One of the most prominent theoretical physicists of our time, Professor Joe Polchinski of the University of Santa Barbara, who has made lasting contributions to our understanding of quantum field theory, of gravity, and of string theory, gave a couple of talks at the Institute for Advanced Study in Princeton this week. The two presentations manifested a certain amusing (anti-)parallel; the first was on a puzzle that was thought to have been mostly solved 20 years ago, but turns out to have only been partly resolved; the second was related to a puzzle that was thought to have been solved last year, but turns out to have been partly solved over 20 years ago.
In the middle of all of this, it was announced that Polchinski was one of several people awarded one of these new-fangled Fundamental Physics Prizes that are getting lots of attention — specifically, one of the Frontiers Prizes, if you’re keeping score. You can read about that elsewhere. Here we’ll try to keep our focus on the science.
Polchinski’s first talk, on Monday, based on work done with Ahmed Almheiri, Don Marolf, and James Sully, addressed a long-standing question regarding the interplay of quantum field theory (the equations we use to describe the behavior of elementary fields and particles) and black holes (objects whose gravity is so strong that even light cannot escape, yet which will evaporate away, by spitting particles off their edges [their ``horizons'']). In the 1970s Stephen Hawking, in his work showing that black holes evaporate, argued that quantum mechanics is violated in this process; the information about how the black hole is produced is lost as it evaporates, which is impossible in a standard (“unitary”) quantum theory. But in 1993, Leonard Susskind, Larus Thorlacius and John Uglum argued that the information is not lost, that there is no problem with quantum mechanics, and that an obvious contradiction is avoided because quantum phenomena as seen by an observer falling into the black hole are different — complementary — to those seen by an observer who remains outside and far from the black hole. “Complementarity” was a nice idea, and it was bolstered by discoveries in string theory (the field theory/string theory [or ``AdS/CFT''] correspondence) that made it apparent that the evaporation of black holes can be described by quantum mechanics. This gave strong evidence that Hawking’s original way of thinking couldn’t be right, but still, the field theory/string theory correspondence didn’t really clarify exactly how complementarity would work. And now it turns out that it doesn’t quite work as expected, if at all; Polchinski and friends have shown it leads to a paradox that has even puzzled Susskind. The debate as to what it all means extended for many hours at the IAS, and continued on Tuesday. Clearly the apparent paradoxes at the nexus of black holes and quantum mechanics will be with us for some time to come.
Polchinski’s second talk, on Tuesday, addressed a long-standing problem in quantum field theory: whether there exist quantum field theories with a certain special property. Since the answer is “no”, I won’t trouble you with the details. (For the experts; the question is whether there exist any unitary scale-invariant field theories, in three spatial and one time dimension, that are not also conformally invariant.) The claim by Polchinski and his co-authors Markus Luty and Riccardo Rattazzi (famous physicists in their own right) is related to an important advance in the field that I described last year, by Komargodski (another prize winner) and Schwimmer. [Note Added: following a comment below, which I encourage you to read, I am urged to mention Fortin, Grinstein and Stergiou, who first claimed to have discovered scale-invariant theories that are not conformally invariant, (going back as far as this paper), and then backed off, instead claiming only they have discovered conformally invariant theories which have cyclically-varying couplings. There were also changes in the Luty et al. paper in their second version, apparently reflecting ongoing discussion between the two sets of authors. Both 2nd versions appeared on November 9th. I am unclear if disagreements between the groups remain.] Among the interesting observations Polchinski made in the talk is that the breakthrough work of Komargodski and Schwimmer was partially presaged by work 20 years ago by Hugh Osborn (also famous), and by Osborn with Ian Jack. Osborn seems to have buried his main result in a long, complex and difficult paper that most people didn’t understand. Well, this kind of thing happens sometimes… In any case, this earlier work deserves some credit, though there’s also much to be said for Komargodski and Schwimmer’s argument, which is more widely applicable and, importantly, is much easier to understand both technically and conceptually.
In attendance were a remarkable number of the world’s experts, along with a healthy number of young experts-in-training. After the talks they clustered in groups and engaged in lengthy conversations, trying to interpret the more confusing elements. For me it was a bit like old times; Monday’s audience included four of my colleagues from my early years as a Rutgers University postdoctoral researcher (in the mid-90s) and many people who were then in Princeton. I was bemused that the conversations concerning black holes seem as confused and confusing now as they were two decades ago. Sometimes progress is made in a slowly ascending spiral, requiring a visit to old territory before further advances can be made.