The KITP at Santa Barbara is holding a conference on QCD and String Theory this week, and the talks have started to appear online.

Of the ones I’ve taken a quick look at so far, there doesn’t seem to be any obvious recent progress on the 30-year old main question that everyone would like the answer to: can one find a reliable analytical technique for dealing with QCD in the infrared regiion where the effective coupling is strong? The best hope for this in recent years has been the AdS/CFT correspondence, but after seven years the state of the art there still seems to be a long ways from solving the problem one wants to solve (although it does give solutions to other problems). I’m looking forward to seeing what some of the later talks will have to say, including Larry Yaffe’s one tomorrow on “Large N gauge theories: old and new”.

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Actually, contrary to what Strominger and Motl may think, some physicists (even string theorists!) do consider it to be among the “top ten “millenium problems” posed:

“Can we quantitatively understand quark and gluon confinement in Quantum Chromodynamics and the existence of a mass gap? ”

http://feynman.physics.lsa.umich.edu/strings2000/millennium.html

Most of the other problems seem to be string theory inspired, and some (like 1 and 7) frankly not answerable, IMHO. They do not have the flavor of Hilbert’s problems of really encompassing the breadth of the subject. I would have expected a better list.

What are some of the more important subjects to pursue? String “phenomenology”? Glad I am not the one being brainwashed and be led astray.

The problem with QCD isn’t that there’s no rigorous proof of confinement, it’s that there’s no controlled analytical approximation that captures what is going on in the theory at long distances. Lacking this, there’s a real sense in which we don’t understand quantum gauge theories. For QCD itself, maybe we don’t care that much since we have lattice calculations, and the evidence for the theory is so overwhelming we don’t really need to extract more predictions from it to compare with experiment.

But it’s an important part of physics and we really should be able to understand what is going on there. The best bet for this kind of understanding has always been to find a consistent 1/N expansion of the theory, probably using string theory methods. AdS/CFT is tantalizingly close to getting somewhere with this, it’s surprising to me there hasn’t been more progress recently. I don’t think Strominger is helping the situation if he is dismissing the importance of this problem and encouraging students instead to concentrate on the doomed project of unifying the Standard Model and gravity via some sort of 11d supersymmetric M-theory.

One thing that Peter is not able to appreciate is that the character of the *important* questions is changing with time, as our understanding becomes deeper.

In fact, this question of confinement was *exactly* the example used by Andy Strominger last week when he was explaining the situation of string theory to the new grad students.

He said that when he was a student, the most important question in theoretical physics was to prove that QCD confines at low energies. “It’s still an interesting question and you may get 1 million USD from the Clay Institute if you solve it,” Strominger said, but he explained that we simply have overwhelming evidence that confinement is true and QCD works, and we must simply move on.

I totally agree with Strominger on this point. There are many other important questions whose answers we really do not know, and answering them would be much more important than to find a rigorous proof of some answers that we’re anyway pretty sure that are correct.

That pretty much summarizes all progress in string theory since the 1980’s: never solving the problems you thought you wanted to solve, but finding solutions to problems it never occured to you to ask.