In physics, M-theory (sometimes also called U-theory) is put forward as the master theory that unifies the five superstring theories. Drawing on the work from a number of string theorists (including Chris Hull, Paul Townsend, Ashoke Sen, Michael Duff, and John Schwarz), Edward Witten of the Institute for Advanced Study proposed the existence of this physical model at a conference at USC in 1995, explaining a number of previously observed dualities and sparking a flurry of new research in string theory, called the second superstring revolution.

M-theory

Apart from the fact that instead of one there are five different, healthy theories of strings (three superstrings and two heterotic strings) there was another difficulty in studying these theories: we did not have tools to explore the theory over all possible values of the parameters in the theory. Each theory was like a large planet of which we only knew a small island somewhere on the planet. But over the last four years, techniques were developed to explore the theories more thoroughly, in other words, to travel around the seas in each of those planets and find new islands. And only then it was realized that those five string theories are actually islands on the same planet, not different ones! Thus there is an underlying theory of which all string theories are only different aspects. This was called M-theory. The M might stand for Mother of all theories or Mystery, because the planet we call M-theory is still largely unexplored.

There is still a third possibility for the M in M-theory. One of the islands that was found on the M-theory planet corresponds to a theory that lives not in 10 but in 11 dimensions. This seems to be telling us that M-theory should be viewed as an 11 dimensional theory that looks 10 dimensional at some points in its space of parameters. Such a theory could have as a fundamental object a Membrane, as opposed to a string.