Cheers,

-cvj

P.S. There might have been some discussion of the species issue in Starinets’ talk at the conference… in response to a question… I do not know if the microphones picked up the discussion, but it is worth listening to find out. If you can decipher the content of the discussion (which I admit that I do not recall), feel free to come back here and mention what you found. Might be a good launching point for ideas or discussion.

They are equivalent to having a very large number of species. In some of the counter-examples, there are explicitly a very large number of species, while in another variant there is a rather weird inter-particle potential that has the effect of producing a very large entropy by allowing a very large number of very long-lived resonant states.

This line of work by the string community (and others) is very very exciting, and really cool mathematical physics, and I hope the quantitative nature of the universality of the results will be better understood soon!

Aleksey

I think that these are all interesting questions that nobody has answers to yet. There is an exciting feeling of adventure here…. people are experimenting with many examples, and patterns are beginning to emerge. I suspect that a whole new definition of “universality” will apply here - not the one relevant in critical phenomena. I wish I had more answers, but on the other hand, I don’t think anyone does.

I honestly don’t know what to think about the contrived counterexamples to the proposed bound of Kovtun et al. Do the examples preserve unitarity, by the way? I’ve not read the papers to really have a good handle on this - is this equivalent to an extraordinarily large number of species?

Sorry to be not so useful…

Cheers,

-cvj

I’ve a couple of questions about the universality class argument that is given to support applying the stringy results to QCD (and other strongly-coupled systems, as other people have tried to do.) I’ve previously thought that in the statistical mechanics context, systems in the same universality class have the same critical exponents - but do not necessarily share other properties. Why would one expect transport coeffiecients to match? Is this wrong?

I am also curious just how wide this universality class that (might) include supersymmetric field theories with gravity duals and QCD should be taken to be. For instance, it’s been shown by Kovtun, Son, and Starinets and others that \eta/s >= 1/4\pi in a certain class of field theories with gravity duals, with the bound being saturated when the ‘t Hooft coupling is infinite. This has then been conjectured to be true more broadly (that is, that the bound is ‘universal’). That is, it’s been conjectured that this bound born in a stringy context applies to _all_ fluids, or perhaps just those described by sensible quantum field theories. (This would include the sQGP that’s produced at RHIC.)

This would be extremely exciting if it were true, but at least in some cases (hep-th/0702136) it is possible to construct (contrived) counter-example systems that violate the bound by essentially having a large entropy s. So it’s not clear in what _precise_ sense such a bound is universal - that is, it’s not clear to what the bound might apply outside of the setting where it was derived.

As a disclaimer, Tom Cohen, whose paper I cited above, is my advisor here at UMD, and we just finished a paper on this issue.

Aleksey Cherman

The use of Mathematica, Maple, and other such packages along side the usual techniques (pen and paper and so forth) is quite routine in lots of areas of physics research now. They are especially useful for automating a long and tedious computation that you might have done the first time by hand to get right, but then want to run several times with different starting points, etc. You change the first line, run it again at the click of a button. Also, their use has allowed tedious computations that would take years of chugging away to be done in very short times, allowing amazing strides to be made in many areas of research.

Make no mistake… they are not a substitute for thinking. you have to know what your doing or you’ll just get garbage out for the garbage put in. Yes, that is still true, and always will be.

Best,

-cvj

Now that you have come full circle, do you still do it ‘by hand’, or surrender the greater part of the toil to a deus ex machina like Maple? And is the assistance provided by computer algebra widespread in string and other theories these days?

Actually we use it all the time, especially exactly the same notions from statistical physics. In fact, most of the traditional formulation of string theory itself is based upon conformal field theories (in a certain gauge, consistency tells you that the background in which the string is propagating is consistent if you have a conformal field theory)… and you’ve met those theories before yourself, probably… they exist precisely at transition points in various statistical models, where the universal physics lives. This is one of the really nice things about this field, it gets one to learn a whole lot of physics from all sorts of wonderful sources… and field theory and stat mech and all that good stuff are such fundamental concepts in doing string theory.

Cheers,

-cvj

-cvj