String Theory
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All Roads Lead to String Theory (Polchinski)
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Prior to the First Superstring Revolution
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Early History
| S-Matrix Theory Regge Trajectory | |

Bosonic String Theory
| Worldsheet String Bosonic String Theory String Perturbation Theory Tachyon Condensation | |

Supersymmetric Revolution
| Supersymmetry RNS Formalism GS Formalism BPS | |

Superstring Revolutions
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First Superstring Revolution
| GSO Projection Type II String Theory Type IIB String Theory Type IIA String Theory Type I String Theory Type H String Theory Type HO String Theory Type HE String Theory | |

Second Superstring Revolution
| T-Duality D-Brane S-Duality Horava-Witten String Theory M-TheoryHolographic Principle N=4 Super-Yang-Mills Theory AdS CFT BFSS Matrix Theory Matrix String Theory (2,0) Theory Twistor String Theory F-Theory String Field Theory Pure Spinor Formalism | |

After the Revolutions
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Phenomenology
| String Theory Landscape Minimal Supersymmetric Standard Model String Phenomenology | |

This article is or was once a |

**M-Theory** is basically a strongly-coupled theory in 11 dimensions from which other String Theories can be derived from.

## MotivationEdit this section

### DualitiesEdit this section

In the Second Superstring Revolution, it was realised that the ordinary 5 String Theoryies were connected by 2 dualities, namely T-Duality and S-Duality/. To be specific, these dualities can be portrayed on a map as you can see in the image here.

This means that some of these String Theoryies are in fact equivalent, so that there are really just 2 distinct String Theoryies, the Type IIA String Theory, and the Type HE String Theory.

At strong coupling (take the S-Dual), these become M-theory on a circle and the Horava-Witten String Theory, respectively. Various physicists, including Edward Witten, Ashok Sen, Nathan Seiberg, etc., postulated (see for example, ^{[1]}, ^{[2]} ^{[3]}) that these two String Theories are actually a \new theory, which came to be known as **M-Theory** compactified on a circle, and a line segment, respectively. This is also when Michael Duff coined the term "The Theory Formerly known as Strings" ^{[4]}.

### BranesEdit this section

While this is certnaily not proclaimable to be a motivation for **M-Theory**, it is worth stating here that the fundamental objects of **M-Theory** are not Strings, but actually 2-Branes and 5-Branes. This was not such a great surprise, because D-Branes already existed in ordinary String Theories.

### SupergravityEdit this section

Following the "Dualities" section, it is obvious that **M-Theory** needs to be an 11-Dimensional theory. Now, 11-Dimensional Supergravity is of course, an 11-Dimensional Theory. Various Supergravity Theoryies, including $ \mathcal{N} =2 $ Supergravity, for example, are low-energy limits of Superstring Theoryies. Thus, it was therefore perceived as possible, that 11-Dimensional $ \mathcal N=8 $Supergravity lies in the low-energy limit of **M-Theory**/ .

## Non-Perturbative FormulationEdit this section

*Main Article: BFSS Matrix Theory*

The non-perturbative formulation of **M-Theory** is BFSS Matrix Theory ^{[5]}.

## ReferencesEdit this section

- ↑ Witten, Edward (1996). "Five-Branes and M-Theory on an orbifold".
*Nucl.Phys.B***463**(6): 382-397. http://arxiv.org/pdf/hep-th/9512219v3.pdf. - ↑ Sen, Ashok (1996). "Unification of String Dualities".
*Nucl.Phys.Proc.Suppl.***58**(6): 5-19. http://arxiv.org/pdf/hep-th/9609176.pdf. - ↑ Townsend, Paul (1996). "Brane Surgery".
*Nucl.Phys.Proc.Suppl.***58**(6): 163-175. http://arxiv.org/pdf/hep-th/9512219v3.pdf. - ↑ Duff, Michael.
*"The Theory Formerly known as Strings"*. http://arxiv.org/pdf/hep-th/9608117v3.pdf. - ↑ Susskind, Leonard; Banks, T., Fischler, W., Shenker, S..
*M Theory as a Matrix model: A conjecture*. http://arxiv.org/pdf/hep-th/9610043v3.pdf.