I spent most of last week back in Cleveland, at Case Western Reserve University, where I spent three delightful years working with Tanmay Vachaspati, Glenn Starkmann, and Lawrence Krauss (who recently left). The occasion was a workshop on Tests of Gravity and Gravitational Physics, organized by the Center for Education and Research in Cosmology and Astrophysics (CERCA). An interesting mix of theoretical cosmologists, relativists, particle physicists, observers and experimentalists participated, and the aim was to pull and tug at the loose threads in various ideas of modifying gravity, while talking about how we might perform sensible tests of the theory in new regimes in the near future. Most of the program consisted of talks, both theoretical and experimental, which you can download from the main site. The theory talks ranged from Nima Arkani-Hamed's talk on why general relativity (GR) is so remarkably robust, with the take-home message "Don't modify gravity - understand it", to Bill Unruh's description of his "dumb Hole" analog models for black holes, and the discussion of the associated analogous phenomenon to Hawking radiation. On the experimental side, there were great talks covering tabletop tests of sub-millimeter gravity - like the one by Blayne Heckel - all the way up to tests of gravity on the scale of galaxies and above - like Stacy McGaugh's talk on tests of MOND. As well as taking part in the general discussions and chairing a session, my role in all this was to sit on a panel for a discussion of "Modified Gravity: What does it buy? and at What Price?"
Nima moderated the discussion, and the other panel members were Eanna Flanagan, Nemanja Kaloper, Glenn Starkman and Bob Wald. While everyone had their own particular take on this, a common theme was that the theoretical and observational constraints are such that we currently do not have any modifications to GR that are compelling, despite our attempts to find a way to address the cosmological constant problem, explain cosmic acceleration, and search for alternatives to the dark matter paradigm. This doesn't mean it isn't worth trying, of course, although Nima made a spirited argument that it is hard to maintain the important holographic (and hence nonlocal) features of general relativity in any modified theory that isn't of the simple scalar-tensor type, and that this is an argument that we shouldn't expect nontrivial modifications. Rather, he argued, we should search for a dual S-matrix description that applies in flat space (unlike AdS/CFT) and that may allow us a better understanding of, for example, the cosmological constant problem. This was all heavy stuff, but it didn't consume our entire time in Cleveland. Tuesday evening saw the workshop dinner and it turned out that our hosts had arranged some after dinner entertainment, in the form of an improvisational comedy team. These guys were big on audience participation, and I was one of the first people chosen to mildly (I hope) embarrass themselves. At first this seemed like a heavy price to pay for dinner. However, trust me, subsequently seeing Tanmay Vachaspati, Bill Unruh and George Pickett up there made me realize it was well worth every blush and squirm. Update: Dmitry Podolsky has a couple of very nice posts over at NEQNET with quite a few details about the talks, including Nima's.