Editing AdS/CFT correspondence (section)

=== String theory and nuclear physics ===

{{Main|History of string theory|1/N expansion}}

The discovery of the AdS/CFT correspondence in late 1997 was the culmination of a long history of efforts to relate string theory to nuclear physics.{{sfn|ps=|Zwiebach|2009|p=525}} In fact, string theory was originally developed during the late 1960s and early 1970s as a theory of [[hadron]]s, the [[subatomic particle]]s like the [[proton]] and [[neutron]] that are held together by the [[strong nuclear force]]. The idea was that each of these particles could be viewed as a different oscillation mode of a string. In the late 1960s, experimentalists had found that hadrons fall into families called [[Regge trajectories]] with squared [[energy]] proportional to [[angular momentum]], and theorists showed that this relationship emerges naturally from the physics of a rotating [[Principle of relativity|relativistic]] string.{{sfn|ps=|Aharony|Bergman|Jafferis|Maldacena|2008|loc=sec. 1.1}}

On the other hand, attempts to model hadrons as strings faced serious problems. One problem was that string theory includes a [[mass]]less [[Spin (physics)|spin-2]] particle whereas no such particle appears in the physics of hadrons.{{sfn|ps=|Zwiebach|2009|p=525}}  Such a particle would mediate a force with the properties of gravity. In 1974, [[Joël Scherk]] and [[John Henry Schwarz|John Schwarz]] suggested that string theory was therefore not a theory of nuclear physics as many theorists had thought but instead a theory of quantum gravity.{{sfn|ps=|Scherk|Schwarz|1974}} At the same time, it was realized that hadrons are actually made of quarks, and the string theory approach was abandoned in favor of quantum chromodynamics.{{sfn|ps=|Zwiebach|2009|p=525}}

In quantum chromodynamics, quarks have a kind of [[charge (physics)|charge]] that comes in three varieties called [[color charge|colors]]. In a paper from 1974, [[Gerard 't Hooft]] studied the relationship between string theory and nuclear physics from another point of view by considering theories similar to quantum chromodynamics, where the number of colors is some arbitrary number ''N'', rather than three. In this article, 't Hooft considered a certain limit where ''N'' tends to infinity and argued that in this limit certain calculations in quantum field theory resemble calculations in string theory.{{sfn|ps=|'t Hooft|1974}}