Things are not going well for string theory on the public relations front. Someone just pointed me to the poll at Wired magazine they call String Theory Smackdown, where the side arguing for string theory is losing the voting by more than 3 to 1.
The argument that seems to be carrying the day with the public is the simple one that a supposedly unified theory that can’t make a single testable prediction, despite more than twenty years of work, must have something really wrong with it. Many string theorists acknowledge that this is the situation the theory is in, but make the case for what they see as promising aspects of the theory that justify continued work on it.
Unfortunately, some string theory partisans have chosen to react to recent criticism not by acknowledging the fact that string theory can’t be tested, but by making misleading claims that the theory does make predictions and is testable. On Monday here at Columbia, Gordon Kane gave a colloquium talk of this kind, with the title String Theory and the Real World — a “new” subfield, string phenomenology. Kane began by quoting David Gross as being highly skeptical about the whole idea of string phenomenology, arguing “we don’t know what string theory is, how can it have a phenomenology?”. Kane’s claim that “string phenomenology” is a new field is rather peculiar, since it was an active subject back in the early 1990s. It is however true that, for better or worse, it has become a more active one the past few years, as string theorists have reacted to their colleague’s complaints that they do mathematics, not physics, by trying to sell themselves as “phenomenologists”.
Kane mostly actually ignored string theory, concentrating on supersymmetry, which he has been promoting for more than 20 years (he had an article about “Is Nature Supersymmetric” in Scientific American back in 1986). He described seeing supersymmetry as essential, pretty much the only way of getting a “window to the Planck scale”. There was some mention of the idea that string theory makes predictions about cosmology, but the “prediction” was just that in “most” string theories, the size of B-mode polarization in the CMB is unobservably small. He put up plots from this recent paper, claiming that one could distinguish different string “backgrounds”, by their “footprints” on LHC data. Looking at the paper, it appears to be based upon a large number of assumptions (e.g. that one just gets the MSSM), designed to provide enough constraints so that one could not get absolutely anything, but not so many as to be forced into contradiction with experiment.
For another exercise of this kind, take a look at Kane’s 1997 Physics Today article entitled String theory is testable, even supertestable. This included an impressive looking detailed, specific spectrum of the masses of superpartners, implying that it was the sort of thing “predicted” by string theory. Only problem is that by now it looks to me as if these “predictions” are almost all in disagreement with experiment. Back in 1997 Kane was arguing against John Horgan that string theory really was testable, that it “would predict a specific spectrum of particles and superpartners that can be compared with experimental data”. He seems to have backed off on that claim, there were no such spectra mentioned in his talk this week. About the landscape and its exponentially large number of possibilities, he had little to say except that we “have to learn how to think about this”.
He repeatedly made the claim that “String theories DO give predictions” and “String theory is falsifiable”, giving as an example work by 3 graduate students of Mary Gaillard that showed that one specific heterotic string compactification scheme gave no light neutrino masses and thus led to models incompatible with experiment. Another repeated point was that the problem with string phenomenology was just a lack of manpower. If more people (especially graduate students) were doing these calculations, great progress would be made. In the question session, asked about the CC, he said that there were lots of ideas about how to solve it, what was needed was just more people doing calculations.
Evidently many agree with him, since the IAS has just announced that next year’s summer program for graduate students and postdocs will be on Strings and Phenomenology.
I decided not to ask any question in the question session, having the overwhelming feeling that arguing with “string phenomenologists” is now just wasting one’s breath. They have made it clear that, no matter how dubious the arguments needed, they’re going to keep promoting this field as predictive and highly relevant to the LHC. The intellectual “dead zone” of “string phenomenology” will be with us no matter what and perhaps even come to dominate particle theory until LHC results are in. May they stay as close as possible to schedule! (Kane estimates first physics collisions next September).