The council's reasoning was that nobody understands apostrophes and their misuse was so common in public signs that they were a hindrance to effective navigation. Anecdotes abounded of ambulance drivers puzzling over how to enter St James's Street into a GPS navigation system while victims of heart attacks, strokes and hit 'n' run drivers passed from this world into the (presumably apostrophe-free) next.

Why the confusion? Part of the reason is that apostrophes are not particularly common in the English language: In French they occur at a rate of more than once per sentence on average. In English, they occur about once in every 20 sentences. So English speakers get less practice.

But the rules governing apostrophes are also more complex in English. In both French and English, apostrophes indicate a missing letter, such as the missing i in that's or the v in e'er. But in English, apostrophes also indicate the possessive (or genitive) case. They are used to show that one noun owns another: St James's Street is the street belonging to St James.

The complexity is compounded because in English, the plural is often formed by adding an s. So the word boys means more than one boy. How then do you form the possessive to indicate, for example, a ball belonging to the boys? Is it the boy's ball or the boys's ball or the boys' ball?

And then there are the exceptions. Pronouns, for example, do not take a possessive apostrophe: you can't say I's ball or me's bat. The truth is that knowing when to use an apostrophe is not always easy.

That may be partly because the rules for using apostrophes are evolving. Today, Odile Piton and Hélène Pignot at the University of Panthéon-Sorbonne in Paris present an analysis of the use of apostrophes in English texts from the 17th century and show that the usage was much simpler in those days.

Their main challenge was how to recognize an apostrophe. Apostrophes are often the same as single quotation marks and are entered on a computer keyboard using the same key. So it's easy to get false positives.

Spotting the absence of an apostrophe where there ought to be one can be tricky, too. They give the example of this sentence: "First, that no other mans errors could draw either hatred, or engagement upon me." The automated analysis missed the absent apostrophe in mans, thinking instead that it was the transitive verb "to man."

What Piton and Pignot have yet to study how the use of the apostrophe changes in time. But they now have the automated analysis tools that should make this possible. That could reveal the forces at work that change our language.

For the moment, Piton and Pignot's conclusion is merely that the world was simpler in the 17th century as far as apostrophes go. They say: "The possessive genitive in 's was not very common yet. The apostrophe mostly marks the omission of letters in a wide range of words and the plural of certain words."

Rather like the road signs in Birmingham.

Ref:arxiv.org/abs/1002.0479: "Mind your p's and q's?": or the Peregrinations of an Apostrophe in 17th Century English



February 04, 2010 05:10 AM


Quantum Diaries

Are You an Infovore?

The Urban Dictionary defines an infovore as “A person that has a voracious appetite for information”. This term is pretty popular on the blogosphere these days.
The people who actually coined the term are neuroscientists: Irving Biederman of the University of Southern California in University Park and Edward Vessel of New York University. Their findings show that humans are natural infovores. When the human brain processes information, endorphins are released, giving us a feeling of pleasure.
It is probably not wrong to assume that people who are drawn to a carrier in science have a particularly strong predilection for processing and interpreting information.
Even though I am a theorist, I am fascinated by data, and I love how much data is freely available to the public via the internet. Be it astronomical data via the Sloane Digital Sky Survey which will have a dataset of 230 million celestial objects, or from the Hubble Space Telescope, to mention just two scientific data sources.

I would definitely classify myself as an infovore, and I have a confession to make: I am addicted to the weather forecast. I check the satellite image and the rain radar echos several times a day. I am just thrilled to have access to up to date satellite and radar data. It might sound silly, but when you think about it, it’s actually pretty cool and wouldn’t have been possible 20 years ago. We’re living in a time in which huge amounts of data are being compiled (the LHC alone produces about 1.3GB per second!), a huge wealth for all flavors of science. The challenge is to find ways of effectively extracting useful information from it.

Check out a very cool instance of automated processing of astronomical images which is accessible to everyone: Any picture of the night sky submitted to the pool of the astrometry group on Flickr is run through an engine, which posts a comment to the picture with the astronomical data of the depicted objects, and adds notes directly on the image which identify the visible objects. I think that’s pretty neat!

by susanne at February 04, 2010 01:19 AM


February 03, 2010

Symmetrybreaking - Fermilab/SLAC

Confirmed: Hubble 3D will be awesome

The upcoming Hubble 3D IMAX movie including footage shot during a manned space mission last year is deserving of the adjective "awesome," says our symmetry correspondent, based on a preview screening in New York.

by Calla Cofield at February 03, 2010 11:55 PM


Clifford V. Johnson - Asymptotia

Gunslinging Bohr

bohr_einsteinThis story has come along at just the right time, given that my last post was about Einstein. Seems that Niels Bohr (another giant from the same period, and another one of the founders of the quantum theory) was a big fan of cowboy movies, and thought a lot about gunfights. Yes, really! (There he is in the photo on the left hanging out with his friend Einstein in later years, perhaps 1925. Perhaps they're at a drive-in movie? I got this photo here.) It turns out to be all relevant to new studies about reaction time. The fastest person to draw does not necessarily win the gunfight: [...]

by Clifford at February 03, 2010 11:22 PM


Peter Steinberg - Entropy Bound

I might be wrong, but...

One of my least-well-kept secrets is that I'm mildly obsessed with the old hydrodynamical model invented by Landau and Fermi in the early 50's, which predicts (surprisingly well, IMHO) the total number of particles produced in collisions of nuclei, and even collisions of protons (and angular distributions, and some novel features of the same etc.). The physics is simple (pack all the energy into a tiny volume and let it explode along the beampipe) but the justification is downright bizarre (that all of these microscopic systems form a droplet of fluid as soon as they overlap each other). Of course, RHIC has told us that nuclei seem to form a fluid, and pretty damn quickly -- I'm still looking for a good reason why protons couldn't in principle do the same thing. Of course, making a snappy prediction would help my case.

So the points are nucleus-nucleus data (top trend) and proton-proton data (bottom-trend), and I've tried my hand at extrapolating with a few favorite functions. Unfortunately for me, but congratulations to CMS, the top one (my favorite) is pretty clearly ruled out by the low multiplicity measured by CMS today (4.5 instead of 5.5).

Of course, there are two more experiments to go, and a few more energies to be filled in in the next few years. A man can still dream -- especially since the CMS paper measures the number of particles at 90 degrees relative to the colliding beams, which is *not* the same thing as the total number. Of course, the calculations nominally allow one to predict things at 90 degrees but lots of things can get in the way. No-one said this would be easy!

by Peter (noreply@blogger.com) at February 03, 2010 11:12 PM


Phil Plait - Bad Astronomy

Cosmophobia

Apropos of my recent post showing a Hubble image of two asteroids colliding, the website Word Spy just happened to have a funny choice for their word of the day: cosmophobia, "the strong and irrational fear that in the near future the earth will be destroyed by some cosmic event."

Personally, I figured it’s really for people who don’t like vodka, triple sec, cranberry and lime juice all mixed together, which is silly. Unless it’s headed for you at 30 km/sec. Because that’ll give you a pretty wicked hangover.

by Phil Plait at February 03, 2010 09:09 PM