Highlighting the usefulness of string theory

Although Edward Witten does an admirable job explaining why the mathematical structure of string theory is quite beautiful (Physics Today, November 2015, page 38 ), I think he overstates the evidence that “string theory potentially unifies gravity with the other forces of nature.” In particular, it is unclear whether string theory provides a natural explanation for supersymmetry breaking or the observed properties of elementary particles. In addition, string theory has yet to explain how the unphysical predictions of classical general relativity are eliminated; for example, how the unphysical features of the interior Kerr metric—which describes the spacetime inside the event horizon of a rotating black hole—are eliminated in the course of the gravitational collapse of massive rotating objects.

It is true that string theory has provided a nice model, the anti–de Sitter/ conformal field theory (AdS/CFT) correspondence, for the nature of spacetime just outside an event horizon, but to date string theory has had nothing definitive to say about the nature of spacetime inside an event horizon. A related problem for string theory (and also for classical general relativity) is that it provides no explanation for what the universe looked like prior to the Big Bang, which is unexpected if string theory really provides an underlying quantum theory of elementary particles and gravity.

I completely agree with Witten that two-dimensional CFT is likely to have a critical role in any fundamental theory of spacetime. However, because of the obvious importance of four dimensions in the real world, it seems much more likely that four-dimensional curved twistor spaces ¹ rather than strings play the role of fundamental objects. ² This, I submit, is an even more beautiful idea than string theory.