S-duality
In theoretical
physics,
S-duality is a
symmetry of
M-theory, connecting the various
string theories, which are believed to be special cases of M-theory. An S-duality transformation maps states with coupling constant g in one theory to states with coupling constant 1/g in the dual theory. This has permitted the use of
perturbation theory, normally useful only for "weakly coupled" theories with g less than 1, to also describe the "strongly coupled" (g greater than 1) regimes of string theory, by mapping them onto dual, weakly coupled regimes.
Also see: T-duality, U-duality, S-duality (homotopy theory)