The many versions of string theory meld into one: M-theory
By the late 1980s, certain problems were evident with string theory. Foremost among these was the fact that there was not just one version of the theory but five, rejoicing in the exotic names of Types I, IIA and IIB, Heterotic O and Heterotic E. They all shared basically similar features: their vibrational patterns determined their possible mass and force charges, they each required a total of 10 spacetime dimensions and their curled up dimensions must be in one of the so-called Calabi-Yau shapes; but they differed in how they incorporated supersymmetry as well as in significant details of the vibrational patterns they support: Greene (2000).
Then in 1995, largely as a result of the labours of one Ed Witten, it was realised that these five separate theories were not separate at all, but different versions of the same theory. There is a concept in string theory called the string coupling constant that determines the likelihood that quantum fluctuations will cause a single string to split into two strings, yielding a virtual pair, and it was Witten who first realised that when the coupling constant in any one formulation of the five versions of string theory is dialled ever larger, the theory morphs into one of the other formulations of the theory, but with a coupling dialled ever smaller. For example, if the Type I string coupling is large, it transforms into the Heterotic-O theory with a coupling that’s small. Therefore the five different versions are not different after all, and each theory will transform or morph into one of the others.
Then in 1995, largely as a result of the labours of one Ed Witten, it was realised that these five separate theories were not separate at all, but different versions of the same theory. There is a concept in string theory called the string coupling constant that determines the likelihood that quantum fluctuations will cause a single string to split into two strings, yielding a virtual pair, and it was Witten who first realised that when the coupling constant in any one formulation of the five versions of string theory is dialled ever larger, the theory morphs into one of the other formulations of the theory, but with a coupling dialled ever smaller. For example, if the Type I string coupling is large, it transforms into the Heterotic-O theory with a coupling that’s small. Therefore the five different versions are not different after all, and each theory will transform or morph into one of the others.
This phenomenon is called duality and all 5 string theories are linked through a network of such dualities. This unified theory is called M-theory (no one quite knows why, but it may have something to do with ‘membrane’), and combines insights from all five formulations stitched together through the various duality relationships. According to M-theory, ours is not the only universe. Instead, M-theory predicts that a great many universes were created out of nothing, and their creation does not depend on the existence of a supernatural being or god. Instead, they arise naturally from physical law[1]. They are a prediction of science[2].
[1] Stephen Hawking and Leonard Mlodinow, The Grand Design, Bantam Press (2011), 8
[2] Source for figures 12.1 and 12.2: Greene (2000) 286, 287.