Mtheory
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In theoretical physics, Mtheory is a mathematical proposal that unifies the five tendimensional superstring theories as limits of a single 11dimensional theory. Though a full description of the theory is not yet known, the lowenergy dynamics are known to be supergravity interacting with 2 and 5dimensional membranes. This theory is the unique supersymmetric theory in eleven dimensions, with its lowenergy matter content and interactions fully determined.
Drawing on the work of a number of string theorists (including Ashoke Sen, Chris Hull, Paul Townsend, Michael Duff and John Schwarz); Edward Witten of the Institute for Advanced Study suggested its existence at a conference at USC in 1995, and used Mtheory to explain a number of previously observed dualities, sparking a flurry of new research in string theory called the second superstring revolution.
According to Witten and others, the M in Mtheory could stand for master, mathematical, mother, mystery, membrane, magic, or matrix. Witten reluctantly admits the M in Mtheory can also stand for murky because the level of understanding of the theory is so primitive. However, originally the letter was taken from membrane, but since Witten was more skeptical to membranes than his colleagues, he just kept the "M". Later, he let the meaning be a matter of taste for the user of the word "Mtheory".
In the early 1990s, it was shown that the various superstring theories were related by dualities, which allow physicists to relate the description of an object in one super string theory to the description of a different object in another super string theory. These relationships imply that each of the super string theories is a different aspect of a single underlying theory, proposed by Witten, and named "Mtheory".
Mtheory is not yet complete; however it can be applied in many situations (usually by exploiting string theoretic dualities). The theory of electromagnetism was also in such a state in the mid19th century; there were separate theories for electricity and magnetism and, although they were known to be related, the exact relationship was not clear until James Clerk Maxwell published his equations, in his 1864 paper A Dynamical Theory of the Electromagnetic Field. Witten has suggested that a general formulation of Mtheory will probably require the development of new mathematical language. However, some scientists have questioned the tangible successes of Mtheory given its current incompleteness, and limited predictive power, even after so many years of intense research.
In late 2007, Bagger, Lambert and Gustavsson set off renewed interest in Mtheory with the discovery of a candidate Lagrangian description of coincident M2branes, based on a nonassociative generalization of Lie Algebra, Nambu 3algebra or Filippov 3algebra. Practitioners hope the BaggerLambertGustavsson action (BLG action) will provide the longsought microscopic description of Mtheory.
History and Development
Prior to May 1995
Prior to 1995 there were five (known) consistent superstring theories (here on referred to as string theories), which were given the names Type I string theory, Type IIA string theory, Type IIB string theory, heterotic SO(32) (the HO string) theory, and heterotic E_{8}×E_{8} (the HE string) theory. The five theories all share essential features that relate them to the name of string theory. Each theory is fundamentally comprised of vibrating, one dimensional strings at approximately the length of the Planck length. Calculations have also shown that each theory requires more than the normal four spacetime dimensions (although all extra dimensions are in fact spatial.) However, when the theories are analyzed in detail, significant differences appear.
Type I string theory and others
The Type I string theory has vibrating strings like the rest of the string theories. These strings vibrate both in closed loops, so that the strings have no ends, and as open strings with two loose ends. The open loose strings are what separates the Type I string theory from the other four string theories. This was a feature that the other string theories did not contain (The Type IIA and Type IIB string theories also contain open strings, however these strings are bound to Dbranes, that is to say, they are tight).
String vibrational patterns
Furthermore, calculations show that the list of string vibrational patterns and the way each pattern interacts and influences others vary from one theory to another. These and other differences hindered the development of the string theory as being the theory that united quantum mechanics and general relativity successfully. Attempts by the physics community to eliminate four of the theories, leaving only one string theory, have not been successful.
Mtheory
Mtheory attempts to unify the five string theories by examining certain identifications and dualities. Thus each of the five string theories becomes a special case of Mtheory.
As the names suggest, some of these string theories were thought to be related to each other. In the early 1990s, string theorists discovered that some relations were so strong that they could be thought of as an identification.
Type IIA and Type IIB
The Type IIA string theory and the Type IIB string theory were known to be connected by Tduality; this essentially meant that the IIA string theory description of a circle of radius R is exactly the same as the IIB description of a circle of radius 1/R, where distances are measured in units of the Planck length.
This was a profound result. First, this was an intrinsically quantum mechanical result; the identification did not hold in the realm of classical physics. Second, because it is possible to build up any space by gluing circles together in various ways, it would seem that any space described by the IIA string theory can also be seen as a different space described by the IIB theory. This implies that the IIA string theory can identify with the IIB string theory: any object which can be described with the IIA theory has an equivalent, although seemingly different, description in terms of the IIB theory. This suggests that the IIA string theory and the IIB string theory are really aspects of the same underlying theory.
Other dualities
There are other dualities between the other string theories. The heterotic SO(32) and the heterotic E_{8}×E_{8} theories are also related by Tduality; the heterotic SO(32) description of a circle of radius R is exactly the same as the heterotic E_{8}×E_{8} description of a circle of radius 1/R. This implies that there are really only three superstring theories, which might be called (for discussion) the Type I theory, the Type II theory, and the heterotic theory.
There are still more dualities, however. The Type I string theory is related to the heterotic SO(32) theory by Sduality; this means that the Type I description of weakly interacting particles can also be seen as the heterotic SO(32) description of very strongly interacting particles. This identification is somewhat more subtle, in that it identifies only extreme limits of the respective theories. String theorists have found strong evidence that the two theories are really the same, even away from the extremely strong and extremely weak limits, but they do not yet have a proof strong enough to satisfy mathematicians. However, it has become clear that the two theories are related in some fashion; they appear as different limits of a single underlying theory.
Only two string theories
Given the above commonalities there appear to be only two string theories: the heterotic string theory (which is also the type I string theory) and the type II theory. There are relations between these two theories as well, and these relations are in fact strong enough to allow them to be identified.
Last step
This last step is best explained first in a certain limit. In order to describe our world, strings must be extremely tiny objects. So when one studies string theory at low energies, it becomes difficult to see that strings are extended objects — they become effectively zerodimensional (pointlike). Consequently, the quantum theory describing the low energy limit is a theory that describes the dynamics of these points moving in spacetime, rather than strings. Such theories are called quantum field theories. However, since string theory also describes gravitational interactions, one expects the lowenergy theory to describe particles moving in gravitational backgrounds. Finally, since superstring string theories are supersymmetric, one expects to see supersymmetry appearing in the lowenergy approximation. These three facts imply that the lowenergy approximation to a superstring theory is a supergravity theory.
Supergravity theories
The possible supergravity theories were classified by Werner Nahm in the 1970s. In 10 dimensions, there are only two supergravity theories, which are denoted Type IIA and Type IIB. This similar denomination is not a coincidence; the Type IIA string theory has the Type IIA supergravity theory as its lowenergy limit and the Type IIB string theory gives rise to Type IIB supergravity. The heterotic SO(32) and heterotic E_{8}×E_{8} string theories also reduce to Type IIA and Type IIB supergravity in the lowenergy limit. This suggests that there may indeed be a relation between the heterotic/Type I theories and the Type II theories.
In 1994, Edward Witten outlined the following relationship: The Type IIA supergravity (corresponding to the heterotic SO(32) and Type IIA string theories) can be obtained by dimensional reduction from the single unique elevendimensional supergravity theory. This means that if one studied supergravity on an elevendimensional spacetime that looks like the product of a tendimensional spacetime with another very small onedimensional manifold, one gets the Type IIA supergravity theory. (And the Type IIB supergravity theory can be obtained by using Tduality.) However, elevendimensional supergravity is not consistent on its own — it does not make sense at extremely high energy, and likely requires some form of completion. It seems plausible, then, that there is some quantum theory — which Witten dubbed Mtheory — in elevendimensions which gives rise at low energies to elevendimensional supergravity, and is related to tendimensional string theory by dimensional reduction. Dimensional reduction to a circle yields the Type IIA string theory, and dimensional reduction to a line segment yields the heterotic SO(32) string theory.
Same underlying theory
Mtheory would implement the notion that all of the different string theories are different special cases and/or different presentations of the same underlying theory (Mtheory). Thus the concept of string theory is expanded. Unfortunately little is known about Mtheory, but there is a great deal of interest in the concept from the theoretical physics community. Computations in Mtheory and string theory in general are extremely complex, so concrete results are very difficult to produce. It may be some time before the full implications of these theories are known.
The promise of Mtheory is that all of the different string theories would become different limits of a single underlying theory.
Nomenclature
There are two issues to be dealt with here:
 When Witten named Mtheory, he did not specify what the "M" stood for, presumably because he did not feel he had the right to name a theory which he had not been able to fully describe. According to Witten himself, "'M' stands for "magic," "mystery" , or "matrix", according to taste." According to the BBC/ TLC documentary Parallel Universes, the M stands for "membrane". Other suggestions by people such as Michio Kaku, Michael Duff and Neil Turok in that documentary are "mother" (as in "mother of all theories"), and "master" theory.
Cynics have noted that the M might be an upside down "W", standing for Witten. Others have suggested that for now, the "M" in Mtheory should stand for Missing or Murky. The various speculations as to what "M" in "Mtheory" stands for are explored in the PBS documentary based on Brian Greene's book The Elegant Universe.
 The name Mtheory is slightly ambiguous. It can be used to refer to both the particular elevendimensional theory which Witten first proposed, or it can be used to refer to a kind of theory which looks in various limits like the various string theories. Ashoke Sen has suggested that more general theory could go by the name Utheory, which might stand for Ur, Uber, Ultimate, Underlying, or perhaps Unified. (It might also stand for Uduality, which is both a reference to Sen's own work and a kind of particle physics pun.)
Mtheory in the following descriptions refers to the more general theory, and will be specified when used in its more limited sense.
Mtheory and membranes
In the standard string theories, strings are assumed to be the single fundamental constituent of the universe. Mtheory adds another fundamental constituent  membranes. Like the tenth spatial dimension, the approximate equations in the original five superstring models proved too weak to reveal membranes.
Pbranes
A membrane, or brane, is a multidimensional object, usually called a pbrane, with p referring to the number of dimensions in which it exists. The value of 'p' can range from zero to nine, thus giving branes dimensions from zero (0brane ≡ point particle) to nine  five more than the world we are accustomed to inhabiting (3 spatial and 1 time). The inclusion of pbranes does not render previous work in string theory wrong on account of not taking note of these pbranes. Pbranes are much more massive ("heavier") than strings, and when all higherdimensional pbranes are much more massive than strings, they can be ignored, as researchers had done unknowingly in the 1970s.
Strings with "loose ends"
Shortly after Witten's breakthrough in 1995, Joseph Polchinski of the University of California, Santa Barbara discovered a fairly obscure feature of string theory. He found that in certain situations the endpoints of strings (strings with "loose ends") would not be able to move with complete freedom as they were attached, or stuck within certain regions of space. Polchinski then reasoned that if the endpoints of open strings are restricted to move within some pdimensional region of space, then that region of space must be occupied by a pbrane. These type of "sticky" branes are called Dirichletpbranes, or Dpbranes. His calculations showed that the newly discovered Dpbranes had exactly the right properties to be the objects that exert a tight grip on the open string endpoints, thus holding down these strings within the pdimensional region of space they fill.
Strings with closed loops
Not all strings are confined to pbranes. Strings with closed loops, like the graviton, are completely free to move from membrane to membrane. Of the four force carrier particles, the graviton is unique in this way. Researchers speculate that this is the reason why investigation through the weak force, the strong force, and the electromagnetic force have not hinted at the possibility of extra dimensions. These force carrier particles are strings with endpoints that confine them to their pbranes. Further testing is needed in order to show that extra spatial dimensions indeed exist through experimentation with gravity.
Membrane Interactions
One of the reasons MTheory is so difficult to formulate is that the numbers of different types of membranes in the various dimensions increases exponentially. For example once you get to 3 dimensional surfaces you have to deal with solid objects with knot shaped holes, and then you need the whole of knot theory just to classify them. Since MTheory is thought to operate in 11 dimensions this problem then becomes very difficult. But just like string theory, in order for the theory to satisfy causality, the theory must be local, and so the topology changing must occur at a single point. The basic orientable 2brane interactions are easy to show. Orientable 2branes are tori with multiple holes cut out of them.
Matrix theory
The original formulation of Mtheory was in terms of a (relatively) lowenergy effective field theory, called 11dimensional Supergravity. Though this formulation provided a key link to the lowenergy limits of string theories, it was recognized that a full highenergy formulation (or "UVcompletion") of Mtheory was needed.
Analogy with water
For an analogy, the Supergravity description is like treating water as a continuous, incompressible fluid. This is effective for describing longdistance effects such as waves and currents, but inadequate to understand shortdistance/highenergy phenomena such as evaporation, for which a description of the underlying molecules is needed. What, then, are the underlying degrees of freedom of Mtheory?
9 matrices
Banks, Fischler, Shenker and Susskind (BFSS) conjectured that Matrix theory could provide the answer. They demonstrated that a theory of 9 very large matrices, evolving in time, could reproduce the Supergravity description at low energy, but take over for it as it breaks down at high energy. While the Supergravity description assumes a continuous spacetime, Matrix theory predicts that, at short distances, noncommutative geometry takes over, somewhat similar to the way the continuum of water breaks down at short distances in favour of the graininess of molecules.
Books
 Brian Greene has written books explaining string theory and Mtheory for the layperson in 1999, The Elegant Universe, ISBN 0375708111 and in 2004, The Fabric of the Cosmos, ISBN 0375412883.
 Kaku, Michio (1999). Strings, Conformal Fields, and MTheory. New York: Springer. ISBN 0387988920. for a more advanced introduction.