Commenter Shantanu points to this video of a recent colloquium by Andy Strominger at Harvard, which includes some extensive comments on the current state of string theory. Strominger is one of the most prominent string theorists in the business, and has been working in the field for more than a quarter-century since the first “Superstring Revolution” of 1984. In the talk (at about the 52 minute point), Strominger gives a “report card” for string theory, where he assigns it 3 As, 2 Bs, 3 Ds and 2 Fs, for an average grade of about C. It gets an F for making no unambiguous testable predictions, a D for prospects of saying anything about LHC, an F for the CC (Strominger isn’t sold on the anthropic landscape) and a D for cosmology. Some of the high grades are debatable, with an audience member pointing out to him that there was a tension between his A for “Not being ruled out as theory of nature” and F for no testable predictions. Strominger repeatedly claimed that most string theorists would agree with him on these grades (except maybe the F for the CC).
As an overall evaluation, he said that it was debatable whether this was a passing or failing report card, then arguing:
But this is the only student in class, so if you flunk her you have to shut the school down.
Along the same lines, a bit earlier in the talk one of his slides characterizes all that theorists can do as “go home and watch TV” if they believe in the landscape (“String theory is everything”) or if they think string theory is a failure (“String theory is nothing”). The positive argument he was trying to make is that there still is something for string theorists to do even after they are forced to give up on particle physics: they can try applying AdS/CFT and black holes to other areas of physics (nuclear physics, solid state physics, fluid mechanics).
I think Strominger is right that his grades and point of view about string theory are now conventional wisdom among leading theorists. What I find striking about this is the argument that if you are forced to give up on string theory, you have to “shut the school down” or “go home and watch TV”. More than 25 years of working on string theory has left Strominger and others somehow believing that there is no conceivable alternative. The failure of string theory as a theory of particle physics leads them to the conclusion that they must not abandon string theory, but instead must abandon particle physics and try and apply string theory to other fields. The obvious conclusion that string theory is just one speculative idea, and that its failure just means you have to try others, is one that they still do not seem willing to face up to.