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 put forth the idea that each was a manifestation of a single underlying theory.

Witten 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.

### The Second String Theory Revolution

In the 1980's and early 1990's, string theory had reached something of a problem due to an abundance of riches. By applying supersymmetry to string theory, into the combined superstring theory, physicists (including Witten himself) had explored the possible structures of these theories, and the resulting work had shown 5 distinct versions of superstring theory. Research further showed that you could use certain forms of mathematical transformations, called S-duality and T-duality, between the different versions of string theory. Physicists were at a loss

At a physics conference on string theory, held at the University of Southern California in spring of 1995, Edward Witten proposed his conjecture that these dualities be taken seriously. What if, he suggested, the physical meaning of these theories is that the different approaches to string theory were different ways of mathematically expressing the same underlying theory. Though he did not have the details of that underlying theory mapped out, he suggested the name for it, M-Theory.

Part of the idea at the heart of string theory itself is that the four dimensions (3 space dimensions and one time dimension) of our observed universe can be explained by thinking of the universe as having 10 dimensions, but then "compactifying" 6 of those dimensions up into a sub-microscopic scale that is never observed. Indeed, Witten himself was one of the people who had developed this method back in the early 1980's! He now suggested doing the same thing, by assuming additional dimensions that would allow for the transformations between the different 10-dimensional string theory variants.

The enthusiasm of research that sprung out of that meeting, and the attempt to derive the properties of M-Theory, inaugurated an era that some have called the "second string theory revolution" or "second superstring revolution."

### 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 if Witten's conjecture turns out to be true:

- 11 dimensions of spacetime (these extra dimensions should not be confused with the idea in physics of a multiverse of parallel universes)
- 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 does the "M" Stand 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, led in large part by Leonard Susskind, have developed Matrix Theory, which they believe could eventually co-opt the M if it is ever shown to be true.

### Is M-Theory True?

M-Theory, like the variants of string theory, has the problem that it is at present makes no real predictions that can be tested in an attempt to confirm or refute the theory. Many theoretical physicists continue to research this area, but when you have over two decades of research with no solid results, enthusiasm undoubtedly wanes a bit. There is no evidence, however, that strong argues that Witten's M-Theory conjecture is false, either. This may be a case where a failure to disprove the theory, such as by showing it to be internally contradictory or inconsistent in some way, is the best that physicists can hope for at the time being.