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Mtheory is a theory in physics that unifies all of the consistent versions of superstring theory. The existence of such a theory was first conjectured by Edward Witten at the string theory conference at the University of Southern California in the summer of 1995. Witten's announcement initiated a flurry of research activity known as the second superstring revolution.
Prior to Witten's announcement, string theorists had identified five different versions of superstring theory. Although these theories appeared at first to be very different, work by a number of different physicists including Ashoke Sen, Chris Hull, Paul Townsend, and Michael Duff showed that the theories were related in intricate and nontrivial ways. In particular, physicists found that the apparently distinct theories were identified by mathematical transformations called Sduality and Tduality. Witten's conjecture was based in part on the existence of these dualities and in part on the relationship of the string theories to a gravitational theory called elevendimensional supergravity.
Although a complete formulation of Mtheory is not known, the theory should describe two and fivedimensional objects called branes and should be approximated by elevendimensional supergravity at low energies. Modern attempts to formulate Mtheory are typically based on matrix theory or the AdS/CFT correspondence. According to Witten, the M in Mtheory can stand for "magic", "mystery", or "matrix" according to taste, and the true meaning of the title should be decided when a more fundamental formulation of the theory is known.
Investigations of the mathematical structure of Mtheory have spawned a number of important theoretical results in physics and mathematics. More speculatively, Mtheory may provide a framework for developing a unified theory of all of the fundamental forces of nature. Attempts to connect Mtheory to experiment typically focus on compactifying its extra dimensions to construct approximate models of our fourdimensional world.
One of the deepest problems in modern physics is the problem of quantum gravity. Our current understanding of gravity is based on Albert Einstein's general theory of relativity, which is formulated within the framework of classical physics. However, nongravitational forces are described within the framework of quantum mechanics, a radically different formalism for describing physical phenomena based on probability.^{[1]} A quantum theory of gravity is needed in order to reconcile general relativity with the principles of quantum mechanics,^{[2]} but difficulties arise when one attempts to apply the usual prescriptions of quantum theory to the force of gravity.^{[3]}
String theory is a theoretical framework that attempts to reconcile gravity and quantum mechanics. In string theory, the pointlike particles of particle physics are replaced by onedimensional objects called strings. These strings look like small segments or loops of ordinary string. String theory describes how strings propagate through space and interact with each other. On distance scales larger than the string scale, a string will look just like an ordinary particle, with its mass, charge, and other properties determined by the vibrational state of the string. One of the vibrational states of a string gives rise to the graviton, a quantum mechanical particle that mediates gravitational interactions.^{[4]}
There are several versions of string theory known as type I, type IIA, type IIB, and the two flavors of heterotic string theory (SO(32) and E_{8}×E_{8}). The different theories allow different types of strings, and the particles that arise at low energies exhibit different symmetries. For example, the type I theory includes both open strings (which are segments with endpoints) and closed strings (which form closed loops), while the type II theories include only closed strings. Each of these five string theories arises as a special limiting case of Mtheory. This theory, like its string theory predecessors, is an example of a quantum theory of gravity. It describes a force just like the familiar gravitational force subject to the rules of quantum mechanics.
In everyday life, there are three familiar dimensions of space (up/down, left/right, and forward/backward), and there is one dimension of time (later/earlier). Thus, in the language of modern physics, one says that spacetime is fourdimensional.^{[5]}
Despite the obvious relevance of fourdimensional spacetime for describing the physical world, there are several reasons why physicists often consider theories in other dimensions. In some cases, by modeling spacetime in a different number of dimensions, a theory becomes more mathematically tractable, and one can perform calculations and gain general insights more easily.^{[6]} There are also situations where theories in two or three spacetime dimensions are useful for describing phenomena in condensed matter physics. Finally, there exist scenarios in which there could actually be more than four dimensions of spacetime which have nonetheless managed to escape detection.^{[7]}
One notable feature of string theory and Mtheory is that these theories require extra dimensions of spacetime for their mathematical consistency. In string theory, spacetime is tendimensional, while in Mtheory it is elevendimensional. In order to describe real physical phenomena using these theories, one must therefore imagine scenarios in which these extra dimensions would not be observed in experiments.^{[8]}
Compactification is one way of modifying the number of dimensions in a physical theory. In compactification, some of the extra dimensions are assumed to "close up" on themselves to form circles.^{[9]} In the limit where these curled up dimensions become very small, one obtains a theory in which spacetime has effectively a lower number of dimensions. A standard analogy for this is to consider a multidimensional object such as a garden hose. If the hose is viewed from a sufficient distance, it appears to have only one dimension, its length. However, as one approaches the hose, one discovers that it contains a second dimension, its circumference. Thus, an ant crawling on the surface of the hose would move in two dimensions.^{[10]}
Theories that arise as different limits of Mtheory turn out to be related in highly nontrivial ways. One of the relationships that can exist between these different physical theories is called Sduality. This is a relationship which says that a collection of strongly interacting particles in one theory can, in some cases, be viewed as a collection of weakly interacting particles in a completely different theory. For example, type I string theory turns out to be equivalent by Sduality to the SO(32) heterotic string theory. Similarly, type IIB string theory is related to itself in a nontrivial way by Sduality.^{[11]}
Another relationship between different string theories is Tduality. Here one considers strings propagating around a circular extra dimension. Tduality states that a string propagating around a circle of radius is equivalent to a string propagating around a circle of radius in the sense that all observable quantities in one description are identified with quantities in the dual description. For example, a string has momentum as it propagates around a circle, and it can also wind around the circle one or more times. The number of times the string winds around a circle is called the winding number. If a string has momentum and winding number in one description, it will have momentum and winding number in the dual description. For example, one can show that type IIA string theory is equivalent to type IIB string theory via Tduality and also that the two versions of heterotic string theory are related by Tduality.^{[12]}
In general, the term duality refers to a situation where two seemingly different physical systems turn out to be equivalent in a nontrivial way. If two theories are related by a duality, it means that one theory can be transformed in some way so that it ends up looking just like the other theory. The two theories are then said to be dual to one another under the transformation. Put differently, the two theories are mathematically different descriptions of the same phenomena.
Another important theoretical idea that plays a role in Mtheory is supersymmetry. This is a mathematical relation that exists in certain physical theories between a class of particles called bosons and a class of particles called fermions. Roughly speaking, bosons are the constituents of radiation, while fermions are the constituents of matter. In theories with supersymmetry, each boson has a counterpart which is a fermion, and vice versa. When supersymmetry is imposed as a local symmetry, one automatically obtains a quantum mechanical theory that includes gravity. Such a theory is called a supergravity theory.^{[13]}
A theory of strings that incorporates the idea of supersymmetry is called a superstring theory. There are several different versions of superstring theory which are all subsumed within the Mtheory framework. At low energies, the superstring theories are approximated by supergravity in ten spacetime dimensions. Similarly, Mtheory is approximated at low energies by supergravity in eleven dimensions.
In string theory and related theories such as supergravity theories, a brane is a physical object that generalizes the notion of a point particle to higher dimensions. For example, a point particle can be viewed as a brane of dimension zero, while a string can be viewed as a brane of dimension one. It is also possible to consider higherdimensional branes. In dimension p, these are called pbranes. Branes are dynamical objects which can propagate through spacetime according to the rules of quantum mechanics. They have mass and can have other attributes such as charge. A pbrane sweeps out a (p+1)dimensional volume in spacetime called its worldvolume. Physicists often study fields analogous to the electromagnetic field which live on the worldvolume of a brane. The word brane comes from the word "membrane" which refers to a twodimensional brane.^{[14]}
In string theory, the fundamental objects that give rise to elementary particles are the onedimensional strings. Although the physical phenomena described by Mtheory are still poorly understood, physicists know that the theory describes two and fivedimensional branes. Much of the current research in Mtheory attempts to better understand the properties of these branes.
General relativity does not place any limits on the possible dimensions of spacetime. Although the theory is typically formulated in four dimensions, one can write down the same equations for the gravitational field in any number of dimensions. Supergravity is more restrictive because it places an upper limit on the number of dimensions.^{[15]} In 1978, work of Werner Nahm showed that the maximum spacetime dimension in which one can formulate a consistent supersymmetric theory is eleven.^{[16]} In the same year, Eugene Cremmer, Bernard Julia, and Joel Scherk of the École Normale Supérieure showed that supergravity not only permits up to eleven dimensions but is in fact most elegant in this maximal number of dimensions.^{[17]}^{[18]}
Initially, many physicists hoped that by compactifying elevendimensional supergravity, it might be possible to construct realistic models of our fourdimensional world. The hope was that such models would provide a unified description of the four fundamental forces of nature: electromagnetism, the strong and weak nuclear forces, and gravity. Interest in elevendimensional supergravity soon waned, however, as various flaws in this scheme were discovered. One of the problems was that the laws of physics appear to distinguish between left and right, a phenomenon known as chirality. As emphasized by Edward Witten and others, this chirality property cannot be readily derived by compactifying from eleven dimensions.^{[19]}
In the first superstring revolution in 1984, many physicists turned to string theory as a unified theory of particle physics and quantum gravity. Unlike supergravity theory, string theory was able to accommodate the chirality of the standard model, and it provided a theory of gravity consistent with quantum effects.^{[20]} Another feature of string theory that many physicists were drawn to in the 1980s and 1990s was its high degree of uniqueness. In ordinary particle theories, one can consider any collection of elementary particles whose classical behavior is described by an arbitrary Lagrangian. In string theory, the possibilities are much more constrained, and there are only a few consistent formulations of the theory. Indeed, by the 1990s, physicists had identified five consistent supersymmetric versions of the theory.
Although there was only a handful of consistent superstring theories, it remained a mystery why there was not just one consistent formulation. However, as physicists began to examine string theory more closely, they began to realize that these theories are related in intricate and nontrivial ways.
In the late 1970s, Claus Montonen and David Olive,^{[21]} had conjectured a special property of a quantum field theory called N = 4 supersymmetric Yang–Mills theory. This theory describes particles similar to the quarks and gluons that make up atomic nuclei. The strength with which the particles of this theory interact is measured by a number called the coupling constant. The result of Montonen and Olive, now known as Montonen–Olive duality, states that N=4 supersymmetric Yang–Mills theory with coupling constant is equivalent to the same theory with coupling constant . In other words, a system of strongly interacting particles (large coupling constant) has an equivalent description as a system of weakly interacting particles (small coupling constant) and vice versa.^{[22]}
In 1990, several theorists generalized Montonen–Olive duality to a relationship called Sduality which connects different string theories. For example, type IIB string theory with a large coupling constant is equivalent via Sduality to the same theory with small coupling constant. Theorists also found that different string theories may be related by a totally different kind of duality known as Tduality. This duality implies that strings propagating on completely different spacetime geometries may be physically equivalent.^{[23]}
String theory extends ordinary quantum field theory by promoting zerodimensional point particles to onedimensional objects called strings. In the late 1980s, it was natural for theorists to attempt to formulate other extensions of quantum field theory in which particles are replaced by twodimensional supermembranes or by higherdimensional objects called branes. Such objects had been considered as early as 1962 by Paul Dirac, and they were reconsidered by a small but enthusiastic group of physicists in the 1980s.^{[24]}
Supersymmetry severely restricts the possible number of dimensions of a brane. In 1987, Eric Bergshoeff, Ergin Sezgin, and Paul Townsend showed that elevendimensional supergravity includes twodimensional branes.^{[25]} Intuitively, these objects look like sheets or membranes propagating through the elevendimensional spacetime. Shortly after this discovery, Michael Duff, Paul Howe, Takeo Inami, Kellogg Stelle considered a particular compactification of elevendimensional supergravity with one of the dimensions curled up into a circle.^{[26]} In this setting, one can imagine the membrane wrapping around the circular dimension. If the radius of the circle is sufficiently small, then this membrane looks just like a string in tendimensional spacetime. In fact, Duff and his collaborators showed that this construction reproduces exactly the strings appearing in type IIA superstring theory.^{[27]}
In 1990, Andrew Strominger published a similar result which suggested that strongly interacting strings in ten dimensions might have an equivalent description in terms of weakly interacting fivedimensional branes.^{[28]} Initially, physicists were unable to prove this relationship for two important reasons. On the one hand, the Montonen–Olive duality was still unproven, and so Strominger's conjecture was even more tenuous. On the other hand, there were many technical issues related to the quantum properties of fivedimensional branes.^{[29]} The first of these problems was solved in 1993 when Ashoke Sen established that certain physical theories require the existence of objects with both electric and magnetic charge which were predicted by the work of Montonen and Olive.^{[30]}
In spite of this progress, the relationship between strings and fivedimensional branes remained conjectural because theorists were unable to quantize the branes. Starting in 1991, a team of researchers including Michael Duff, Ramzi Khuri, Jianxin Lu, and Ruben Minasian considered a special compactification of string theory in which four of the ten dimensions curl up. If one considers a fivedimensional brane wrapped around these extra dimensions, then the brane looks just like a onedimensional string. In this way, the conjectured relationship between strings and branes was reduced to a relationship between strings and strings, and the latter could be tested using already established theoretical techniques.^{[31]}
Speaking at the string theory conference at the University of Southern California in 1995, Edward Witten of the Institute for Advanced Study made the surprising suggestion that all five superstring theories were in fact just different limiting cases of a single theory in eleven spacetime dimensions. Witten's announcement drew together all of the previous results on S and Tduality and the appearance of two and fivedimensional branes in string theory.^{[32]} In the months following Witten's announcement, hundreds of new papers appeared on the Internet confirming that the new theory involved membranes in an important way.^{[33]} Today this flurry of work is known as the second superstring revolution.
One of the important developments following Witten's announcement was Witten's work in 1996 with string theorist Petr Hořava.^{[34]}^{[35]} Witten and Hořava studied Mtheory on a special spacetime geometry with two tendimensional boundary components. Their work shed light on the mathematical structure of Mtheory and suggested possible ways of connecting Mtheory to real world physics.^{[36]}
Initially, some physicists suggested that the new theory was a fundamental theory of membranes, but Witten was skeptical of the role of membranes in the theory. In a paper from 1996, Hořava and Witten wrote
As it has been proposed that the elevendimensional theory is a supermembrane theory but there are some reasons to doubt that interpretation, we will noncommittally call it the Mtheory, leaving to the future the relation of M to membranes.^{[37]}
In the absence of an understanding of the true meaning and structure of Mtheory, Witten has suggested that the M should stand for "magic", "mystery", or "matrix" according to taste, and the true meaning of the title should be decided when a more fundamental formulation of the theory is known.^{[38]}
The application of quantum mechanics to physical objects such as the electromagnetic field, which are extended in space and time, is known as quantum field theory.^{[39]} In particle physics, quantum field theories form the basis for our understanding of elementary particles, which are modeled as excitations in the fundamental fields. Quantum field theories are also used throughout condensed matter physics to model particlelike objects called quasiparticles.^{[40]}
One approach to formulating Mtheory and studying its properties is provided by the antide Sitter/conformal field theory (AdS/CFT) correspondence. Proposed by Juan Maldacena in late 1997, the AdS/CFT correspondence is a theoretical result which implies that Mtheory is in some cases equivalent to a quantum field theory.^{[41]} In addition to providing insights into the mathematical structure of string and Mtheory, the AdS/CFT correspondence has shed light on many aspects of quantum field theory in regimes where traditional calculational techniques are ineffective.^{[42]}
In the AdS/CFT correspondence, one considers string theory or Mtheory on an antide Sitter background. This means that the geometry of spacetime is described in terms of a certain vacuum solution of Einstein's equation called antide Sitter space.^{[43]} In very elementary terms, antide Sitter space is a mathematical model of spacetime in which the notion of distance between points (the metric) is different from the notion of distance in ordinary Euclidean geometry. It is closely related to hyperbolic space, which can be viewed as a disk as illustrated on the left.^{[44]} This image shows a tessellation of a disk by triangles and squares. One can define the distance between points of this disk in such a way that all the triangles and squares are the same size and the circular outer boundary is infinitely far from any point in the interior.^{[45]}
Now imagine a stack of hyperbolic disks where each disk represents the state of the universe at a given time. The resulting geometric object is threedimensional antide Sitter space.^{[46]} It looks like a solid cylinder in which any cross section is a copy of the hyperbolic disk. Time runs along the vertical direction in this picture. The surface of this cylinder plays an important role in the AdS/CFT correspondence. As with the hyperbolic plane, antide Sitter space is curved in such a way that any point in the interior is actually infinitely far from this boundary surface.^{[47]}
This construction describes a hypothetical universe with only two space and one time dimension, but it can be generalized to any number of dimensions. Indeed, hyperbolic space can have more than two dimensions and one can "stack up" copies of hyperbolic space to get higherdimensional models of antide Sitter space.^{[48]}
An important feature of antide Sitter space is its boundary (which looks like a cylinder in the case of threedimensional antide Sitter space). One property of this boundary is that, locally around any point, it looks just like Minkowski space, the model of spacetime used in nongravitational physics.^{[49]} One can therefore consider an auxiliary theory in which "spacetime" is given by the boundary of antide Sitter space. This observation is the starting point for AdS/CFT correspondence, which states that the boundary of antide Sitter space can be regarded as the "spacetime" for a quantum field theory. The claim is that this quantum field theory is equivalent to the gravitational theory on the bulk antide Sitter space in the sense that there is a "dictionary" for translating calculations in one theory into calculations in the other. Every entity in one theory has a counterpart in the other theory. For example, a single particle in the gravitational theory might correspond to some collection of particles in the boundary theory. In addition, the predictions in the two theories are quantitatively identical so that if two particles have a 40 percent chance of colliding in the gravitational theory, then the corresponding collections in the boundary theory would also have a 40 percent chance of colliding.^{[50]}
One particular realization of the AdS/CFT correspondence states that Mtheory on the product space is equivalent to the socalled (2,0)theory on the sixdimensional boundary.^{[51]} In this example, the spacetime of the gravitational theory is effectively sevendimensional (hence the notation ), and there are four additional "compact" dimensions (encoded by the factor). In the real world, spacetime is fourdimensional, at least macroscopically, so this version of the correspondence does not provide a realistic model of gravity. Likewise, the dual theory is not a viable model of any realworld system since it describes a world with six spacetime dimensions.
Nevertheless, the (2,0)theory has proven to be important for studying the general properties of quantum field theories.^{[52]} Indeed, this theory subsumes a large number of mathematically interesting effective quantum field theories and points to new dualities relating these theories. For example, Luis Alday, Davide Gaiotto, and Yuji Tachikawa showed that by compactifying this theory on a surface, one obtains a fourdimensional quantum field theory, and there is a duality known as the AGT correspondence which relates the physics of this theory to certain physical objects associated with the surface itself.^{[53]} More recently, theorists have extended these ideas to study the theories obtained by compactifying down to three dimensions.^{[54]}^{[55]}
In addition to its applications in quantum field theory, the (2,0)theory has spawned a number of important results in pure mathematics. For example, the existence of the (2,0)theory was used by Witten to give a "physical" explanation for a conjectural relationship in mathematics called the geometric Langlands correspondence. Witten's results also gave an intuitive geometric explanation of a well known fact from classical electrodynamics, namely the invariance of Maxwell's equations under the interchange of electric and magnetic fields.^{[56]} In subsequent work, Witten showed that the (2,0)theory could be used to understand a concept in mathematics called Khovanov homology.^{[57]} Developed by Mikhail Khovanov around 2000, Khovanov homology provides a tool in knot theory, the branch of mathematics that studies and classifies the different shapes of knots.^{[58]}
Another realization of the AdS/CFT correspondence states that Mtheory on is equivalent to a quantum field theory called the ABJM theory in three dimensions. In this version of the correspondence, seven of the dimensions of Mtheory are curled up, leaving four noncompact dimensions. Since the spacetime of our universe is fourdimensional, this version of the correspondence provides a somewhat more realistic description of gravity.^{[59]}
The ABJM theory appearing in this version of the correspondence is also interesting for a variety of reasons. Introduced by Aharony, Bergman, Jafferis, and Maldacena, it is closely related to another quantum field theory called ChernSimons theory. The latter theory was popularized by Witten in the late 1980s because of its applications to knot theory.^{[60]} In addition, the ABJM theory serves as a useful toy model for solving problems that arise in condensed matter physics.^{[61]}
In addition being an idea of considerable theoretical interest, Mtheory provides a framework for constructing models of real world physics that combine general relativity with the standard model of particle physics. Phenomenology is the branch of theoretical physics in which physicists construct realistic models of nature from more abstract theoretical ideas. String phenomenology is the part of string theory that attempts to construct realistic models of particle physics based on string and Mtheory.
Typically, such models are based on the idea of compactification.^{[62]} Starting with the ten or elevendimensional spacetime of string or Mtheory, physicists postulate a shape for the extra dimensions. By choosing this shape appropriately, they can construct models roughly similar to the standard model of particle physics, together with additional undiscovered particles.^{[63]} One popular way of deriving realistic physics from string theory is to start with the heterotic theory in ten dimensions and assume that the six extra dimensions of spacetime are shaped like a sixdimensional Calabi–Yau manifold. This is a special kind of geometric object named after mathematicians Eugenio Calabi and ShingTung Yau.^{[64]} Calabi–Yau manifolds offer many ways of extracting realistic physics from string theory. Other similar methods can be used to construct realistic models of our fourdimensional world based on Mtheory.^{[65]}
Partly because of theoretical and mathematical difficulties and partly because of the extremely high energies needed to test these theories experimentally, there is so far no experimental evidence that would unambiguously point to any of these models being a correct fundamental description of nature. This has led some in the community to criticize these approaches to unification and question the value of continued research on these problems.^{[66]}
In one approach to Mtheory phenomenology, theorists assume that the seven extra dimensions of Mtheory are shaped like a G_{2} manifold. This is a special kind of sevendimensional shape constructed by mathematician Dominic Joyce of the University of Oxford.^{[67]} These G_{2} manifolds are still poorly understood mathematically, and this fact has made it difficult for physicists to fully develop this approach to phenomenology.^{[68]}
For example, physicists and mathematicians often assume that space has a mathematical property called smoothness, but this property cannot be assumed in the case of a G_{2} manifold if one wishes to recover the physics of our fourdimensional world. Another problem is that G_{2} manifolds are not complex manifolds, so theorists are unable to use tools from the branch of mathematics known as complex analysis. Finally, there are many open questions about the existence, uniqueness, and other mathematical properties of G_{2} manifolds, and mathematicians lack a systematic way of searching for these manifolds.^{[69]}
Because of the difficulties with G_{2} manifolds, most attempts to construct realistic theories of physics based on Mtheory have taken a more indirect approach to compactifying elevendimensional spacetime. One approach, pioneered by Witten, Hořava, Burt Ovrut, and others, is known as heterotic Mtheory. In this approach, one imagines that one of the eleven dimensions of Mtheory is shaped like a circle. If this circle is very small, then the spacetime becomes effectively tendimensional. One then assumes that six of the ten dimensions form a Calabi–Yau manifold. If this Calabi–Yau manifold is also taken to be small, one is left with a theory in fourdimensions.^{[70]}