Recent Results in String Duality

Until recently, one might have expected that string theories at strong coupling would involve new and exotic physics. For example, since string theory includes gravity, one might have expected a phase with large fluctuations of the spacetime geometry. But now it appears that string theory at strong coupling is not so exotic. Rather, string duality is the principle that any string theory at strong coupling simply looks like another string theory at weak coupling. This sort of duality is familiar in low dimensional field theories. That it might happen in string theory has been put forward by a number of people over the years, and was extensively pursued by Duff, Sen, Schwarz, and others. Until recently though, it seemed unlikely to many of us that such a thing could happen even in a nontrivial four dimensional field theory, and much more unlikely that it could happen in a theory as complicated as string theory. But the work of Seiberg and others has made it clear that this can indeed happen in four-dimensional gauge theories, and now it appears overwhelmingly likely that it happens in string theory as well. String duality is a revolution, shaking our understanding of the foundations of the theory. Many of the world-sheet properties that previously received great emphasis now appear to be technical features of string perturbation theory, not preserved by duality. On the other hand, spacetime ideas such as supergravity are playing a more prominent role in determining the structure of the theory. At this point it is not clear where we are headed or whether the final result will still be called ‘string theory.’ The main theme of my talk is “Should you believe it?” Along