Editing String theory (section)

=== Applications to quantum gravity ===

The discovery of the AdS/CFT correspondence was a major advance in physicists' understanding of string theory and quantum gravity. One reason for this is that the correspondence provides a formulation of string theory in terms of quantum field theory, which is well understood by comparison. Another reason is that it provides a general framework in which physicists can study and attempt to resolve the paradoxes of black holes.<ref name="de Haro et al. 2013, p.2"/>

In 1975, Stephen Hawking published a calculation which suggested that black holes are not completely black but emit a dim radiation due to quantum effects near the [[event horizon]].<ref name=Hawking1975/> At first, Hawking's result posed a problem for theorists because it suggested that black holes destroy information. More precisely, Hawking's calculation seemed to conflict with one of the basic [[postulates of quantum mechanics]], which states that physical systems evolve in time according to the [[Schrödinger equation]]. This property is usually referred to as [[Unitarity (physics)|unitarity]] of time evolution. The apparent contradiction between Hawking's calculation and the unitarity postulate of quantum mechanics came to be known as the [[black hole information paradox]].<ref name=Susskind2008/>

The AdS/CFT correspondence resolves the black hole information paradox, at least to some extent, because it shows how a black hole can evolve in a manner consistent with quantum mechanics in some contexts. Indeed, one can consider black holes in the context of the AdS/CFT correspondence, and any such black hole corresponds to a configuration of particles on the boundary of anti-de Sitter space.<ref>[[#Zwiebach|Zwiebach]], p. 554</ref> These particles obey the usual rules of quantum mechanics and in particular evolve in a unitary fashion, so the black hole must also evolve in a unitary fashion, respecting the principles of quantum mechanics.<ref>[[#Maldacena2005|Maldacena 2005]], p. 63</ref> In 2005, Hawking announced that the paradox had been settled in favor of information conservation by the AdS/CFT correspondence, and he suggested a concrete mechanism by which black holes might preserve information.<ref name=Hawking2005/>