M-theory

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For a generally accessible and less technical introduction to the topic, see Introduction to M-theory.

M-theory 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 S-duality and T-duality. 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 eleven-dimensional supergravity.

Although a complete formulation of M-theory is not known, the theory should describe two- and five-dimensional objects called branes and should be approximated by eleven-dimensional supergravity at low energies. Modern attempts to formulate M-theory are typically based on matrix theory or the AdS/CFT correspondence. According to Witten, the M in M-theory 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 M-theory have spawned a number of important theoretical results in physics and mathematics. More speculatively, M-theory may provide a framework for developing a unified theory of all of the fundamental forces of nature. Attempts to connect M-theory to experiment typically focus on compactifying its extra dimensions to construct approximate models of our four-dimensional world.

Background[edit]

Quantum gravity and strings[edit]

Main articles: Quantum gravity and String theory
A wavy open segment and closed loop of string.
The fundamental objects of string theory are open and closed strings.

One of the deepest problems in modern physics is 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. On the other hand, the 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 point-like particles of particle physics are replaced by one-dimensional 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 E8×E8). 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 M-theory. 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.

Number of dimensions[edit]

An example of compactification: At large distances, a two dimensional surface with one circular dimension looks one-dimensional.

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 four-dimensional.[5]

Despite the obvious relevance of four-dimensional 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 M-theory is that these theories require extra dimensions of spacetime for their mathematical consistency. In string theory, spacetime is ten-dimensional, while in M-theory it is eleven-dimensional. 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]

Dualities[edit]

Main articles: S-duality and T-duality
A diagram of string theory dualities. Yellow arrows indicate S-duality. Blue arrows indicate T-duality.

Theories that arise as different limits of M-theory turn out to be related in highly nontrivial ways. One of the relationships that can exist between these different physical theories is called S-duality. 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 S-duality to the SO(32) heterotic string theory. Similarly, type IIB string theory is related to itself in a nontrivial way by S-duality.[11]

Another relationship between different string theories is T-duality. Here one considers strings propagating around a circular extra dimension. T-duality states that a string propagating around a circle of radius R is equivalent to a string propagating around a circle of radius 1/R 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 p and winding number n in one description, it will have momentum n and winding number p in the dual description. For example, one can show that type IIA string theory is equivalent to type IIB string theory via T-duality and also that the two versions of heterotic string theory are related by T-duality.[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.

Supersymmetry[edit]

Main article: Supersymmetry

Another important theoretical idea that plays a role in M-theory 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 M-theory framework. At low energies, the superstring theories are approximated by supergravity in ten spacetime dimensions. Similarly, M-theory is approximated at low energies by supergravity in eleven dimensions.

Branes[edit]

Main article: Brane

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 higher-dimensional branes. In dimension p, these are called p-branes. 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 p-brane 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 two-dimensional brane.[14]

In string theory, the fundamental objects that give rise to elementary particles are the one-dimensional strings. Although the physical phenomena described by M-theory are still poorly understood, physicists know that the theory describes two- and five-dimensional branes. Much of the current research in M-theory attempts to better understand the properties of these branes.

History and development[edit]

Early work on supergravity[edit]

Main article: Supergravity

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 eleven-dimensional supergravity, it might be possible to construct realistic models of our four-dimensional 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 eleven-dimensional 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.

Relationships between string theories[edit]

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 g is equivalent to the same theory with coupling constant 1/g. 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 S-duality which connects different string theories. For example, type IIB string theory with a large coupling constant is equivalent via S-duality 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 T-duality. This duality implies that strings propagating on completely different spacetime geometries may be physically equivalent.[23]

Membranes and fivebranes[edit]

String theory extends ordinary quantum field theory by promoting zero-dimensional point particles to one-dimensional 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 two-dimensional membranes or by higher-dimensional 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 eleven-dimensional supergravity includes two-dimensional branes.[25] Intuitively, these objects look like sheets or membranes propagating through the eleven-dimensional spacetime. Shortly after this discovery, Michael Duff, Paul Howe, Takeo Inami, Kellogg Stelle considered a particular compactification of eleven-dimensional 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 ten-dimensional 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 five-dimensional 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 five-dimensional 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 five-dimensional 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 five-dimensional brane wrapped around these extra dimensions, then the brane looks just like a one-dimensional 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]

Second superstring revolution[edit]

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 T-duality and the appearance of two- and five-dimensional 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 M-theory on a special spacetime geometry with two ten-dimensional boundary components. Their work shed light on the mathematical structure of M-theory and suggested possible ways of connecting M-theory to real world physics.[36]

Origin of the term[edit]

Unlike most of the theories that physicists study, M-theory cannot be analyzed using the mathematical technique known as perturbation theory. As a result, little is known about the mathematical structure of M-theory and the physics that it describes. In the absence of an understanding of the true meaning and structure of M-theory, 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.[37]


Notes[edit]

  1. ^ For a standard introduction to quantum mechanics, see Griffiths 2004.
  2. ^ The necessity of a quantum mechanical description of gravity follows from the fact that one cannot consistently couple a classical system to a quantum one. See Wald 1984, p. 382.
  3. ^ From a technical point of view, the problem is that the theory one gets in this way is not renormalizable and therefore cannot be used to make meaningful physical predictions. For more information, see Zee 2010, p. 72.
  4. ^ For an accessible introduction to string theory, see Greene 2000.
  5. ^ Wald 1984, p. 4
  6. ^ For example, in the context of the AdS/CFT correspondence, theorists often formulate and study theories of gravity in unphysical numbers of spacetime dimensions.
  7. ^ Zwiebach 2009, p. 9
  8. ^ Zwiebach 2009, p. 8
  9. ^ Yau and Nadis 2010, Ch. 6
  10. ^ This analogy is used for example in Greene 2000, p. 186
  11. ^ Becker, Becker, and Schwarz 2007
  12. ^ Becker, Becker, and Schwarz 2007
  13. ^ Duff 1998, p. 64
  14. ^ Moore 2005
  15. ^ Duff 1998, p. 64
  16. ^ Nahm 1978
  17. ^ Cremmer, Julia, and Scherk 1978
  18. ^ Duff 1998, p. 65
  19. ^ Duff 1998, p. 65
  20. ^ Duff 1998, p. 65
  21. ^ Montonen and Olive 1977
  22. ^ Duff 1998, p. 66
  23. ^ Duff 1998, p. 67
  24. ^ Duff 1998, p. 65
  25. ^ Bergshoeff, Sezgin, and Townsend 1987
  26. ^ Duff et al. 1987
  27. ^ Duff 1998, p. 66
  28. ^ Strominger 1990
  29. ^ Duff 1998, pp 66–7
  30. ^ Sen 1993
  31. ^ Duff 1998, p. 67
  32. ^ Witten 1995
  33. ^ Duff 1998, pp. 67–8
  34. ^ Hořava and Witten 1996a
  35. ^ Hořava and Witten 1996b
  36. ^ Duff 1998, p. 68
  37. ^ Duff 1996, sec. 1

References[edit]

  • Becker, Katrin; Becker, Melanie; Schwarz, John (2007). String theory and M-theory: A modern introduction. Cambridge University Press. 
  • Greene, Brian (2000). The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. Random House. ISBN 978-0-9650888-0-0. 
  • Griffiths, David (2004). Introduction to Quantum Mechanics. Pearson Prentice Hall. ISBN 978-0-13-111892-8. 
  • Duff, Michael (1996). "M-theory (the theory formerly known as strings)". International Journal of Modern Physics A 11 (32): 6523–41. 
  • Duff, Michael (1998). "The theory formerly known as strings". Scientific American 278 (2): 64–9. 
  • Duff, Michael; Howe, Paul; Inami, Takeo; Stelle, Kellogg (1987). "Superstrings in D=10 from supermembranes in D=11". Nuclear Physics B 191 (1): 70–74. 
  • Zee, Anthony (2010). Quantum Field Theory in a Nutshell (2nd ed.). Princeton University Press. ISBN 978-0-691-14034-6. 
  • Zwiebach, Barton (2009). A First Course in String Theory. Cambridge University Press. ISBN 978-0-521-88032-9. 

Further reading[edit]

  • Greene, Brian (2000). The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. Random House. ISBN 978-0-9650888-0-0. 
  • Witten, Edward (1998). "Magic, mystery, and matrix". Notices of the AMS 45 (9): 1124–1129. 

External links[edit]

  • The Elegant Universe—A three-hour miniseries with Brian Greene on the series Nova (original PBS broadcast dates: October 28, 8–10 p.m. and November 4, 8–9 p.m., 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. References on string theory and M-theory for the layperson and expert.
  • Not Even WrongPeter Woit's blog on physics in general, and string theory in particular.