Five years ago the 20th anniversary of the “First Superstring Revolution” was being celebrated, and I wrote some postings about this history (see here, here and here). This month is the quarter-century anniversary, and I haven’t seen any evidence of anyone celebrating.
25 years later, it’s pretty clear that the anomaly cancellation discovered by Green and Schwarz doesn’t actually provide at all the kind of explanation of some aspects of the standard model that people got excited about back then. 1984 also saw the beginning of endlessly repeated and overhyped hand-waving arguments that string theory is needed to cure the perturbative ills of quantum gravity. It turns out that these arguments don’t actually work either. The latest issue of Science has an article about recent discoveries showing that N=8 supergravity amplitudes are much better behaved than expected. Zvi Bern compares the widespread belief that string theory is needed to deal with perturbative divergences to the belief that ulcers are caused by spicy food. While you could make a plausible hand-waving argument for this, it turns out that the real culprit is a bacterium, not enchiladas. According to the article:
The work doesn’t disprove string theory, but it has string theorists backpedaling a bit in their criticism of quantum field theory. “At certain points, our understanding has been incomplete, and we may have said things that weren’t right,” says John Schwarz of the California Institute of Technology in Pasadena. “That being said, the fact is that we still need string theory.”
Forced to concede that a quarter-century’s worth of argument about perturbative amplitudes was wrong, Schwarz tries to shift ground by claiming that the QFT problems are non-perturbative. That may very well be, but until one actually has a viable non-perturbative version of string theory, it will be hard to argue that string theory is the only way to go.
A new review article about quantum gravity describes how things are 25 years after the revolution, with many if not most string theorists having given up hope that string theory has anything to say about unification:
A personal anecdote might best convey the current state of affairs. Early in the spring 2007 semester my University of Florida colleague, Charles Thorn, began a seminar by announcing his belief that:
String theory is just a technique for summing the leading terms in the 1/N expansion of QCD.
After years of hearing more ambitious assessments this was so shocking that I checked to be sure I had understood correctly. Charles confirmed that I had; in his current view, the effort to regard superstrings as a fundamental theory of everything was a blind alley. Later that year I related Charles’ pronouncement to string theory colleagues on three continents and solicited their own opinions. About half of them agreed with him, more often the younger people.
Update: One more, from Martha Stewart, Some Pearls of Wisdom on String Theory.
