Editing String theory (section)

=== Second superstring revolution ===

In 1995, at the annual conference of string theorists at the University of Southern California (USC), [[Edward Witten]] gave a speech on string theory that in essence united the five string theories that existed at the time, and giving birth to a new 11-dimensional theory called [[M-theory]]. M-theory was also foreshadowed in the work of [[Paul Townsend]] at approximately the same time. The flurry of activity that began at this time is sometimes called the [[second superstring revolution]].<ref name="Duff 1998"/>

[[Image:JuanMaldacena.jpg|right|thumb|upright|[[Juan Maldacena]]]]

During this period, [[Tom Banks (physicist)|Tom Banks]], [[Willy Fischler]], [[Stephen Shenker]] and [[Leonard Susskind]] formulated matrix theory, a full holographic description of M-theory using IIA D0 branes.<ref name=Banks/> This was the first definition of string theory that was fully non-perturbative and a concrete mathematical realization of the [[holographic principle]]. It is an example of a gauge-gravity duality and is now understood to be a special case of the [[AdS/CFT correspondence]]. [[Andrew Strominger]] and [[Cumrun Vafa]] calculated the entropy of certain configurations of D-branes and found agreement with the semi-classical answer for extreme charged black holes.<ref name="Strominger and Vafa 1996"/> [[Petr Hořava (theorist)|Petr Hořava]] and Witten found the eleven-dimensional formulation of the heterotic string theories, showing that orbifolds solve the chirality problem. Witten noted that the effective description of the physics of D-branes at low energies is by a supersymmetric gauge theory, and found geometrical interpretations of mathematical structures in gauge theory that he and [[Nathan Seiberg]] had earlier discovered in terms of the location of the branes.

In 1997, [[Juan Maldacena]] noted that the low energy excitations of a theory near a black hole consist of objects close to the horizon, which for extreme charged black holes looks like an [[anti-de Sitter space]].<ref name=Maldacena1998/> He noted that in this limit the gauge theory describes the string excitations near the branes. So he hypothesized that string theory on a near-horizon extreme-charged black-hole geometry, an anti-de Sitter space times a sphere with flux, is equally well described by the low-energy limiting [[gauge theory]], the [[N = 4 supersymmetric Yang–Mills theory]]. This hypothesis, which is called the [[AdS/CFT correspondence]], was further developed by [[Steven Gubser]], [[Igor Klebanov]] and [[Alexander Markovich Polyakov|Alexander Polyakov]],<ref name=Gubser/> and by Edward Witten,<ref name=Witten1998/> and it is now well-accepted. It is a concrete realization of the [[holographic principle]], which has far-reaching implications for [[black hole]]s, [[Principle of locality|locality]] and [[information]] in physics, as well as the nature of the gravitational interaction.<ref name="de Haro et al. 2013, p.2"/> Through this relationship, string theory has been shown to be related to gauge theories like [[quantum chromodynamics]] and this has led to a more quantitative understanding of the behavior of [[hadron]]s, bringing string theory back to its roots.{{Citation needed|reason=see talk on source 84|date=June 2018}}