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Definition: M-Theory is the name for a unified version of string theory, proposed in 1995 by the physicist Edward Witten. At the time of the proposal, there were 5 variations of string theory, but Witten believed that each was a manifestation of a single underlying theory.

He and others identified several forms of duality between the theories which, together with certain assumptions about the nature of the universe, could allow for them to all be one single theory: M-Theory. One of the major components of M-Theory is that it required adding yet another dimension on top of the already-numerous extra dimensions of string theory, so that the relationships between the theories could be worked out.

Properties of M-Theory

Though physicists have still not uncovered the secrets of M-Theory, they have identified several properties that the theory would have:
  • 11 dimensions of spacetime
  • contains strings and branes (originally called membranes)
  • methods of using compactification to explain how the extra dimensions reduce to the four spacetime dimensions we observe
  • dualities and identifications within the theory that allow it to reduce to special cases of the string theories known, and ultimately into the physics we observe in our universe

What is the M For?

It is unclear what the M in M-Theory is meant to stand for, though it is likely that it originally stood for "Membrane" since these had just been discovered to be a key element of string theory. Witten himself has been enigmatic on the subject, stating that the meaning of the M can be selected for taste. Possibilities include Membrane, Master, Magic, Mystery, and so on. A group of physicists have developed Matrix Theory, which they believe could eventually co-opt the M if it is ever shown to be true.
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