Editing String theory (section)

== History ==
{{Main|History of string theory}}

=== Early results ===

Some of the structures reintroduced by string theory arose for the first time much earlier as part of the program of classical unification started by [[Albert Einstein]]. The first person to add a [[Five-dimensional space|fifth dimension]] to a theory of gravity was [[Gunnar Nordström]] in 1914, who noted that gravity in five dimensions describes both gravity and electromagnetism in four. Nordström attempted to unify electromagnetism with [[Nordström's theory of gravitation|his theory of gravitation]], which was however superseded by Einstein's general relativity in 1919. Thereafter, German mathematician [[Theodor Kaluza]] combined the fifth dimension with [[general relativity]], and only Kaluza is usually credited with the idea. In 1926, the Swedish physicist [[Oskar Klein]] gave [[Kaluza–Klein theory|a physical interpretation]] of the unobservable extra dimension—it is wrapped into a small circle. Einstein introduced a [[Antisymmetric tensor|non-symmetric]] [[metric tensor]], while much later Brans and Dicke added a scalar component to gravity. These ideas would be revived within string theory, where they are demanded by consistency conditions.

[[File:LeonardSusskindStanford2009 cropped.jpg|left|thumb|upright|[[Leonard Susskind]]]]

String theory was originally developed during the late 1960s and early 1970s as a never completely successful theory of [[hadron]]s, the [[subatomic particle]]s like the [[proton]] and [[neutron]] that feel the [[strong interaction]]. In the 1960s, [[Geoffrey Chew]] and [[Steven Frautschi]] discovered that the [[meson]]s make families called [[Regge trajectories]] with masses related to spins in a way that was later understood by [[Yoichiro Nambu]], [[Holger Bech Nielsen]] and [[Leonard Susskind]] to be the relationship expected from rotating strings. Chew advocated making a theory for the interactions of these trajectories that did not presume that they were composed of any fundamental particles, but would construct their interactions from [[bootstrap model|self-consistency conditions]] on the [[S-matrix]]. The [[S-matrix theory|S-matrix approach]] was started by [[Werner Heisenberg]] in the 1940s as a way of constructing a theory that did not rely on the local notions of space and time, which Heisenberg believed break down at the nuclear scale. While the scale was off by many orders of magnitude, the approach he advocated was ideally suited for a theory of quantum gravity.

Working with experimental data, R. Dolen, D. Horn and C. Schmid developed some sum rules for hadron exchange. When a particle and [[antiparticle]] scatter, virtual particles can be exchanged in two qualitatively different ways. In the s-channel, the two particles annihilate to make temporary intermediate states that fall apart into the final state particles. In the t-channel, the particles exchange intermediate states by emission and absorption. In field theory, the two contributions add together, one giving a continuous background contribution, the other giving peaks at certain energies. In the data, it was clear that the peaks were stealing from the background—the authors interpreted this as saying that the t-channel contribution was dual to the s-channel one, meaning both described the whole amplitude and included the other.

[[File:GabrieleVeneziano.jpg|right|thumb|upright|[[Gabriele Veneziano]]]]

The result was widely advertised by [[Murray Gell-Mann]], leading [[Gabriele Veneziano]] to construct a [[Veneziano scattering amplitude|scattering amplitude]] that had the property of Dolen–Horn–Schmid duality, later renamed world-sheet duality. The amplitude needed poles where the particles appear, on straight-line trajectories, and there is a special mathematical function whose poles are evenly spaced on half the real line—the [[gamma function]]—which was widely used in Regge theory. By manipulating combinations of gamma functions, Veneziano was able to find a consistent scattering amplitude with poles on straight lines, with mostly positive residues, which obeyed duality and had the appropriate Regge scaling at high energy. The amplitude could fit near-beam scattering data as well as other Regge type fits and had a suggestive integral representation that could be used for generalization.

Over the next years, hundreds of physicists worked to complete the [[Bootstrap model|bootstrap program]] for this model, with many surprises. Veneziano himself discovered that for the scattering amplitude to describe the scattering of a particle that appears in the theory, an obvious self-consistency condition, the lightest particle must be a [[tachyon]]. [[Miguel Ángel Virasoro (physicist)|Miguel Virasoro]] and Joel Shapiro found a different amplitude now understood to be that of closed strings, while [[Ziro Koba]] and [[Holger Bech Nielsen|Holger Nielsen]] generalized Veneziano's integral representation to multiparticle scattering. Veneziano and [[Sergio Fubini]] introduced an operator formalism for computing the scattering amplitudes that was a forerunner of [[world-sheet conformal theory]], while Virasoro understood how to remove the poles with wrong-sign residues using a constraint on the states. [[Claud Lovelace]] calculated a loop amplitude, and noted that there is an inconsistency unless the dimension of the theory is 26. [[Charles Thorn]], [[Peter Goddard (physicist)|Peter Goddard]] and [[Richard Brower]] went on to prove that there are no wrong-sign propagating states in dimensions less than or equal to 26.

In 1969–1970, [[Yoichiro Nambu]], [[Holger Bech Nielsen]], and [[Leonard Susskind]] recognized that the theory could be given a description in space and time in terms of strings. The scattering amplitudes were derived systematically from the action principle by [[Peter Goddard (physicist)|Peter Goddard]], [[Jeffrey Goldstone]], [[Claudio Rebbi]], and [[Charles Thorn]], giving a space-time picture to the vertex operators introduced by Veneziano and Fubini and a geometrical interpretation to the [[Virasoro algebra|Virasoro conditions]].

In 1971, [[Pierre Ramond]] added fermions to the model, which led him to formulate a two-dimensional supersymmetry to cancel the wrong-sign states. [[John Henry Schwarz|John Schwarz]] and [[André Neveu]] added another sector to the fermi theory a short time later. In the fermion theories, the critical dimension was 10. [[Stanley Mandelstam]] formulated a world sheet conformal theory for both the bose and fermi case, giving a two-dimensional field theoretic path-integral to generate the operator formalism. [[Michio Kaku]] and [[Keiji Kikkawa]] gave a different formulation of the bosonic string, as a [[string field theory]], with infinitely many particle types and with fields taking values not on points, but on loops and curves.

In 1974, [[Tamiaki Yoneya]] discovered that all the known string theories included a massless spin-two particle that obeyed the correct [[Ward identities]] to be a graviton. John Schwarz and [[Joël Scherk]] came to the same conclusion and made the bold leap to suggest that string theory was a theory of gravity, not a theory of hadrons. They reintroduced [[Kaluza–Klein theory]] as a way of making sense of the extra dimensions. At the same time, [[quantum chromodynamics]] was recognized as the correct theory of hadrons, shifting the attention of physicists and apparently leaving the bootstrap program in the [[dustbin of history]].

String theory eventually made it out of the dustbin, but for the following decade, all work on the theory was completely ignored. Still, the theory continued to develop at a steady pace thanks to the work of a handful of devotees. [[Ferdinando Gliozzi]], Joël Scherk, and [[David Olive]] realized in 1977 that the original Ramond and Neveu Schwarz-strings were separately inconsistent and needed to be combined. The resulting theory did not have a tachyon and was proven to have space-time supersymmetry by John Schwarz and [[Michael Green (physicist)|Michael Green]] in 1984. The same year, [[Alexander Markovich Polyakov|Alexander Polyakov]] gave the theory a modern path integral formulation, and went on to develop conformal field theory extensively. In 1979, [[Daniel Friedan]] showed that the equations of motions of string theory, which are generalizations of the [[Einstein equations]] of [[general relativity]], emerge from the [[renormalization group]] equations for the two-dimensional field theory. Schwarz and Green discovered T-duality, and constructed two superstring theories—IIA and IIB related by T-duality, and type I theories with open strings. The consistency conditions had been so strong, that the entire theory was nearly uniquely determined, with only a few discrete choices.

=== First superstring revolution ===

[[File:Edward Witten.jpg|right|thumb|upright|[[Edward Witten]]]]

In the early 1980s, [[Edward Witten]] discovered that most theories of quantum gravity could not accommodate [[chirality (physics)|chiral]] fermions like the neutrino. This led him, in collaboration with [[Luis Álvarez-Gaumé]], to study violations of the conservation laws in gravity theories with [[Gravitational anomaly|anomalies]], concluding that type I string theories were inconsistent. Green and Schwarz discovered a contribution to the anomaly that Witten and Alvarez-Gaumé had missed, which restricted the gauge group of the type I string theory to be SO(32). In coming to understand this calculation, Edward Witten became convinced that string theory was truly a consistent theory of gravity, and he became a high-profile advocate. Following Witten's lead, between 1984 and 1986, hundreds of physicists started to work in this field, and this is sometimes called the [[first superstring revolution]].{{Citation needed|date=September 2020}}

During this period, [[David Gross]], [[Jeffrey A. Harvey|Jeffrey Harvey]], [[Emil Martinec]], and [[Ryan Rohm]] discovered [[heterotic strings]]. The gauge group of these closed strings was two copies of [[E8 (mathematics)|E8]], and either copy could easily and naturally include the standard model. [[Philip Candelas]], [[Gary Horowitz]], [[Andrew Strominger]] and Edward Witten found that the Calabi–Yau manifolds are the compactifications that preserve a realistic amount of supersymmetry, while [[Lance Dixon]] and others worked out the physical properties of [[orbifolds]], distinctive geometrical singularities allowed in string theory. [[Cumrun Vafa]] generalized T-duality from circles to arbitrary manifolds, creating the mathematical field of [[mirror symmetry (string theory)|mirror symmetry]]. [[Daniel Friedan]], [[Emil Martinec]] and [[Stephen Shenker]] further developed the covariant quantization of the superstring using conformal field theory techniques. [[David Gross]] and Vipul Periwal discovered that string perturbation theory was divergent. [[Stephen Shenker]] showed it diverged much faster than in field theory suggesting that new non-perturbative objects were missing.{{Citation needed|date=September 2020}}

[[File:Joseph Polchinski.jpg|left|thumb|upright|[[Joseph Polchinski]]]]

In the 1990s, [[Joseph Polchinski]] discovered that the theory requires higher-dimensional objects, called [[D-brane]]s and identified these with the black-hole solutions of supergravity. These were understood to be the new objects suggested by the perturbative divergences, and they opened up a new field with rich mathematical structure. It quickly became clear that D-branes and other p-branes, not just strings, formed the matter content of the string theories, and the physical interpretation of the strings and branes was revealed—they are a type of black hole. [[Leonard Susskind]] had incorporated the [[holographic principle]] of [[Gerardus 't Hooft]] into string theory, identifying the long highly excited string states with ordinary thermal black hole states. As suggested by 't Hooft, the fluctuations of the black hole horizon, the world-sheet or world-volume theory, describes not only the degrees of freedom of the black hole, but all nearby objects too.

=== 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}}