Editing Heterotic string theory (section)

==String duality==
[[String duality]] is a class of symmetries in physics that link different string theories. In the 1990s, it was realized that the strong coupling limit of the HO theory is [[type I string theory]] &mdash; a theory that also contains [[Open string (physics)|open strings]]; this relation is called [[S-duality]]. The HO and HE theories are also related by [[T-duality]].

Because the various superstring theories were shown to be related by dualities, it was proposed that each type of string was a different limit of a single underlying theory called [[M-theory]].

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