M-theory is a theory conjectured to exist by Edward Witten of the Institute for Advanced Study, that would unify the five different superstring theories: Type IIA string theory, Type IIB string theory, Heterotic string theory, SO(32) string theory, and heterotic E8×E8 string theory; all of these seemingly different string theories are simply different limits of the same underlying M-theory. Edward Witten has used M-theory to explain a number of observed dualities, such as the T-duality, U-duality and S-duality. The discovery of M-theory is hailed by string theorists as the second superstring revolution.
In M-theory, the fundamental constituents of the universe are membranes, or p-brane where p is the dimensionality of the membrane. Thus the strings theorized in String theory belongs in the class of 1 dimensional membranes, the 1-branes.
So far, no one has found any relation between M-theory and gravity, electricity, photons, or any other known aspect of the real world, nor has anyone shown that anything can be consistently calculated in M-theory. Scientists interviewed by BBC such as Neil Turok from Cambridge University referred to enigmatic and profound M-theory with some sense of humour as "Magical mystery, madness".
- Parallel Universes. BBC (14 February 2002). “NARRATOR: The tiny invisible strings of String Theory was supposed to be the fundamental building blocks of all the matter in the Universe, but now, with the addition of the eleventh dimension, they changed. They stretched and they combined. The astonishing conclusion was that all the matter in the Universe was connected to one vast structure: a membrane. In effect our entire Universe is a membrane. The quest to explain everything in the Universe could begin again and at its heart would be this new theory. It was dubbed Membrane Theory, or M Theory, but so enigmatic and profound did the idea seem that some thought M should stand for other things.... MICHAEL DUFF: Where M stands for magic, mystery or membrane. ... PAUL STEINHARDT: Physicists get kind of dreamy-eyed when they talk about M Theory. ... MICHIO KAKU: Maybe M stands for mother, the mother of all strings. Maybe it's magic. Maybe it's the majesty, the majesty of a comprehensive theory of the Universe. ... NEIL TUROK (Cambridge University): Magical mystery, madness.”