Thursday, April 24, 2008

Some physics news

Because of the limited target audience, I am not going to spend too much time with refining formal aspects of this posting.

Dmitry wrote some nice texts about some serious topics. Let me start with them.

Dark matter causes global warming

I needed a catchy title :-) that will be explained soon. As Dmitry writes and other people in the media and blogs have mentioned, the DAMA/LIBRA collaboration (Dama stands for Dark Matter much like Lumo stands for Luboš Motl haha) claims nothing less than an eight-sigma ("extremely certain") discovery of the particles of dark matter.

These experimenters use scintillators with a lot of pure NaI(Tl); future projects should be based on Xenon, with added amounts of Xe 129 and Xe 136. And they study how much these gadgets scintillate ;-) during the year. It turns out that there is a modulation of the signal, i.e. a dependence on seasons, that was measured and confirmed during 7 years of work. Now, the key statement is that they can't imagine any other explanation than the interaction with the dark matter particles - a halo in our Galaxy. It follows, they say, that what they see is dark matter interacting with their scintillator.

Well, pretty convincing but not quite. What they see are some ill-understood and therefore - almost by definition - dark matters. ;-) But whether these are the same dark matters as the dominant particle of dark matter in cosmology remains to be seen. I chose the title involving global warming because their argument seems somewhat similar to the argument that we can't fully explain the 20th century global warming by the first natural model we write down so it must be caused by the first "unnatural" but convenient effect we think about, namely the greenhouse effect.

But there could exist other justifications of my title, too. If the "dark matter wind" can modulate their scintillations, why couldn't it modulate the Earth's climate? You know, there are places with a lot of dark matter and places with less dark matter and these particles could do something dark to the climate system. OK, let me emphasize that I am mostly kidding.

Because their interpretation is consistent with various constraints, I would estimate the probability that their experiment is evidence of dark matter to be 25%. It's a lot but it's not enough to establish the interpretation without an experiment with a more characteristic, dark-matter-like signal. But you shouldn't be shocked by my high number. The existence of dark matter is less mysterious and uncertain than the adjective indicates.

NSR superstring amplitudes

Another NeqNET posting talks about Morozov's article we saw a few days ago.

It reviews the current progress in multiloop Neveu-Schwarz-Ramond amplitudes - progress that we discussed in the context of four-loop amplitudes. Morozov puts the progress into the historical perspective - he tries to explain what are the real technical reasons why these pretty "straightforward" procedures and possibilities were overlooked 20 years ago ("straightforward" for the ultra-experts who follow the details only).

Well, these are truly delicate subtleties - but even more important could have been the wrong idea that it is "unlikely" that a special solution to a mathematical problem exists. Sometimes, special and unexpected solutions do exist and are important. The whole structure of string theory is an example.

I could also recommend you some of Dmitry's lectures on de Sitter space and inflation from a hydrodynamical viewpoint. But let us look elsewhere.

Black hole information paradoxes

A well-known notorious critic of theoretical physics wrote his 680th hostile rant against theoretical physics in general and string theory in particular. What an anniversary. ;-)

As a reader has pointed out, Peter Woit used an idiosyncratic interpretation of a Hollywood scene and a bitter blogging economist as the main sources of information about the validity of string theory. ;-/ Those who have known Woit for quite some time will fail to be surprised. I am amazed by the people who deliberately keep on opening the pile of manure called Not Even Wrong - it must be due to a really nasty deviation of theirs that dwarves pedophilia. Fortunately, the number of pageviews of Not Even Wrong is now well below the numbers of TRF.

The owner must be disappointed that some physicists have used the space for a meaningful discussion about the black hole information puzzle. They even managed to explain the correct key answer: while we might be dissatisfied with our "local" understanding "how" the information gets out of the black hole, we know that it does. The unitary AdS/CFT correspondence and Matrix theory are enough to give us the correct answer.

The research in these frameworks has shown us that the process is as messy as burning books and the information gets scrambled. The idea that the process is easy and transparent is the very wrong assumption that underlies a wrong alternative answer - the answer that the information is lost because there doesn't seem to be any "easy" information in the black hole, right? Wrong. Black holes have a lot of information inside - the maximum amount of information you can squeeze into the same volume.

And because the basic properties of black holes are local in nature, this qualitative conclusion is almost certainly universal. The information is always preserved. For example, black holes in a flat space almost certainly behave similarly to black holes in a nearly flat anti de Sitter space. Moshe R, David B, and to some extent Aaron B have explained these things to Peter Shor which doesn't prove that the latter has understood it. Moshe has even politely expected Peter Shor as a quantum information expert to tell us something relevant about the black holes but he has probably confused Peter Shor with John Preskill who has something to say. Peter Shor is just a Peter Woit lite, a producer of mentally retarded, un-thoughtful attacks that can only be helpful to those who like to eat dinosaur manure.

But the defeat of the aggressive people by sane people at Not Even Wrong is a good news. Even if you're a cute innocent bunny facing a venomous snake such as Peter Woit, you may deal with it if you grab it from the proper end:



"Proving" typicality

Today, the arXiv contains a paper by Don Page who thinks that he has "derived" typicality, i.e. essentially proved the anthropic principle. Of course that the paper is complete nonsense. He essentially assumes that all possible answers to a question always form a complete set, much like microstates in a thermal equilibrium, and are thus equally likely. With these assumptions, after a few pages, he is able to prove that all possible answers to a question are equally likely.

Prof Page, well, your assumptions are wrong. Different values of quantities are not equally likely and things that are not microstates in thermal equilibrium usually don't behave as microstates in thermal equilibrium. That's why most alternatives in Nature have different probabilities - sometimes very different. And that's why sets of macroscopic observables never form a complete set of quantum observables to describe a system. Whoever has used Page's wrong assumption is nearly guaranteed to have ended up with wrong answers. Proper science never depends on the wrong assumption.

Page also uses the unusually mad "result" by Bousso, Freivogel, and Yang that we discussed in a text about Bayesian inference. Recall that these three otherwise smart guys argued that the anthropic principle is a key pillar of all of science. Their statement was based on their inability - or perhaps a lack of will - to distinguish the sentence "a particular detector or planet leads to a particular outcome" from the sentence "at least one detector or one planet in the Universe leads to a particular outcome".

Well, with these assumptions and (deliberate?) mistakes, one can't be surprised that Page's paper is just another example of the GIGO rule: garbage in, garbage out.

On the origin of time and the Universe

The second paper in the hep-th list seems obscure, too. It is about the arrow of time and is based on similar absurd assumptions as the previous paper by Page. Jejjala and 3 otherwise sane collaborators propose a "novel solution to the low [initial] entropy puzzle". Well, the low entropy of the Universe is not a puzzle, it is an experimental fact and a simple theoretical consequence of the second law of thermodynamics. What does it mean to "solve" a fact is not quite clear to me. It is like the "fight against climate change" or the fight against other laws of Nature and other facts.

Clichés about solutions sound mysterious but their paper doesn't. It is one incorrect sentence followed by another. For example, they implicitly use Page's wrong "microstate" assumptions and argue that the probability of the Big Bang is 1/10^{10^{123}}. A pretty small number, indeed. Write 0.0000 ... now add 10^{123} zeroes (this is 1,000,000 ... 000 zeroes) ... and finally you can write "1" or anything you like. Well, because I estimated the probability of the Big Bang to be 99.9%, you shouldn't be surprised that I don't consider their paper to be compatible with my knowledge of the Universe, especially if their error is more than obvious.

It is simply not true that all possible initial microstates of the Universe are equally (or comparably) likely. The people who claim that they are equally likely not only have zero bits of evidence but they also face a virtually infinite amount of counter-evidence.

These particular authors eventually "solve" their "problem" by combining Matrix theory with freezing by (global) warming, dissipative dynamical systems, and the Fubini-Study metric from a mathematical paper by Mumford and Franziska Michor's father. Great, the entropy is low and the temperature was high and one can see similar bizarre systems in math. And what? How could one possibly say anything more substantial about it without having an explicit formula for the initial state?

OK, I don't believe that anything valuable can arise from this colorful combination of ideas, especially if the very "problem" that they try to "solve" is just their idiosyncratic, psychological problem. So despite my admiration for Djordje Minic, I would put the paper to the Lee Smolin category.

M5 from M2

The paper by Ho and Matsuo is more material. It is a part of the membrane minirevolution. They study the "classical limit" of the large 3-algebras that involves the Nambu-Poisson bracket. What is it? Well, recall how the membrane Hamiltonian is derived from the D0-brane Hamiltonian in Matrix theory. The commutator becomes a Poisson bracket and you get two new dimensions.

Because the Bagger-Lambert-Gustavsson Lagrangian has 3-brackets, you will analogously get three new dimensions and many M2-branes (with a large 3-algebra) become an M5-brane. Ho and Matsuo can even derive the self-dual 3-form field strength in the M5-brane worldvolume as various objects in the 2+1-dimensional theory or their Hodge duals. They explain why M5-brane is the only new object one can get in this way: it boils down to the highly constraining mutated Jacobi ("fundamental") identity.

Inflation

Yang and Ma construct something that could be called a highly realistic, garden-variety model of inflation in string theory. It gives a GUT-like inflation scale, scale-invariant spectrum with n=0.96 or so, and other good things, starting with a manifold with moderate values of the Hodge numbers.

Enhanced near-horizon symmetry

Mike Duff argues that the symmetry of the near-horizon geometry of the heterotic string gets interestingly enhanced to OSp(2|8) - and he might really mean OSp(8|2) which is not the same thing - and how this enhanced symmetry can solve a 21-year-old paradox in SUGRA solutions, one whose full importance is probably fully comprehensible only to those who worked on it 21 years ago. ;-)
See a table of the symmetries for various d,D
There are also other papers about the integrability of N=4 SYM, integrability of some spin chains, integrability of noncommutative chiral non-linear sigma models, SUSY breaking in SUGRA, actions with non-polynomial functions of curvature f(R), Coulomb scattering in de Sitter space, scale-invariant Liouville-like theories that are not conformally invariant, Casimir energy from inhomogeneities, and some kinematics and geometry of the Rindler space.

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