M-Theory

physicsstring theorytikz

One of the pivotal insights of the 2nd superstring revolution point was Edward Witten's formulation of M-Theory in 1995. Before that, there had been 5 different types of string theories, each 10-dimensional: Type I, Type IIa and IIb, and two different heterotic theories. Witten showed that all of these theories are simply different limiting cases of a single overarching string theory (without showing what that underlying theory actually is). See Sean Carroll's excellent post on M-Theory for details.