Friday, 19 December 2008

UK Annual Theory Meeting, day 1.

Neil Lambert --- Multiple Brane Dynamics: D-branes to M-branes.

Apparently string theory needs promoting again. Presumably at the behest of the organisers, the talk began with a 10 minute introduction to string theory which would not have taxed a PhD student. He began with the usual force unification picture, some trouser diagrams, and then ending with the string dualities and the hints of M-theory. All very old stuff.

There were some strange, to be honest sloppy, statements, such as the existence of 10 dimension being the "great prediction" of string theory -- this is clearly wrong. First, it's not a prediction of string theory, it's a requirement, and secondly, if we discover there are 6 hidden dimensions it neither proves nor disproves string theory. Sadly, no-one called him on this. Sigh.

There were some other statements I was unsure of -- apparently you still can't get the exact standard model as one of the plethora of string vacua, but I thought that perhaps Hui-He and collaborators had managed this some years ago. Unsure. Also, there is "great evidence that magnetic charges exist in nature, but they are very massive". Really? Can someone enlighten me? He's not talking about branes, is he?

After this dubious introduction, the talk turned to the Bagger--Lambert model. It was not thought possible, until recently, to be able to write down actions for (the low energy effective theory of) D-branes (2 and 5) in M--theory. The way around this is to take the symmetries of the theory to be generated not my Lie algebras, but by three algebras -- which just means you have a bracket [A,B,C] rather than [A,B], but where the bracket need not be totally antisymmetric. The nice example given was the space of M by N matrices, which are not a Lie algebra, but are a 3 algebra with the bracket [A,B,C]=AB^\dagger C-CB^\dagger A. A nice description of these 3--algebras is as a lie algebra with matter in the bi-fundamental, all packaged up into one object. Using this you can write down actions with the right symmetries of M--theory, but apparently you have to then do a bit more work to get actions which describe the branes you're interested in.

So yes, there were snippets of interesting information, but the talk would have benefited enormously from having a gentle introduction to M--theory and its branes, before progressing, rather than having a trivial introduction to string theory before leaping tall buildings.

Edward Shuryak -- Strongly coupled quark-gluon plasma.

RHIC has seen events involving sQGP (as in the title), I think. That's what I managed to pull out of the ultra-dense slides of this extremely busy talk. And then I lost interest, until half way through when there was a slide of diagrams which looked very much like the trajectories of a particle in an electromagnetic vortex (must check that), and then I lost interest until the conclusions. There were slides with pages of text taken from his research papers -- this is not the way to give a talk.

So, the conclusions -- QGP behaves like a very good liquid because of "magnetic bottle trapping", for T<1.4 Tc it is dominated by magnetic monopoles. But forget all that, the important point to take away is that QGP does not behave the way people expected it to. Apparently this chap is a candidate for a Nobel prize for discovering this, and it was nice to hear about this `negative' result. But t was obvious. The people who first suggested that QGP was not going to agree with expectations will, of course, never get the credit. Why? Well, this comes down to the assumption that you lose confinement in QGP. And that's wrong. Why? Well, confinement means the absence of physical, colour charged states (to a field theorist) or a confining potential (to a lattice guy). The latter might well fail at high temp when you get QGP, you almost certainly have some other form of potential, but the former is a consequence of the global (non pert) properties of the Yang Mills configuration space, for example Gribov copies. None of this is altered by going to high temperature, so individual quarks and gluons aren't suddenly going to become observable.

But yeah, there we go. No-one listens to whispers.

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