Posted by Matt
I am at the American Physical Society "April Meeting" (May 1 - 4) in Denver, Colorado. It's been very enjoyable. For one thing, just getting a chance to relax and spend time with some friends who are also physics students has been nice. We even managed to get $10 gift certificates to Amazon by easily winning a "physics trivia quiz" at the student reception tonight. In one case we even corrected the quiz writer. A question asked for the most recent Nobel prize given in particle physics, before the recent one given to Ray Davis, Masatoshi Koshiba, and Riccardo Giacconi. The answer they were looking for was the 1995 prize given to Perl and Reines for the tau lepton and the neutrino, respectively. We pointed out that 't Hooft and Veltman had won more recently, for showing that the Standard Model is renormalizable. It's fun to be able to combine my physics and trivia dorkiness, especially when I get prizes out of it.
This conference has given me a good chance to learn a bit more about things that are going on in string theory. My knowledge of string theory is rather limited, but I have enough of the basics down to have an intuitive picture and follow along in talks, even if I don't get the details. At the DPF prize session, after Peter Onyisi gave his talk on his work with CDF that won him a well-deserved Apker Award, there were two good string theory talks aimed at a broad audience. Veneziano gave a very nice talk on string theory from a historical perspective, and Maldacena talked on the AdS/CFT correspondence.
What interested me more on the string theory front, though, were talks on recent ideas. One such development has been the discussion of the "string theory landscape." Roughly, the idea is that string theory is not enough to specify a complete theory; you have to specify a "vacuum" to expand around. For instance, some vacua might correspond to flat space, others to inflationary universes, and so on. In what has become referred to as the "KKLT" paper (for its authors, Kachru, Kallosh, Linde, and Trivedi), a model of a metastable de Sitter vacuum in string theory was proposed. This would be a universe that expands as ours seems to. Its "metastability" means that it is unstable; at some point, the universe would tunnel to a lower vacuum state. This would be catastrophic, but you don't really need to worry; even if the idea is correct, the lifetime of the universe would far outlast humanity. Giddings has argued (e.g., here) that this idea is in fact more generic, and that in any theory with extra dimensions (as string theory, for instance, says we should have) and a positive cosmological constant (as experiment tells us we have), our four-dimensional universe will be catastrophically unstable (again, nothing to worry about). Here in Denver I got to hear Giddings speak about this "landscape" picture, and also Tom Banks speak about his own views. (Mostly he focused on other topics, but he briefly criticized the landscape picture.)
Most of the controversy revolves around the idea that there would be very many of these deSitter vacua, so that string theory would not give unique predictions for physics. Instead one might have to turn to the anthropic principle to explain why physical parameters have the values that they do. Tonight another paper has been posted about this topic. In this paper, Daniel Robbins and Savdeep Sethi of Chicago argue that the metastable deSitter vacua are not as generic as others have proposed. They make this argument by using nonperturbative ideas in string theory rather than the simple effective field theory pictures that others have used. Explaining more of the technical details would only serve to make plain my own lack of sufficient technical sophistication. It's a very interesting debate, though, and it's fun to try to figure out what is going on. Over the next year as I study string theory more seriously, hopefully I'll have more to say about it.
Aside from those things, I have met some interesting people. I briefly got to chat with Juan Maldacena, who developed the idea of the AdS/CFT correspondence, roughly that 5-dimensional quantum gravity (i.e., a string theory) on a certain space is equivalent to a well-understood 4-dimensional quantum field theory (i.e., a theory of point particles) on the boundary of the 5-d space. This is a concrete realization of the interesting idea of "holography." It has also inspired a lot of phemonological ideas, in which people approach "technicolor"-like models (proposing that strongly interacting physics at a certain energy scale [which will soon be probed by the Large Hadron Collider in Switzerland] will explain certain riddles in the Standard Model) by considering their duals as "brane-world" scenarios in 5 dimensions. I told Maldacena that I'm interested in such phenomenological ideas, and he sounded rather skeptical of them, but I didn't get a chance to ask him why.
There was also a nice public lecture by John Bahcall on the solar neutrino problem. I should try to make a post explaining it along similar lines sometime. Tomorrow morning I might get to hear some phenomenology lectures before I run to catch my flight back to Chicago (just in time for Scavhunt).
Also while here, I saw Kill Bill 2, which I enjoyed, and I got a chance to do some CD shopping. I only regret that I didn't have a chance to get away to the mountains for some hiking; I did get a nice view of them from the 22nd floor of the hotel, but that wasn't so satisfying.Posted by Matt at May 3, 2004 11:53 PM