S-duality

In string theory, strings are strongly interacting if they combine and decay frequently. Strings are weakly interacting if they combine and decay, less frequently. In S-duality, a collection of strongly interacting strings can be viewed as being a collection of weakly interacting strings. They are equivalent. Type I string theory is related to the Heterotic SO (32) string theory by S-duality and Type IIB string theory is related to itself by S-duality.

S-duality is a strong-weak duality. This is when a strongly coupled string theory is dual, or shown to be physically identical to a weakly coupled string theory. A coupling constant is strong if it is more than 1 and weak if it is less than 1. The coupling constant of string theory is a positive number that will tell us the probability of a collection of strings joining or splitting. This is the most basic process in string theory. Type I string theory is related to be heterotic SO(32) string theory by S-duality. Type IIB string theory is also related to itself by S-duality. 

The N=4 supersymmetric Yang-Mills theory describes particles like the quarks and gluons. The first kind of S-duality relationship was proposed by Claus Montonen and David Olive. They proposed that the N=4 supersymmetric Yang-Mills theory with the coupling constant "g" is equivalent to the same theory with the coupling constant "1/g". This result was known as Montonen-Olive duality. This was generalized to the S-duality relationship by some theorists in the 1990s. 

• Ashoke Sen, studied 4-dimensional heterotic strings, using S-duality. 

• Chris Hull and Paul Townsend, showed that type IIB string theory, with a large coupling constant is equivalent to itself, by S-duality, to the same theory with a small coupling constant.