I spent the last two days up in Cambridge, mainly attending the conference in honor of Sidney Coleman. Sadly, Coleman is in poor health, suffering from Parkinson’s disease, and was unfortunately unable to attend the talks in his honor. They were videotaped so that he could watch them later.
For me as for many particle theorists, taking Coleman’s quantum field theory course at Harvard was one of the great intellectual experiences of my life. Another such experience was reading and learning from his great Erice lectures, both the late seventies ones as they came out, as well as going back to his earlier ones that started in 1966. These were collected in 1985 in the book “Aspects of Symmetry”, allowing me and many others to replace a stack of dog-eared Xeroxes with a more durable volume. The fact that Coleman stopped giving these lectures after 1979 was to me one of the first indications that particle theory was entering a much less promising phase of its history. Coleman never really warmed to the topics of supersymmetry and string theory.
For much of his career Coleman played the role of guru for the particle theory community, generously sharing his unmatched insights into quantum field theory. He would sleep through the morning (famously announcing that he couldn’t teach a 9am class because he couldn’t stay up that late), get into his office late in the afternoon, then spend hours dealing with a long line of people waiting to talk to him to try and get some help with whatever problem they were working on. Steven Weinberg spoke for many people at the conference when he said that Coleman was the single person he had learned the most physics from.
The conference was extremely well-attended, with the large lecture hall in the physics building at Harvard overflowing on Saturday. I don’t think I’ve ever seen so many Nobel prize winning particle theorists in one place. They included Gell-Mann, Glashow, Weinberg, ‘t Hooft, Gross, Wilczek, Wilson, as well as Fields medalist Edward Witten. One of the few living Nobel particle theorists who couldn’t make it was David Politzer, who very much directly owes his prize to Coleman.
I won’t describe the talks in detail, this has been done pretty accurately already by Lubos Motl (who I got to meet in person for the first time). Physics weblogging was very well represented at the conference: besides Lubos, Jacques Distler was liveblogging from one corner of Science Center B on Friday, and Serkan Cabi was also there. Sean Carroll also has some comments about Coleman.
Among the more historical talks, perhaps the most interesting was that of Gerard ‘t Hooft (Lubos seems to have missed ‘t Hooft’s comment that he shouldn’t be referred to as “Gerardus”, a formal version of his name that appears on his Nobel citation and his passport, but is otherwise not much used). ‘t Hooft gave his version of the asymptotic freedom story. He said that he had computed the Yang-Mills beta function a couple years before Gross-Wilczek-Politzer, but didn’t realize that this result wasn’t known to the experts. He pointed out that everyone else had experience only in computing the scaling behavior of non-asymptotically free theories, whereas the first theory he did the computation for was an asymptotically free one, so he thought this was unremarkable. He did say that Gross-Wilczek-Politzer deserved the Nobel since (besides being the ones to publish the beta-function result) they had understood how to use this to explain Bjorken scaling, something that he hadn’t known about. He said his advisor Martin Veltman had told him that the Yang-Mills scaling behavior wasn’t relevant to experiment since experimentalists only cared about what happens on mass-shell. Luckily Veltman was one of the few Nobel theorists not in attendance, since he would likely have blown a gasket if he had been there to hear some of the things ‘t Hooft had to say about him. ‘t Hooft went on to say that he had learned one important thing from this episode: always immediately publish any new result you have.
There was significant mention of string theory in only two talks, those of Gross and Witten. Gross gave essentially the same talk he gave last October at the 25th anniversary of the KITP. He joked that he had managed to time the award of the Nobel with the KITP celebration by every year for the last thirty years writing to the Nobel Committee and asking them to wait a while before awarding him the prize, something they had been happy to do. At the point of his talk when he said that the question “What is String Theory” was one of the big questions for the future, he stopped to defensively note that since we don’t know what string theory is, it is an idea that can’t be killed, no matter how much certain members of the audience wanted to do this. He went on to claim that since AdS/CFT kind of connects string theory with QCD, string theory is in some sense part of the standard model, so it’s importance is secure. This argument seemed to me pretty disingenuous, since presumably he’s well aware that the problem most critics have with string theory is not with the idea of using it as a dual representation of QCD, but with the idea of getting a TOE out of it, a project which some have called a “colossal failure”. He didn’t have anything to say either about this failure or about the whole Landscape mania.
The last talk was Witten’s, entitled “Emergent Phenomena in Condensed Matter and Particle Physics”. He started by saying that he was afraid the title of his talk might be more exciting than the talk itself. By “emergent phenomena” he meant roughly non-perturbative phenomena in QFT, where the long distance degrees of freedom one observes are not directly related to the local degrees of freedom. He gave QED as an example of a non-emergent theory, QCD an emergent one, with the nature of the electroweak theory still up in the air until we know more about the origin of electroweak symmetry breaking.
He went on to say that gravity messes up this distinction between local and emergent phenomena, since one doesn’t have diffeomorphism invariant local observables. He then quoted his 1980 work with Weinberg (and with help from Coleman) to the effect that you can’t get a massless spin two bound state in a theory with a local stress-energy tensor, saying that this showed that you can’t start with a local theory in Minkowksi space and generate Einstein gravity as an emergent phenomenon. For him the lesson is that if you want gravity as an emergent phenomenon, you need to find a way to first get space-time as an emergent phenomenon, and he believes that whatever the primoridial M-theory underlying string theory is, it should do this. While such a theory doesn’t now exist, he went on to give the AdS/CFT correspondence as the kind of thing he had in mind. There the Weinberg-Witten argument is evaded since a QFT in 4 dimensions is related to a gravity theory in a different number of dimensions (5).
