On Price on the Arrow of Time

Apologies for the lack of posting recently. I had made my mind to write my long-delayed review of Huw Price's book Time's Arrow and Achimedes' Point, but rereading it made me think so much about the issues it raises that I decided insteadt to write two or more separate posts discussing them. My evaluation of the book is simply that it is a must read if you are intereseted in the philosophy of time and/or conceptual problems at the foundations of physics.

Price assumes more or less the "atemporal" or "block universe" perspective in the philosophy of time, and discusses the asymmetries to be found within time, their physical and conceptual relation to each other and the possible ultimate explanations for them. It is a commonplace that all the asymmetries we see in ordinary life (broken glasses that do not reassemble spontaneously, etc.) are ultimately traceable to the second law of thermodynamics and the growth of entropy. (The fundamental dynamical laws are time-symmetrical, with the exception of some very specific particles subject to weak interactions and that so far as we know play no role in ordinary physics.) Price explains very clearly that the growth of entropy means only the tendency to macrostates of greater probability, and as such requires no particular explanation; what does require an explanation is the surprising fact that entropy is so low to begin with, when the "natural" state of the universe is a thermal equilibrium with maximum entropy. Boltzmann suggested once that the original state of the universe was really a high entropy one and that our low-entropy universe was just product of a fluctuation. This proposal has unacceptable quasi-solipsistic consequences, explained by Sean Carroll some weeks ago: by its reasoning it is far more likely that I have sprung directly into existence by a small fluctuation that created my present brain, not by a large one that created a whole universe in which my brain could eventually appear (the "Boltzmann's Brain" problem).

Price also does an excellent job of noting the "double standard" many authors commit in discussing time asymmetries: applying different criteria of plausability in different temporal directions. For example, a common assumption is that "incoming influences are independent": sistems interacting and with no common cause have their properties uncorrelated before the interaction but correlated after it. This seems entirely plausible and commonsensical, but if dynamical laws are time-symmetrical there is no good justification for it. A priori we should find the correlation of systems prior to interacting and uncorrelation afterwards to be just as possible. For macroscopic systems the principle can be justified from the second law and shown to arise from the special low-entropy state at the beginning of the universe (instead of positing it to explain the second law, as some physicists used to hope), whereas for microscopical systems the situation is more obscure (more on this later). Price is extraordinarily good at thinking in "atemporal" terms and uncovering hidden assumptions and double standards in the way we usually think about these questions.

The third chapter of the book contains a discussion of the so-called Arrow of Radiation (we see "retarded" waves going out from emmiting systems and not "advanced" wave focusing spontaneously in absorbing systems, a situation which is the time-mirror image of the first and just as possible on dynamical grounds). Price explains the well-known Wheeler-Feynman theory for explaining this asymmetry and gives a thought-provoking reinterpretation and reassessment of it. Alas, he does not discuss the how his version of the theory fares when quantum electrodynamics replaces classical electrodynamics. Can anybody point to a good discussion of a quantum version of the Wheeler-Feynman theory, if such thing exists?

The fourth chapter discusses cosmology, adressing the fundamental question: "What could be a possible explanation for the unnatural, extraordinarily low-entropy state of the early universe?" After considering and subjecting to criticism the opinions of many well-known physicists (the names of Davies, Penrose and Hawking come up repeatedly) Price concludes that there are four possible alternatives:

1) The anthropic approach: there is a multiverse with vast, vast, number of other universes, with all possible initial conditions, and ours is just a lucky one that got a sufficently-low entropy initial condition to allow for life to develope. Two objections to this view are that it postulates a huge number of unobservable entities, and that it is not clear whether a universe like ours is really the "least costly" way of creating life; if it isn't, we may face a Boltzmann's Brain problem here.

2) The asymmetric law proposal. Roger Penrose is the main contemporary defender of this idea, which essentially postulates a fundamental asymmetric physical law that constrains initial singularities (like the Big Bang) to have low entropy but puts no such constraint on final singularities (like stars collapsing into black holes, or a possible eventual Big Crunch). The main problem with this idea is how arbitrary it seems, especially if the dynamical laws are time-symmetric. (It may become more palatable if a future Unified Theory shows the tiny time asymmetry in weak interactions to derive from some deep time asymmetry at a fundamental level, but this is, I think, pure speculation at the moment. Do string theory folks out there have any idea about the possible status of T-symmetry in M theory?

3) The Gold universe (named for cosmologist Thomas Gold, the first one to propose it). There is a fundamental constraint that makes singularities have a low entropy, but it applies symmetrically. If the universe recollapses to a Big Crunch entropy would revert and decrease in the collapsing phase (at least from the point of view of our present standpoint; from the point of view of observers in the collapsing phase it may seem that the universe in expanding and increasing entropy, if as it seems likely our psychological arrow of time depends on the thermodynamical one). Also black holes should are constrained to have low entropy. Price favours this kind of view for its attractive stmmetry, but most physicists reject it. I will discuss below who has the upper ground in this discussion.

4) The "corkscrew" view. Like a factory that produces an equal number of left-handed and right-handed corkscrews, it might be that the fundamental theory of the universe is time-symmetric but only allows strongly time-asymmetric realizations. Price understands Stephen Hawking's present views (after retracting from a Gold-like view in the 1980s) as an example of this kind of view. Hawking claims that by using his "no boundary proposal" for Euclidean path-integral quantum gravity it can be proven that one extreme of the universe must have low entropy. He further agues that the connection between the thermodynamical and psychological arrows of time warrant calling the low-entropy extreme the "initial" one instead of the "final" one; and that once we have the universe securely expanding to a higher-entropy future we can use ordinary statistical arguments that say that a reversal of entropy is overwhelmly improbable, so the final state will be high-entropy instead of low-entropy.

Setting aside the anthopic view for the reasons outlined, Price favours the symmetrical Gold view (3) over Penrose's and Hawking's asymmetrical views, or at least argues that the Gold universe deserves a more serious consideration that most physicists are willing to give it. The standard argument against the Gold view is that the reversal of entropy requires events of huge statistical imporbability (broken glasses spontaneosly mending themselves, etc.) in the contracting phase. Price argues that this rebuttal invoves a Double Standard: we know already that the argument does not apply in one time direction, so why apply it confidently in the other? From an atemporal perspective, a glass breaking is a trajectory in configuration space precisely as improbable as a glass "unbreaking". We assume that an initial boundary condition of low entropy, whether imposed by fiat as in Penrose or dynamically as in Hawking, can override statistical considerations in one direction; so why not in the other? If we have a thoery saying that a "natural" initial boundary condition is a low entropy one, then the theory will have more symmetry, therefore be more plausible, if it also applies to final boundary conditions. Price devotes a lot of space to showing that there are (and are likely to be) no empirical contradictions to the Gold universe, at least until and if the universe actually starts recollapsing.

I think love of symmetry takes Price to far away in this. His reasoning neglects a very simple fact: we have lots of empirical evidence supporting a low-entropy past, none for a low-entropy future. Thus an explanation for the low entropy past is the only thing we need -and according to Ockham's Razor, we better take the simplest explanation that we can find. If we take Penrose's "Weyl Curvature Hypothesis" or Hawking's dynamically generated low-entropy singularity to be satisfactory explanations, then the question is: does the additional "simplicity" of temporal symmetry gained by extending them to the future as per Gold's universe compensate the additional "non-simplicity" of this making the universe's history far statistically unlikelier than it is without this extension? Statistical considerations are usually very powerful; they enable us, after all, to make reliable predictions from the Second Law in all ordinary situations. It is my opinion that they should not be overriden by a mere desire for symmetry. Absent any empirical evidence that the future is low-entropy (as we have regarding the past) the statistical unlikeliness of such a future takes precedence for me over symmetry, making me reject Gold's universe.

There is an additional consideration: it seems pretty certain that the universe will not in fact recollapse to a Big Crunch, but continue its expansion indefinitely at an ever greater rate, driven by dark energy. Morevorer, it seems to me as plausible as not that this is not an "accident", but a necessary feature of universe. (To have a Big Crunch, the dark energy content of the universe would have to be much lower and the matter energy content much larger; but the closeness of the actual values of both quantities, which are of the same order of magnitude, suggests that there may be a dynamical reason for it, which would make recollapse impossible). If this speculation is solid, then either Penrose's or Hawking's theories would support the "corkscrew" result of a naturally asymmetric universe. Price mentions the possibility of the universe expanding forever, but does not give it the importance he should in my opinion. (The book is written before the discovery of dark energy.) Price argues that even if the unverse does not recollapse there are local collapses to black holes; but I am not sure if these local singularities must be subjected to the same conditions than the global one. (For example, does anybody know if Hawking's "no boundary condition" can be applied to a black/white hole as well as to the universe? If yes, there is indeed a question of why does it not imply that collapsed black holes have low entropy; but I suspect the answer must be no, or Hawking would have realized this!)

There is a fifth possiblity which Price does not discuss because it is more recent than his book. It is suggested by Sean Carroll and Jennifer Chen in this article, which Sean is fond of citing in his Cosmic Variance posts. It claims to give a model for the emergence of the universe as a statistical fluctuation in a high-energy background, but avoiding not only the usual problems of the Anthopic Principle but also, and crucially, the Boltzmann Brain problem as well. I do not really understand how it manages to do so; it has appearently something to do with a distinction between entropy and entropy density, but I am not sure how this does the required job. I have asked Sean about it yesterday in this comment, but the question seems to have been lost among the discussion of sexism in science that highjacked the thread.

My next post on Price's book will discuss how he frames the question of "independence of incoming influences" with respect to microsystems, and the radical interpretation of quantum mechanics and Bell's theorem this leads him to. Stay tuned.


UPDATE: In case you are interested, this paper contains most of the the ideas of Price on cosmology I have discussed here. Check his list of publications for more material on this and other philosophical problems. I find Price to be one of the most interesting contemporary philosophers.

Pluto no longer a planet

So says the definitive resolution of the International Astronomical Union. The provisional decision of last week, which kept Pluto's status as a planet and added three more planets to the Solar System by definitional fiat (see the discussions linked in this post) has been overturned by pressure of the "hard-liners". The main argument for the decision was, of course, that Pluto is known now to be just one more in a large class of trans-Neptunian objects, with no particular distinction among them justifying a special status for it. This is not clear, however, from the text of the press report:

Much-maligned Pluto doesn't make the grade under the new rules for a planet: "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit."

Pluto is automatically disqualified because its oblong orbit overlaps with Neptune's.

Instead, it will be reclassified in a new category of "dwarf planets," similar to what long have been termed "minor planets." The definition also lays out a third class of lesser objects that orbit the sun -- "small solar system bodies," a term that will apply to numerous asteroids, comets and other natural satellites.

The new definition makes little sense to me. What does "clear the neighborhood around its orbit" mean exactly? How does the overlap between Pluto's orbit and Neptune's disqualify "automatically" Pluto but not Neptune? I suspect there is some sloppy journalism going around here, and that the official resolution must be clearer.

Language Log's comment is:

The International Astronomical Union has spoken, and Pluto is no longer to be called a "planet." It is, however, still considered a "dwarf planet." Don't be fooled by any preconceptions you might have about English hyponymy: a dwarf planet is not, in fact, a planet.

Snakes on a Plane

I saw it a few days ago. I loved it. It will probably become the cult film of the decade, and one of the few cult films -if not the only one- that was created to be a cult film.

This post is not a film review. (What review do you need? The title together with the main actor say everything: there is a plane, it is full of snakes, and Samuel Jackson has to battle them, end of the story.) It is a reflection on an aesthetic problem the movie presents. The movie is not a normal mainstream action/horror film. It is not a normal class B action/horror film, either, though it is much closer to being one; the difference is that its intention is to provoke laughs. A parody of normal class B action/horror films, then? Not that either; a parody film has moments which are clear satirical jokes, that are there to prompt the audience into laughing with the film, at the victim of the parody. Snakes on a Plane (SoaP), by contrast, has no such parodical jokes; its comic relief moments -in the conventional sense of the term- are few, commonplace for action movies, and rather unfunny. And yet one finds oneself laughing through the whole film -but laughing at the film, not with it.

What SoaP is, in a nutshell, is a typical cheesy, cliched, über-class B, bad action/horror film that is so cheesy, cliched, über-class B and bad that it provokes laughs. Seeing it is the same kind of experience as gathering with geeky friends to see Plan 9 from Outer Space just to laugh at it. The only difference is that SoaP is made deliberately to be laughed at. And only by acknowledging this can one say that the film is brilliantly executed, and indeed a masterpiece. It does not omit one cliché from the list: the cast of characters includes a sadistic Asian mobster, an effeminate male flight attendant, a slutty female flight attendant, another flight attendant on her last flight before retirement, a young couple that meet a gruesome death as punishment for dabbling in Sex & Drugs, and a snobbish passenger that meets an even more gruesome death as punishment for being obnoxious (among many others -I mean, both clichés and gruesome deaths). The plot is full of holes, the dialogue is predictable, the jokes are unfunny, the snakes are unrealistic (but still scary enough to not make the film an overt parody, something it never turns into) and even the shooting seems badly edited at many moments, especially in the scenes added by pressure of the Internet fans (like Jackson's already classic line "I've had it with these motherf**king snakes on this motherf**king plane!") . Everything is carefully crafted to look and feel like the Platonic archetype of the cheesy class B movie, and succeeds in showing almost no sign of postmodern self-awareness; of course it is really full of it, and that is the joke, but it is not obvious from how the movie looks -it is only obvious considering the cultural context in which it is aired and the unique publicity camapign behind it. If I saw the same movie on TV, and subtracting the actors and modern cultural and technological references, I would easily believe it to be a honest, really bad class B action movie from the early 80s.

And there is where the aesthetic problem lies. The same film, produced in 1981 is an awfully bad one, and produced a quarter-century later and with the contribution of Internet fans is an excellent one. But shouldn't the criticism of a work of art consider it intrinsically, without reference to the circumstances of its creation?

Leaving that question for you to ponder, I conclude with a advice: Don't even think of going to see this movie to the cinema alone, or even worse, seeing it later on DVD completely alone. You should go watch it with all your geeky friends -preferrably, after "getting in the mood" by talking days or weeks about what an inspired, supremely silly premise for a film this is, and discussing endless variations on its title (Kangaroos on a transatlantic? Tarantulas on the Popemobile? You get the idea.) And then, when you see the film all together, I bet you will not be able to resist laughing at every scene starting from the titles. Nor will you want to.

August blogroll update

New links that you can find on the sidebar:


The n-Category Café, group blog starring John Baez, Dave Corfield and Urs Schreiber, which according to Jacques Distler in the introductory post "will focus on that heady interface between Physics, Mathematics and Philosophy"

Asymptotia, solo blog starring Clifford Johnson where he can post all the things he wants without worrying about unbalancing the group blog at Cosmic Variance.

Conscious Entities, an excellent blog focused on philosophy of mind.

A couple of blogs "of general interest" which I find myself checking often enough to merit inclusion: Majikthise, and Obsidian Wings. The latter has some invaluable discussions on the Israel-Lebanon conflict, over which I have been brooding a lot recently (but not enough to dare posting about it as Scott did!)

One more of my friends has started a blog: Alejo's Remando el Tsunami.

Two sites for the "Other" category: the Slate online magazine, and Autoliniers, which features daily comic strips by Argentinian humorist Liniers. (Well yes this last one is a blog, but it didn't fit any preexisting category and I wasn't going to create one for it. Any problems with that?)

A Bemused Comment on the Pitfalls of Scientific Nomenclature

A bit of news that has appeared recently has set me thinking about how human thought in general, and science in particular, need to divide the world into categories according to concepts, and how the ultimately pragmatic nature of those concepts may backfire, when what seems to be a useful name for a kind becomes a bad one and unexpected problems arise...

Eh? You think I am talking about the polemic about the status of Pluto and our brand-new definition of "planet"? No, why would I get into that topic that has been beaten to death already? (I will only say that minutes ago chating with a friend on MSN I asked him if he knew our solar system had 12 planes now, and when he said he didn't I said "yes, they discovered 3 more made of dark matter which is why they hadn't noticed them before". I think he believed it for half a second.) I want to talk about a much humbler thing that planets -beetles.

There is a species of beetle which is being hunted down to extintion. This is despite the fact that there are 41 other species in the same genus, with only minor differences that could be of interest only to a very dedicated entomologist. The beetle is being hunted just for having the name it has.

Which is? Anophthalmus hitleri

Named to honour the Führer himself by the German collector who discovered it in 1933, and victim of the unbreakable rules of taxonomy which enshrine the name first given to the species. It is not being hunted down by odium towards the Nazis, but (depressingly) by the opposite: neo-Nazi sympathizers are willing to pay a large amount of money for them and use them as symbols.

Spanish-speaking readers can find more information in the Clarín article that prompted this post; English-speakers could go, as usually, to Wikipedia.

As Oscar Wilde put it...

"All that I desire to point out is the general principle that Life imitates Art far more than Art imitates Life, and I feel sure that if you think seriously about it you will find that it is true. Life holds the mirror up to Art, and either reproduces some strange type imagined by painter or sculptor, or realises in fact what has been dreamed in fiction." (The Decay of Lying).




















From a story in the Argentinian newspaper Clarin (my translation and emphasis):

A man has 37 children and asks for an operation to not have any more

His name is Cleto Ruiz Díaz, he is 44 years old and lives in Corrientes. He has no permanent job. He has so many children he cannot remember their names. He wants to get a vasectomy to eliminate his reproductive capacity.

The story does not include the children's names, but who can have any doubt that they were Tiffany, Heather, Cody, Dylan, Dermot, Jordan, Taylor, Brittany, Wesley, Rumer, Scout, Cassidy, Zoe, Chloe, Max, Hunter, Kendall, Caitlin, Noah, Sasha, Morgan, Kyra, Ian, Lauren, Q-bert, and Phil? At least some of them.



(Hat tip goes to my cynical friend, who told me about the article without posting it but insisted on being credited anyway )

Some Google searches leading to this blog

The power of Sitemeter uncovers the following queries bringing people here:


the earth's are really rotate?

Yes, it really rotates, and you need remedial classes in grammar as well as astronomy.


cats legal responsibility

I don't think they have any.


Why does the earth turn around the sun?

Because the Schwarzschild metric has geodesics whose spatial projection are closed curves, to a good approximation.


dembski evil

Well, I do think the guy is an anti-science propagandist who spreads lies and confusion about evolution and tries to damage educational standards to favour his religious views... but "evil" is perhaps too strong a word, don't you think?


weirdest google searches

I get a surprising number of searches for this. People must be really bored.

Big Bang =/= Creation!

A recent interview on Francis Collins, renowned biologist and leader of the Human Genome Project, has been much discussed around the blogosphere for its statements on the relation of science to religion. Collins is an outspoken evangelican Christian and, though keeping a clear distance from ID pseudoscientists, he does see his scientific work as leading to God, in a way that has attracted the expected scathing criticism by PZ Myers and excellent comments by Amanda Marcotte.

Both PZ and Amanda point out the double standard that Collins uses: against both creationists and atheists like Richard Dawkins he says that science can never disprove the existence of a trascendent God that does not intervene in the Universe but created it entirely, so science and evolution are compatible with belief in God (something I have argued for extensively) but next he contradicts his premise in two different ways: first he tries to argue for the existence of God from scientific facts such as the Big Bang, and second he states his belief in miracles such as the Virgin Birth and the Resurrection. The first involves a double standard in that if Collins can take God as a scientific hypothesis to explain particular aspects of the universe, then Dawkins and other atheists are allowed to do the same, and by all scientific standards the God hypothesis comes out as a terrible one, untestable and explaining everything and nothing. That is why I think fideism is practically the only consistent position for a religious scientist. The second involves a double standard in that Collins has (rightly) ruled out miraculous divine intervention as a viable explanation in biology, but now allows it in matters of Biblical history just because he regards them as more central to his faith. If biologists studying evolution must rely on methodological naturalism and reject divine intervention as explanation for biological facts, then Bible scholars and historians studying the life of Jesus must also rely on methodological naturalism and reject claims of miracles. Fair is fair.

I want signal out for criticism the way Collins uses the Big Bang in support of theism, first because neither PZ nor Amanda deal with it, and second because it will allow me to clear several misconceptions the general public has about the theory. Collins says:

First of all, we have this very solid conclusion that the universe had an origin, the Big Bang. Fifteen billion years ago, the universe began with an unimaginably bright flash of energy from an infinitesimally small point. That implies that before that, there was nothing. I can't imagine how nature, in this case the universe, could have created itself. And the very fact that the universe had a beginning implies that someone was able to begin it. And it seems to me that had to be outside of nature. And that sounds like God.

The Big Bang is most definitely not an unimaginably bright flash of energy from an infinitesimally small point. As remarked by the Angry Astronomer in his much-linked to post on a similar topic, it is more like an expansion that an explosion. The universe is spatially infinite according to the standard contemporary theory, so it did not start form one very small point and expand from it. Instead, it started from one very dense state, and it became less dense by space itself expanding, not by matter spreading apart. The best analogy I know of is an infinite chessboard, in which the size of the squares doubles at a fixed rate. Then, imagining a particle of matter at the centre of each square, all the particles are moving away from each other and each one sees all the others moving away from itself. In the past particles were much densely packed together, but the universe itself was always infinite, not smaller -though any specific finite region within the universe was indeed smaller in the past, and started off from a point, at least in this idealized model.

This, however, is a side issue of little relevance for the religious argument. (Of even less relevance, though I still can't resist remarking it, is that radiation "decoupled" from matter only 300,000 years after the Big Bang, so there was no "free" light around in the primordial universe. So much for "Let light be made"!) The main point I want to make is that the Big Bang cannot be identified with Creation, and that the Big Bang theory does not give any special support to the notion of divine creation (although it does not contradict it).

First, from the philosophical point of view, since God is supposed to be outside time, I can't see why a universe that has started in time squares better with a Creator than a universe that has existed forever. An eternal and omnipotent God could surely have created a universe with an infinite time span as easily as one with a finite one. But of course what those who equate the Big Bang with Creation are looking for is not any Creation but one that resembles, even in the most vague possible way, the narration of Genesis. Well, let's move on.

The second and main point is that even though the model we use for the present universe, extrapolated backwards, does lead to a "t=0" instant where the universre started, we know that the model cannot be trusted for very early times. When the density of matter becomes so large as to approach the so-called Planck density (more than 10^96 kilograms per cube meter!) then our present theories cannot make reliable predictions anymore, and some yet-undeveloped theory of quantum gravity would be necessary. We do not know what such theory would imply for times earlier than those we can reach through our present theories. It might be that the universe was more or less stationary in such a dense state until a random quantum fluctuation provoked an expansion. Or it might be that a previous large universe collapsed to the Plack density and then "bounced", as some (preliminary!) results in the "Loop Quantum Cosmology" approach suggest. The only thing most physicists agree on is that physics does not "end" at the Big Bang, as in the naive model many people have and imagine that fits well with their religion. Beyond the Big Bang, or rather, beyond the Planck density scale (as there is no real event we can call the Big Bang, aside form the general fact of a very dense past) there is not God but just presently unknown physics.

An analogy: imagine a cannonball being fired; after it leaves the cannon, it is in a fixed parabolic trajectory (neglecting friction). Intelligent ants that appeared in the ball a while after it left the cannon could calculate the parabolic trajectory, and conclude that the cannonball had been touching the ground a definite and calculable time ago. However, this would be a wrong conclusion; in the first moments of the trajectory, the ball was inside the cannon, constrained by it to move in a straight line, and being moved not only by gravity and inertia but also by the pressure of the exploding gases. Different physics took over in the first instants, in other word, though the simple physics of parabolic motion is a correct and excellent description for the part of the motion the ants have experienced. If for a philosophical ant it is a matter of great importance whether the ball has ever been in touch with the ground, however, it should refrain to conclude that it had just because the parabolic motion leads to that inference. It should study the physics of cannons to discover what the ball had been doing at times when the simple theory breaks down. And it would discover the the ball was initially not in touch with the ground, but with the bottom of the cannon's inside.

Using the Big Bang to argue for God, is, in effect, a straightforward example of the God of the Gaps fallacy. It is not recognising our present knowledge to be limited and expandable with hard work, and lazily calling up God to replace the part we don't yet understand. It is a testimony to the power of belief to overcome reason that Collins, who evidently recognizes the fallacy when IDers apply it in his area of expertise, applies is so carelessly himself in a different area.

Report on MG11

The Marcel Grossmann Meeting is a huge conference.

A really, really huge one.


Number of plenary talks: 35 (in 6 mornings)

Number of parallel sessions: about 20 in each of 4 afteroons, that is, about 80.

Number of talks per parallel session: about 10 or 12. Total number of talks, then: about 850.

Total number of registered participants: 957.

Number of parallel sessions one was interested in attending in each afteroon: about 4 or 5.

Possibilities of doing this: 0.

Percentage of talks attended that actually included interesting or useful information: about 30%. Percentage of the talks spent sleeping, or trying to decide whether to move to another room or to give the next speaker in this room a chance: 70%.


OK, that was the easy part of the report. Now comes the tough part: an actual day by day report of that 30% of interesting things. (I know nobody forces me to write it, but I feel not doing it would be betraying my Gentle Readers). So:


Monday 24/7

Morning: Opening remarks by Remo Ruffinni et. al. Roy Kerr receives a Marcel Grossmann Award, which is an artistic work inspired in, coincidentally, geodesics on the Kerr metric. More awards given, more opening remarks. Coffee break in which I somehow find myself, with some friends, getting a photo with Kerr himself:

Also, the discovery is made that no food is served with the coffee, which begs the question of where has all the money of registration fees gone to. (Yes, I know that "begs the question" does not really mean that).

Later in the morning: Talks by Thibault Damour and Hermann Nicolai on hidden symmetries in near-singularity (string theory-based) cosmology. Hard stuff. Also: Polyakov gives a nice introduction to basic issues in string theory. The main message of his talk is that the gravitational field is not fundamental, but a mean field generated by completely different (Yang-Mills) objects, according to the string-gauge duality.

Afternoon: Time spent switching between parallel sessions on LQG, on Cosmological Singularities, and on Quantum Fields. Probably I should have sticked to the latter, which is my actual area of research, but there was a second QFT session on Tuesday and I had to go to that one becasue my talk was scheduled for it, so I decided to try some other stuff on Monday. I should say that moving between rooms was not trivial as they where distributed among more than ten buildings, and it could take up to ten minutes to go from one to another.

I started in the LQG session, where Thomas Thiemann opened giving a standard introduction to the theory, stressing his own Master Constraint approach to the Hamiltonian constraint. He said that the Hamiltonian constraint in LQG has nothing to do with "generation of time translations" and that misunderstanding of this creates the artificial "problem of time"; according to him, it is possible to define an operator H which is a Dirac observable and generates physical time translations, but is unrelated to the Hamiltonian constraint. I didn't get the reference he mentioned for this; does anyone know it? He also talked a bit about the results in LQC (loop quantum cosmology) where it is proven that for coherent states peaked on the classical FRW solution the expected value of the curvature operator is finite, even if the operator itself is not bounded. The next talks in this session where very technical and quite over my head, making me regret not having moved to the QFT room earlier. I passed by the Cosmological Singularities room where a friend gave a nice and understandable talk on the validity of different energy conditions for various kinds of cosmological "events" (Big Bang, Crunch, Rip, Bounce, etc.)

After the afternoon coffee break I went to the Quantum Fields session, but I was already very tired and my notes are scarce. The talk I have more notes on was one by Deborah Konkowski on whether quantum mechanics "heals" singularities, in the sense that, for a classical singular background, the evolution of a quantum wave packet is well defined without adding special boundary conditions at the singularity. It seems that for a rather large class of singularities, it is.


Tuesday 25/7

Morning: More talks on string theory and cosmological singularities. To bad I can understand so little of such a hot topic.

Later: Joe Polchinski talks about cosmic superstrings. Explaining that a cusp in them can provoke a burst of gravitational waves like the crack of a whip, he takes out a whip and proceeds to demonstrate his words, Indiana Jones-wise. Laughs in the audience.

Later: Abhay Ashtekar gives yet another standard introduction to LQG, including a list of FAQs at the end which I wrote down completely and include here, because they will sure be of interest to many readers:

• Q: Shouldn't Einstein equations have quantum corrections? A: Yes, and they do; this is clearly visible in LQC where the Friedmann equation gets corrected in hte semicalssical regime.

• Q: Aren't there quantization ambiguities in LQG, which reflect the ambiguities in the perturbative treatment due to non-renormalizability? A: Yes, there are ambiguities and they are being worked upon; but they just reflect ignorance about the proper method of quantizing in background-independent contexts. They are not the same as those in the perturbative treatment; those arise for inadequacy of the Gaussian fixed point, and there is evidence that a non-Gaussian fixed point exists. (This is a common criticism of LQG I find in Lubos' and Jacques' blogs, and I suspect they would not be satisfied by this answer. Is it wishful thinking or is there solid evidence for it?)

• Q: If the Hamiltonian constraint has not been solved, does this not mean there has been no progress? A: By the same token there has been no progress in reaching a non-perturbative formulation of string theory. (Tu quoque!)

• Q: Isn't LQC too restrictive because of symmetry assumptions? A: Yes, of course. Current work is being done on inhomogeneities; LQC provides useful heuristic intuition.

• Q: In black hole entropy, what is the justification for assuming a Boltzmann statistics for the punctures? A: Misconception! No such assumption is made. The horizon states are counted exactly. Counting punctures with a Boltzmann statistic is an pedagogical presentation device.

Unfortunately, if the intention of the talk was to provoke a fruitful debate between stringers and LQGers, this didn't come to pass. There was only one question of little interest, and then time constraints forced to move on to the next talk.

Afternoon: My talk was scheduled for 16:00 on the Quantum Fields session, so I headed for there after lunch. Nothing worth mentioning in the talks previous to mine. Two of those speakers didn't appear so my talk was moved 40 minutes earlier, casuing several friends who were coming to the session just for it to miss it. The talk itself went fine and there were no challenging or enlightning questions afterwards.

After coffee break I passed quickly by the Geometric Calculus session; this is a kind of mathematical language for physics made famous by David Hestenes, though it goes back to Clifford, and which I found intriguing since reading this online introduction. There was a talk on that session on application of GC to Rindler observers; the notes I took there will possibly be of use to my work. I went later to the LQG room, where Stephen Zhoren gave an interesting talk on entropy in causal set theory. He has shown that counting the states of a causal set within a given volume gives, in the continuum limit, an entropy proportional to the area (consistent with the holographic bound) in any number of dimensions. The coefficent depends on a numerical parameter (the ratio of the discreteness scale to the Plack length) which is fixed by the requirement S=A/4 and is universal for any volume in a given dimension, but depends on the dimension. However, d going to infinity, the coefficient approaches 1! I have no idea if this has a deep physical meaning, but it sure looked cool.

In the evening we had a popular lecture on "The Fate of the Universe". Lots of cool animations of structure formation. Quote of the day: "In 10^27 years, white dwarfs destroy dark matter". If it doesn't strike you as funny, repeat it to a friend innocent of astrophysics.


Wednesday 26/7

There were talks on the morning on gravitational wave detectors. Unfortunately from now on the lights in the plenary room were turned off during the talks (this was done to reduce the heat) and so I didn't take notes for them. I will start writing directly on the afternoon sessions each day from now on.

Oh, but on Wednesday we had the afternoon free. In the evening we had a conference banquet in the Ritz hotel, which answers at last where all the money that couldn't be spent in biscuits for the coffe breaks went. It was perhaps the poshest dinner I ever attended to, although an English friend said the exact word is not "posh" but "swanky", a term I was ignorant of before.


Thursday 27/7

I started the afternoon in the "Analog Models of and for GR" session, which began with a good talk by Grigory Volovik. This talk first introduced analog (sonic) models for black holes, and then launched a polemical attack against the "cosmological constant problem". According to Volovik, a condensed matter analogy suggests that the natural value for the vacuum energy density of the universe is not the Planxk scale as usually said, but the scale of the matter energy as in fact observed. The reason is that microscopic degrees of freedom (atomic, in the condensed matter case) store energy which cancels the huge value found by cutting off the calculation at the Planck scale (which would be the interatomic distance in the analogy). The argument is developed in this paper. I suspect high-energy physicists would be more convinced if Volovik could provide a concrete microscopical model for vacuum spacetime, instead of just relying on the analogy. Interesting stuff though.

I turned later to the Quantum Gravity Phenomenology session, chaired by Giovanni Amelino-Camelia. The first talk I heard there was by Ralf Lehnert, who sketched a comprehensive strategy for analysing Lorentz and CPT invariance violations in beyond Standard Model physics. There are two kinds of these violations: a kinematical one, with modified transformations between inertial frames alla DSR, and a nontrivial vacuum that picks a preferred frame (dynamical violation; e.g. a fundamental vector field). In this second case passive but not active Lorentz invariance is preserved; changing coordinates between frames does not alter the equations, but rotating an experiment gives different results. Without clear input from fundamental theories on what to expect, the strategy suggested by Ralf was to write the most general Lagrangian which corrects the SM by terms of the form (tensorial background) x (SM field), assume the background is constant at low energies, and study which terms thus generated can be experimentally probed and constrained.

A second interesting talk in the same sesssion was by Jerzy Kowalski-Glikman on DSR. According to Jerzy, the best way to think of DSR is not as "a deformation of SR with two observer-independent scales" (a formulation which makes physical interpretation obscure) but as "an effective theory of particles coupled to gravity, when spacetime is flat and local degrees of freedom of gravity are switched off". Thus, the physical interpretation must be imported from a theory of gravity. In 3D QG coupled to particles it is already proven that when the (topological) gravitational degrees of freedom are integrated out then the net result is that the energy-momentum space for the particles becomes curved: de Sitter, in fact. In the effective theory that results positions are non-commutative. In 4D gravity can be expressed as a topological theory + constraints, the latter enclosing the local degrees of freedom, which vanish in the DSR limit. There is thus reason to hope that some kind of deformed particle kinematics comes out in the flat limit, but it's precise form is yet unknown.

There was a third interesting talk in the same session, by Tomasz Konopka, who works at Perimeter and whom I knew from the Loops 05 conference. He suggested a "5D" approach to 4D DSR: embedd the momentum space of a particle in a flat 5D space where it is represented by the de Sitter hyperboloid with scale parameter k, and add the constraint m^2 = P^2 where P is the momentum four-vector. Thus the value of the "extra" momentum coordinate P4 is constrained to be P4^2 = m^2+k^2, which makes it different for each species of particle. Tomasz discussed physical consequences of this model when applied to QED. The audience seemed a bit skeptical about this, because he did not find a Lagrangian for this theory but just postulate reasonable-looking Feynman rules. Which, moreover, required (for consistency with observation) that the deformation constant k be different for each species of particle, and in particular zero for the photon. Hum...


Friday 28/7

Started in the Black Hole Thermodynamics session, chaired and opened by Don Page. He made a general introduction to the field and then discussed the particular problem of the statistical mechanics of extremal black holes. He talked really fast, so after a while I stop trying to take notes. I can only offer the following quote (about how to distinguish between some two possibilities): "Just wait 10^837 years and see whether the radiation has stopped. Unfortunately, this is slightly longer than the time over which a PhD student generally hopes to complete a thesis".

An interesting talk in the session was by Paul Anderson, who is a specialist in stress-energy of quantum fields in black hole spacetimes. He discussed how for many cases the usual analytical approximations show a divergence in the horizon, which if physical ought to alter significantly the geometry via backreaction -but is it physical? In most cases bumerical calculations don't show it. After the talk, during the coffee break, I discussed with him a bit the results of my first degree thesis project (published here), which showed very different results for vacuum energy of massless scalar field around stars than for black holes, and to my surprise he was aware of the paper and had even discussed it with Bob Wald. It seems that work of him of which I was not aware (almost contemporary with our paper) shows that the vacuum energy around black holes is after all similar to the one we found in stars, not different as previous calculations had shown... but there are intriguing discrepancies for all values of the parameter that couples the field to the curvature other than the minimal one (zero) and the conformal one (1/6). So he is thinking carefully about these issues now. It's nice to see that my old work has been noticed -after two years with zero citations I was feeling a bit discouraged.

And this was about it, insofar as my notes cover -in the rest of Friday I chose badly my sessions and talks and did not hear much of interest. Thus ends my extra-long report on hte conference. How many of you have read it complete?

Oh, and one word of advice for those traveling to Berlin: Don't let tourism guides fool you into making the one hour long queue for enetering the Reichstag. The Schwarzschild geometry-like mirror structure inside the dome (seen below), though nice, is not worth the waiting.

Two quick links and promises

Don't miss Sean Carroll's post on Boltzmann and the arrow of time (which ovelaps which a post I promised some time ago but never got around to write, my review of Huw's Price book on the philosophy of time) or Bee's review of Lee Smolin's The Trouble with Physics (which ovelaps with a post I am promising now to make, reviewing the same book... after reading it, which is bound to happen soon).

Back to the Blogging: London pictures

The promised report on the Marcel Grossmann conference I was in last week is not completed yet. (I am making it really, really thorough). I expect to post it tomorrow, or the day after if I get delayed. On other news, after the conference I went two days to London to meet up with Jonás, an Argentinian friend who is traveling around Europe. Here you can see us in Kew Gardens:




Later, strolling by an area near Baker Street, we found just by chance this house where José de San Martín, the foremost Argentine national hero, has appearently lived:




Of course, giving that I lean towards philosophical rationalism more than towards militant nationalism, this discovery was (though amusing) far less exciting than the one I made in my first visit to London almost two years ago. Just a couple of hours after arriving, I found myself by chance staring at Bertrand Russell's house!



And really, what more could you ever ask from a city?