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-DualityHorava-Witten String Theory M-Theory Holographic 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 | |

Like T-Duality, **S-Duality** is a duality, or equivalence, between theories. Unlike T-Duality, **S-Duality** is * not* a purely stringy concept as it also appears in Quantum Field Theorys.

**S-Duality**is useful because equates a Strongly-Coupled theory with a Weakly-Coupled theory. I.e. it equates a theory with coupling constant with a theory with coupling constant .

## S-Duality and the DilatonEdit this section

In String Theory, the coupling constant is related to the Dilaton Field:

Negating the Dilaton Field is equivalent to inverting the coupling constant:

Therefore, **S-Duality** negates the Dilaton Field, in a similar way as to how T-Duality negates the bosonic and fermionic fields, ; and /.

## S-Duality in String TheoryEdit this section

In String Theory, analysing the massless fields and mass spectra of String Theoryies shows that Type I String Theory is **S-Dual** to Type HO String Theory and Type IIB String Theory is **S-Dual** to itself. More specifically, Type I String Theory with D-Branes instead of F1-Strings is **S-Dual** to Type HO String Theory and Type IIB String Theory with D-Branes instead of F1-Strings is **S-Dual** to itself (but with F1 Strings).