S-duality

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.