M-theory is a cutting-edge theory of physics that deals with the extension of superstring theory. It is somewhat contentious in the physics community, in the absence of experimental backup. If ever experimentally verified, M-theory and string theory would represent remarkable advances in science.
To understand M-theory it is necessary to first get some understanding of string theory. For hundreds of years physics has operated on the paradigm that the fundamental particles, like the familiar electron, are point-like or (in mathematical jargon) 0-dimensional. If string theory were to be summed up in a single idea, it is that this assumption is incorrect. Instead, string theory posits that the Universe is fundamentally composed of 1-dimensional objects—things that are similar to a string. These strings would be so small that on even the tiny scale of particles they would seem like points. In string theory, each fundamental particle is created in some sense by different patterns of vibration of the strings. One might ask why physicists have constrained themselves to 0-dimensional points for all this time; the answer is that 1-dimensional objects are much harder to work with and often cause technical problems with causality and violations of special relativity's mandate that nothing can travel faster than the speed of light.
String theory's development has come primarily because of an extremely important problem that has plagued physics for almost 100 years. The problem is that general relativity, the theory developed by Albert Einstein that explains things on very large or cosmological scales, is irreconcilable with quantum mechanics and the Standard Model, which describe the Universe on the small subatomic scale. Additionally, there are problems with the Standard Model: it has around 20 free parameters that must be plugged in by hand, and has a large number of particles it declares fundamental (there are three copies of every particle organized in what are termed as "families" whose only difference from one another is mass). Also, because it can't be reconciled with general relativity, it lacks a description of gravity, the most familiar of the four fundamental forces.
It turns out that using 1-dimensional objects instead of point particles solves many of these problems. The number of free parameters in the theory drops from 20 to one (a parameter that corresponds to the size of the strings), and there is hope that details of the theory will explain why the three families of particles exist. Most importantly, string theorists were delighted to find that string theory necessarily contains gravitons, the particle that causes gravity. This has led Edward Witten, the founder of M-theory, to joke that string theory does have the remarkable experimental evidence that gravity exists all around us. Thus, string theory successfully unites General Relativity with Quantum Mechanics.
However, there are some problems with string theory. First of all, it requires 10 dimensions for the strings to vibrate in as opposed to the 4 (3 space and 1 time) dimensions that we commonly observe. This may seem ludicrous; it is, however, possible if the extra 6 dimensions are extremely tiny and curled up. For example, if you look at a tube from a great distance it seems to be just a line (1-dimensional). Closer inspection of the tube reveals 2-dimensions, though: one that is along the tube (the one we saw from far away) and one that is going around the tube. Thus, the tube is really a 2-dimensional surface despite the fact that it appears to be a line from far away. Template:Unsolved
Another problem with string theory is that there are 5 different formulations of it. This is a key to understanding where M-theory comes in. The 5 theories differ in what kind of strings they allow and in how they implement supersymmetry, a technical part of string theory that leads to the often-used name superstring theory. These 5 different theories (esoterically called Type I, Type IIA, Type IIB, SO(32), and E8×E8 (pronounced E8 "times" E8, not E8 "ex" E8)) are something of an embarrassment of riches for string theorists. Since string theory claims to be a theory of everything, there should really only be one consistent formulation of the theory, but instead there are 5. Here is where M-theory steps in to the rescue.
In 1995, Edward Witten initiated what has been called the Second Superstring Revolution by introducing M-theory to the world. This theory combines the 5 different string theories (along with a previously abandoned attempt to unify General Relativity and Quantum Mechanics called (eleven-dimensional) Supergravity) into one theory. This is accomplished by knitting together a web of relationships between each of the theories called dualities (specifically, S-duality, T-duality, and U-duality). Each of these dualities provides a way of converting one of the string theories into another.
T-duality is probably the most easily explained of the dualities. It has to do with the size, denoted by R, of the curled up dimensions of the string theories. It was discovered that if you take a Type IIA string theory that has a size R and change the radius to 1/R then you will end up getting what is equivalent to a Type IIB theory of size R. This duality, along with the others, creates connections between all 5 (or 6, if you count supergravity) theories.
Factually, these dualities' existence had been known before Witten came up with the idea of M-theory. What Witten did was to predict that the fact that all of these different theories were connected was as a result of there being some underlying theory to which they were all approximations. Additionally, it was found that the equations that required string theory to exist in 10 dimensions were also approximations. The proposed (and somewhat nebulous) M-theory would instead be a theory that took place in 11 dimensions, although the details have not been pinned down.
M-Theory is a variation of String Theory, taking into account all of the 5 String Theories and showing they are basically the same (in a general sense). M-Theory is the most current version of SuperString Theory that uses 11 dimensions, Superstring Theory uses between 8 and 22.
What "M" may stand for
Additional amusement has come for many in guessing what the deliberately ambiguous letter "M" might stand for. Possibilities include Matrix theory, Muffin theory, Mystery theory, Magic theory, Maker theory, Murky theory, Membrane theory, Monstrous theory, and Mother theory. Sheldon Glashow believes that "M" is an upside-down "W" for Witten. Regardless of what the M might possibly mean, M-theory has become one of the most interesting and active areas of research in theoretical physics today.
For a more technical explanation, see M-theory.
- Brian Greene has written books explaining string theory and M-theory for the layman in 1999, The Elegant Universe, ISBN 0-375-70811-1 and in 2004, The Fabric of the Cosmos, ISBN 0-375-41288-3.
- The Elegant Universe - A Three-Hour Miniseries with Brian Greene by NOVA (original PBS Broadcast Dates: October 28th, 8-10 PM and November 4, 8-9 PM, 2003). Various images, texts, videos and animations explaining string theory and M-theory.
- Superstringtheory.com - The "Official String Theory Web Site", created by Patricia Schwarz. Excellent references on string theory and M-theory for the layman and expert.
- Basics of M-Theory is a lecture note on M-Theory.