Editing AdS/CFT correspondence (section)

=== Black holes and holography ===
[[Image:Stephen Hawking.StarChild.jpg|thumb|left|upright=0.6|[[Stephen Hawking]] predicted in 1975 that [[black hole]]s emit [[Hawking radiation|radiation]] due to quantum effects.]]

{{Main|Black hole information paradox|Thorne–Hawking–Preskill bet|Holographic principle}}

In 1975, Stephen Hawking published a calculation that suggested that black holes are not completely black but emit a dim radiation due to quantum effects near the event horizon.{{sfn|ps=|Hawking|1975}} This work extended previous results of [[Jacob Bekenstein]] who had suggested that black holes have a well-defined entropy.{{sfn|ps=|Bekenstein|1973}} At first, Hawking's result appeared to contradict one of the main postulates of quantum mechanics, namely the unitarity of time evolution. Intuitively, the unitarity postulate says that quantum mechanical systems do not destroy information as they evolve from one state to another. For this reason, the apparent contradiction came to be known as the black hole information paradox.{{sfn|ps=|Susskind|2008}}

[[File:LeonardSusskindStanford2009 cropped.jpg|thumb|upright=0.6|[[Leonard Susskind]] made early contributions to the idea of [[holography]] in [[quantum gravity]].]]

Later, in 1993, Gerard 't Hooft wrote a speculative paper on quantum gravity in which he revisited Hawking's work on [[black hole thermodynamics]], concluding that the total number of [[degree of freedom|degrees of freedom]] in a region of spacetime surrounding a black hole is proportional to the [[surface area]] of the horizon.{{sfn|ps=|'t Hooft|1993}} This idea was promoted by [[Leonard Susskind]] and is now known as the [[holographic principle]].{{sfn|ps=|Susskind|1995}} The holographic principle and its realization in string theory through the AdS/CFT correspondence have helped elucidate the mysteries of black holes suggested by Hawking's work and are believed to provide a resolution of the black hole information paradox.{{sfn|ps=|Maldacena|2005|p=63}} In 2004, Hawking conceded that black holes do not violate quantum mechanics,{{sfn|ps=|Susskind|2008|p=444}} and he suggested a concrete mechanism by which they might preserve information.{{sfn|ps=|Hawking|2005}}