Witten’s talk was a rather polemical argument for string theory, in which he laid out his reasons for still feeling that string theory is on the right track. The video is in two parts, with the first part not especially interesting since it is pretty much word-for-word the standard arguments for string theory that he and others have been making since the mid-eighties. The second part has some more interesting content, including Witten’s comments on the evolution of his own personal relationship to the subject. This started in the early eighties, before the 1984 “First Superstring Revolution”, when he began studying string theory, feeling that it was an approach to unification that deserved more attention than it was getting.
A recurring theme in his talk is that “string theory” has gone through unexpected changes in perspective over the 30 years he has been working on it, with the unspoken argument being that some new change in perspective may yet make the current deadly problems of string unification go away. He takes an ambiguous attitude towards attempts in recent years to argue for a change in perspective to the pseudo-scientific “landscape”, explaining the arguments of proponents while not signing on to them. In the podcast he says “I don’t know if this is the right picture of the universe”, in the talk it’s “to my thinking we still need more clues to have a better picture whether that is the right interpretation.”
It’s interesting to compare Witten’s pro-string theory arguments to the somewhat similar ones of his much less mild-mannered thesis advisor David Gross (see here for a posting about a recent talk by Gross). Unlike Witten, Gross is clear where he stands on the anthropic landscape, denouncing it as pseudo-science. One other crucial difference has to do with their discussion of upcoming LHC results. Here, Gross argues strongly that the LHC will see supersymmetry, and is willing to put money on the table to back this up. Witten’s talk barely refers to the LHC, and while he argues that a point in string theory’s favor is its relation to supersymmetry, all he’s willing to say about supersymmetry or extra dimensions at the LHC is “it might happen if we’re fortunate enough.” The next interesting part of the string theory story may very well be what happens in 2013-4 when it becomes clear that supersymmetry and extra dimensions are not going to be seen at LHC energies. After paying off his debts, I wouldn’t be surprised to see Gross take this as an opportunity to back off from the idea of string theory unification. On the other hand, in his talk Witten seems to be positioning himself for carrying on as before, by now making arguments for string theory that don’t at all involve low-energy supersymmetry.
Witten ends his talk with the argument that we still know very little about what string theory is, and this implies that it remains an excellent subject for young physicists to start to work on. I suspect he sees all too well that among physicists the tide has changed, with students turning to other subjects as jobs in string theory dry up and prospects for progress on string theory unification look increasingly dismal. He’s trying to counter this by restating the arguments that have continued to keep him interested in the subject.
Update: Clifford Johnson has some very interesting and accurate comments on Witten and on how others react to him here.