The Council for the Advancement of Science Writing is holding a New Horizons in Science conference right now in Austin. This morning Steven Weinberg gave a talk, now available online, with the title Higgs, dark matter and supersymmetry, what the Large Hadron Collider will tell us. He described the Higgs as something definitely expected, supersymmetry as a much more speculative possibility, but had nothing to say about string theory during the talk. In the question session, Tom Siegfried of Science News asked him about why he hadn’t mentioned string theory, and what its prospects now were, 25 years after first being heavily promoted to the press. Weinberg answered:
It’s developed mathematically, but not to the point where there is any one theory, or to the point that even if we had one theory we would know how to do calculations to predict things like the mass of the electron, or the masses of the quarks. So, I would say, although there has been theoretical progress it’s been, I find it disappointing. One of the hopes would be that the LHC would provide a clue to something we’re missing in superstring theory and I think there supersymmetry is the most likely place to look.
One of the troubles with superstring theory is that although in a sense the theorists think there is only one theory, there are an infinite number of approximate solutions of it and we don’t know which one corresponds to our world. But at least in a large variety of the solutions of superstring theory there is supersymmetry visible at low energies, and if we see supersymmetry at low energies, superstring theorists may be able to derive from it some kind of clue as to how to solve these theories. But I haven’t talked about it in this lecture because I don’t see how that would work, it would be.. I mean I couldn’t say that that was likely with any degree of sincerity, and certainly the LHC and any other accelerator that we can imagine being built will not get up to energies which are high enough so that we can directly see the structures that are described by superstring theory, the strings or the D-branes or whatever it is. Those will not be accessible at the LHC, so any clue we get will be very indirect.
I myself, well I was working on superstring theory in the 80s and gave it up because I… I moved into cosmology, which in the last couple of decades has had the excitement that elementary particle physics had in the 60s and 70s, a wonderful coming together of theory and observation. Cosmology now reminds me of the excitement that I felt when I was younger and doing particle physics.. and it’s a pity that superstring hasn’t developed better. I still think it’s the best hope we have, I don’t know of anything else. My own work very recently has been trying to develop an alternative to superstring theory as a way of making sense out of quantum gravity at very high energies. But even though I’m working on this I still find superstring theory more attractive, but not attractive enough…
Siegfried gives an account of the talk here. It includes a new remarkably convoluted and misleading way of referring to the fact that string theory predicts nothing at all about observable physics:
But despite a quarter century of intense effort, superstring theory has not produced a cohesive and clear guide to testing its fit with all the observable features of physical existence.