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The starting point for string theory is the idea that the [[point particle|point-like particles]] of [[particle physics]] can also be modeled as one-dimensional objects called [[string (physics)|strings]]. String theory describes how strings propagate through space and interact with each other. In a given version of string theory, there is only one kind of string, which may look like a small loop or segment of ordinary string, and it can [[vibration|vibrate]] in different ways. On distance scales larger than the string scale, a string will look just like an ordinary particle consistent with non-string models of elementary particles, with its [[mass]], [[charge (physics)|charge]], and other properties determined by the vibrational state of the string. String theory's application as a form of quantum gravity proposes a vibrational state responsible for the [[graviton]], a yet unproven quantum particle that is theorized to carry gravitational force.<ref name="Becker, Becker 2007, pp. 2">[[#Becker|Becker, Becker and Schwarz]], pp. 2–3</ref>
One of the main developments of the past several decades in string theory was the discovery of certain 'dualities'
Studies of string theory have also yielded a number of results on the nature of black holes and the gravitational interaction. There are certain paradoxes that arise when one attempts to understand the quantum aspects of black holes, and work on string theory has attempted to clarify these issues. In late 1997 this line of work culminated in the discovery of the [[anti-de Sitter/conformal field theory correspondence]]
Since string theory incorporates all of the fundamental interactions, including gravity, many physicists hope that it will eventually be developed to the point where it fully describes our universe, making it a [[theory of everything]]. One of the goals of current research in string theory is to find a solution of the theory that reproduces the observed spectrum of elementary particles, with a small [[cosmological constant]], containing [[dark matter]] and a plausible mechanism for [[cosmic inflation]]. While there has been progress toward these goals, it is not known to what extent string theory describes the real world or how much freedom the theory allows in the choice of details.<ref>[[#Becker|Becker, Becker and Schwarz]], pp. 3, 15–16</ref>
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The currently accepted theory describing elementary particles and their interactions is known as the [[standard model of particle physics]]. This theory provides a unified description of three of the fundamental forces of nature: electromagnetism and the strong and weak nuclear forces. Despite its remarkable success in explaining a wide range of physical phenomena, the standard model cannot be a complete description of reality. This is because the standard model fails to incorporate the force of gravity and because of problems such as the [[hierarchy problem]] and the inability to explain the structure of fermion masses or dark matter.
String theory has been used to construct a variety of models of particle physics going beyond the standard model. Typically, such models are based on the idea of compactification. Starting with the ten- or eleven-dimensional spacetime of string or M-theory, physicists postulate a shape for the extra dimensions. By choosing this shape appropriately, they can construct models roughly similar to the standard model of particle physics, together with additional undiscovered particles.<ref name=Candelas1985/> One popular way of deriving realistic physics from string theory is to start with the heterotic theory in ten dimensions and assume that the six extra dimensions of spacetime are shaped like a six-dimensional Calabi–Yau manifold. Such compactifications offer many ways of extracting realistic physics from string theory.<ref>{{cite arXiv|last1=Cvetic|first1=M|authorlink1=Mirjam Cvetič|last2=Halverson|first2=J.|authorlink2=|last3=Shiu|first3=G.|authorlink3=Gary Shiu|last4=Taylor|first4=W.|authorlink4=|date=2022|title=Snowmass White Paper: String Theory and Particle Physics|pages=|
=== Cosmology ===
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In 1992, [[Richard Borcherds]] constructed a bridge between the theory of modular functions and finite groups and, in the process, explained the observations of McKay and Thompson.<ref>[[#Gannon|Gannon]], p. 8</ref><ref name=Borcherds/> Borcherds' work used ideas from string theory in an essential way, extending earlier results of [[Igor Frenkel]], [[James Lepowsky]], and [[Arne Meurman]], who had realized the monster group as the symmetries of a particular{{which|date=February 2016}} version of string theory.<ref name=FLM/> In 1998, Borcherds was awarded the [[Fields medal]] for his work.<ref>[[#Gannon|Gannon]], p. 11</ref>
Since the 1990s, the connection between string theory and moonshine has led to further results in mathematics and physics.<ref name="Klarreich 2015"/> In 2010, physicists [[Tohru Eguchi]], [[Hirosi Ooguri]], and [[Yuji Tachikawa (physicist)|Yuji Tachikawa]] discovered connections between a different sporadic group, the [[Mathieu group M24|Mathieu group {{math|''M''<sub>24</sub>}}]], and a certain version{{which|date=November 2016}} of string theory.<ref name=EOT/> [[Miranda Cheng]], John Duncan, and [[Jeffrey A. Harvey]] proposed a generalization of this moonshine phenomenon called [[umbral moonshine]],<ref name=CDH/> and their conjecture was proved mathematically by Duncan, Michael Griffin, and [[Ken Ono]].<ref name=DGO/> Witten has also speculated that the version of string theory appearing in monstrous moonshine might be related to a certain simplified model of gravity in three spacetime dimensions.<ref name=Witten2007/>
== History ==
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<ref name=Nekrasov>{{cite journal |last1=Nekrasov |first1=Nikita |last2=Schwarz |first2=Albert |date=1998 |title=Instantons on noncommutative {{math|'''R'''<sup>4</sup>}} and (2,0) superconformal six dimensional theory |journal=Communications in Mathematical Physics |volume=198 |issue=3 |pages=689–703 |doi=10.1007/s002200050490|bibcode=1998CMaPh.198..689N |arxiv = hep-th/9802068 |s2cid=14125789 }}</ref>
<ref name="Polchinski 2007">{{cite journal |last1=Polchinski |first1=Joseph |title=All Strung Out? |journal=American Scientist |volume=95 |pages=72 |date=2007 |url=http://www.americanscientist.org/bookshelf/pub/all-strung-out |access-date=29 December 2016|doi=10.1511/2007.63.72 |url-access=subscription }}</ref>
<ref name=Randall>{{cite journal |last1=Randall |first1=Lisa |last2=Sundrum |first2=Raman |date=1999 |title=An alternative to compactification |journal=Physical Review Letters |volume=83 |issue=23 |pages=4690–4693 |doi=10.1103/PhysRevLett.83.4690 |arxiv=hep-th/9906064 |bibcode = 1999PhRvL..83.4690R |s2cid=18530420 }}</ref>
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* {{Cite book| first = Roger | last = Penrose | year = 2005 | title = The Road to Reality: A Complete Guide to the Laws of the Universe | publisher = Knopf | isbn = 978-0-679-45443-4 | title-link = The Road to Reality: A Complete Guide to the Laws of the Universe }}
* {{Cite book| first = Lee | last = Smolin | year = 2006 | title = The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next | publisher = Houghton Mifflin Co. | ___location = New York | isbn = 978-0-618-55105-7 | title-link = The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next }}
* {{
=== Textbooks ===
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{{Wikiquote}}
* [https://www.bbc.co.uk/science/horizon/2001/paralleluni.shtml BBC's ''Parallel Universes''], 2002 feature documentary by ''[[Horizon (British TV series)|BBC Horizon]]'', episode "[[Parallel Universes (film)|Parallel Universes]]", focuses on history and emergence of M-theory, and scientists involved.
* [https://www.pbs.org/wgbh/nova/
{{String theory topics |state=collapsed}}
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