Shamit Kachru gave the physics colloquium at Rutgers yesterday. His title was “The Theory of More Than Everything” and I heard from people who attended that he was promoting research into the “Landscape” as a new model of how to do theoretical physics, especially cosmology.
I was down there today and heard two talks in the mathematical physics seminar, by Abhay Ashtekar and Tom Banks. Ashtekar’s talk was a standard exposition of a few of the basic ideas of loop quantum gravity, also reviewing an attempt to apply these ideas to cosmology. The talk by Banks was titled “Triumphs and Travails of String Theory”. The first hour was about the triumphs, giving a pretty standard survey of the supposed accomplishments of string theory. Banks emphasized the importance of supersymmetry, and described string theory as not quite background independent, but depending only on a choice of “asymptotic background”. He dealt with matrix models, holography, BPS states, dualities, getting gauge bosons out of string theory, and AdS/CFT.
His talk contained quite a lot of content, unlike some promotional talks of this kind, but it did come off a bit like an hour-long infomercial (“And, there’s even more! It slices, it dices, it ….”). The last five minutes were devoted to the travails of string theory, of which, according to Banks, there is really only one (although he did mention that the lack of observed supersymmetry is also a problem). The one travail is the fact that the cosmological constant seems to be positive, so the universe is de Sitter, not anti de Sitter or flat. This creates well known problems with defining an S-matrix. He went on to explain the “Landscape” idea with its de Sitter states that are only metastable, saying this “leads to a new philosophy of doing physics that many are exploring”. He didn’t seem interested in directly criticizing this new philosophy, but did end by promoting his own, different, ideas about how to deal with the cosmological constant problem.
During the question session afterward, someone asked if there was any overlap between loop quantum gravity and string theory. After some hemming and hawing by the speakers, Michael Douglas spoke from the audience, saying that since string theory was really 20 or so different kinds of approaches to quantum gravity, it was quite plausible that LQG was another related one.