Editing Supergravity (section)

===The second superstring revolution===
Initial excitement about the 10-dimensional theories, and the string theories that provide their quantum completion, died by the end of the 1980s. There were too many [[Calabi–Yau]]s to compactify on, many more than [[Shing-Tung Yau|Yau]] had estimated, as he admitted in December 2005 at the [[23rd International Solvay Conference in Physics]]. None quite gave the standard model, but it seemed as though one could get close with enough effort in many distinct ways. Plus no one understood the theory beyond the regime of applicability of string [[perturbation theory]].

There was a comparatively quiet period at the beginning of the 1990s; however, several important tools were developed. For example, it became apparent that the various superstring theories were related by "[[string dualities]]", some of which relate weak string-coupling - perturbative - physics in one model with strong string-coupling - non-perturbative - in another.

Then the [[second superstring revolution]] occurred. [[Joseph Polchinski]] realized that obscure string theory objects, called [[D-branes]], which he discovered six years earlier, equate to stringy versions of the [[p-branes]] known in supergravity theories. String theory perturbation didn't restrict these [[p-branes]]. Thanks to supersymmetry, p-branes in supergravity gained understanding well beyond the limits of string theory.

Armed with this new [[nonperturbative]] tool, [[Edward Witten]] and many others could show all of the perturbative string theories as descriptions of different states in a single theory that Witten named [[M-theory]]. Furthermore, he argued that M-theory's [[Long Wavelength Limit|long wavelength limit]], i.e. when the quantum wavelength associated to objects in the theory appear much larger than the size of the 11th dimension, needs 11-dimensional supergravity descriptors that fell out of favor with the [[first superstring revolution]] 10 years earlier, accompanied by the 2- and 5-branes.

Therefore, supergravity comes full circle and uses a common framework in understanding features of string theories, M-theory, and their compactifications to lower spacetime dimensions.