In non-technical terms, M-theory presents an idea about the basic substance of the universe. As of 2020, science has produced no experimental evidence to support the concept that M-theory is a description of the real world. Although a complete mathematical formulation of M-theory is not known, the general approach is the leading contender for a universal "Theory of Everything" that unifies gravity with other forces such as electromagnetism. M-theory aims to unify quantum mechanics with general relativity's gravitational force in a mathematically consistent way. In comparison, other theories such as loop quantum gravity are considered by physicists and researchers/students as less elegant because they posit gravity to be completely different from forces such as the electromagnetic force.[1][2][3]


In the early years of the 20th century, the atom – long believed to be the smallest building-block of matter – was proven to consist of even smaller components called protons, neutrons and electrons, which are known as subatomic particles. Other subatomic particles began being discovered in the 1960s. In the 1970s, it was discovered that protons and neutrons (and other hadrons) are themselves made up of smaller particles called quarks. The Standard Model is the set of rules that describes the interactions of these particles.

In the 1980s, a new mathematical model of theoretical physics, called string theory, emerged. It showed how all the different subatomic particles known to science could be constructed by hypothetical one-dimensional "strings", infinitesimal building-blocks that have only the dimension of length, but not height or width.

However, for string theory to be mathematically consistent, the strings must be in a universe of ten dimensions. This contradicts the experience that our real universe has four dimensions: three space dimensions (height, width, and length) and one time dimension. To "save" their theory, string theorists therefore added the explanation that the additional six dimensions exist but cannot be detected directly; this was explained by sophisticated mathematical objects called Calabi–Yau manifolds. The number of dimensions was later increased to 11 based on various interpretations of the 10-dimensional theory that led to five partial theories, as described below. Supergravity theory also played a significant part in establishing the necessity of the 11th dimension.

These "strings" vibrate in multiple dimensions and, depending on how they vibrate, they might be seen in three-dimensional space as matter, light or gravity. It is the vibration of the string that determines whether it appears to be matter or energy, and every form of matter or energy is the result of the vibration of strings.

String theory as described above ran into a problem: another version of the equations was discovered, then another, and then another. Eventually, five major string theories were developed. The main differences between the theories were principally the number of dimensions in which the strings developed, and their characteristics (some were open loops, some were closed loops, etc.). Furthermore, all these theories appeared to be workable. Scientists were not comfortable with five seemingly contradictory sets of equations to describe the same thing.

Speaking at the string theory conference at the University of Southern California in 1995, Edward Witten of the Institute for Advanced Study suggested that the five different versions of string theory might be describing the same thing seen from different perspectives.[4] He proposed a unifying theory called "M-theory", in which the "M" is not specifically defined but is generally understood to stand for "membrane". The words "matrix", "master", "mother", "monster", "mystery" and "magic" have also been claimed. M-theory brought all of the string theories together. It did this by asserting that strings are really one-dimensional slices of a two-dimensional membrane vibrating in 11-dimensional spacetime. Vibrations of higher-dimensional objects (as in three-dimensional vibrating blob or sphere or even more possible dimensions) are certainly a part of M-theory,[5] but the basic theory of branes is still in progress. Higher-dimensional objects are much harder to mathematically calculate than a point in classical physics or a one-dimension string in string theory or two-dimensional membranes in M-theory.

M-theory is not complete, but the mathematics of the approach has been explored in great detail. However, so far no experimental support of the M-theory exists.[1] Some physicists are skeptical that this approach will ever lead to a physical theory describing our real world, due to fundamental issues.[6]

Nevertheless, some cosmologists are drawn to M-theory because of its mathematical elegance and relative simplicity, triggering the hope that the simplicity is a reason why it may describe our world.

One feature of M-theory that has drawn great interest is that it naturally predicts the existence of the graviton, a spin-2 particle hypothesized to mediate the gravitational force. Furthermore, M-theory naturally predicts a phenomenon that resembles black hole evaporation. Competing unification theories such as asymptotically safe gravity, E8 theory, noncommutative geometry, and causal fermion systems have not demonstrated any level of mathematical consistency. M-theory's chief rival is loop quantum gravity, a non-unifying theory; many physicists consider loop quantum gravity to be less elegant than M-theory because it posits gravity to be completely different from the other fundamental forces.[1][2]
See also

History of string theory


Wolchover, Natalie (December 2017). "The Best Explanation for Everything in the Universe". The Atlantic. Archived from the original on 15 November 2020. Retrieved 7 February 2018.
"Physicists and Philosophers Debate the Boundaries of Science | Quanta Magazine". Quanta Magazine. 16 December 2015. Archived from the original on 15 November 2020. Retrieved 7 February 2018.
Devlin, Hannah (5 July 2017). "Tying loose ends? Gravitational waves could solve string theory, study claims". The Guardian. Archived from the original on 15 November 2020. Retrieved 7 February 2018.
"University of Southern California, Los Angeles, Future Perspectives in String Theory, March 13-18, 1995, E. Witten: Some problems of strong and weak coupling". Archived from the original on 2020-11-15. Retrieved 2017-04-08.
"Quantum gravity – Does string/M-theory address higher-dimensional membrane vibration modes?". Archived from the original on 2020-11-15. Retrieved 2017-08-05.

Lee Smolin, April 2007:Response to review of The Trouble with Physics by Joe Polchinski.

Further reading

Greene, B. (1999). The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. W.W. Norton. ISBN 978-0-375-70811-4.
Greene, B. (2004). The Fabric of the Cosmos: Space, Time, and the Texture of Reality. Alfred A. Knopf. ISBN 978-0-375-41288-2.
Miemic, A.; Schnakenburg, I. (2006). "Basics of M-theory". Fortschritte der Physik. 54 (1): 5–72. arXiv:hep-th/0509137. Bibcode:2006ForPh..54....5M. doi:10.1002/prop.200510256. S2CID 98007313.
Musser, G. (2008). The Complete Idiot's Guide to String Theory. Alpha Books. ISBN 978-1-59257-702-6.
Smolin, L. (2006). The Trouble with Physics. Houghton Mifflin. ISBN 978-0-618-55105-7.
Woit, P. (2006). Not Even Wrong: The Failure of String Theory and the Continuing Challenge to Unify the Laws of Physics. Basic Books. ISBN 978-0-465-09275-8.

External links

The Elegant Universe – A Three-Hour miniseries with Brian Greene by NOVA (original PBS Broadcast Dates: October 28 and November 4, 2003). Various images, texts, videos and animations explaining string theory and M-theory. – The "Official String Theory Web Site", created by Patricia Schwarz. Excellent references on string theory and M-theory for the layperson and expert.

Introductory science articles

Introduction to eigenstates Introduction to electromagnetism Introduction to thermodynamic entropy Introduction to evolution Introduction to gauge theory Introduction to general relativity Introduction to genetics Introduction to M-theory Introduction to the mathematics of general relativity Introduction to quantum mechanics Introduction to systolic geometry Introduction to viruses

String theory

Strings History of string theory
First superstring revolution Second superstring revolution String theory landscape



Nambu–Goto action Polyakov action Bosonic string theory Superstring theory
Type I string Type II string
Type IIA string Type IIB string Heterotic string N=2 superstring F-theory String field theory Matrix string theory Non-critical string theory Non-linear sigma model Tachyon condensation RNS formalism GS formalism

String duality

T-duality S-duality U-duality Montonen–Olive duality

Particles and fields

Graviton Dilaton Tachyon Ramond–Ramond field Kalb–Ramond field Magnetic monopole Dual graviton Dual photon


D-brane NS5-brane M2-brane M5-brane S-brane Black brane Black holes Black string Brane cosmology Quiver diagram Hanany–Witten transition

Conformal field theory

Virasoro algebra Mirror symmetry Conformal anomaly Conformal algebra Superconformal algebra Vertex operator algebra Loop algebra Kac–Moody algebra Wess–Zumino–Witten model

Gauge theory

Anomalies Instantons Chern–Simons form Bogomol'nyi–Prasad–Sommerfield bound Exceptional Lie groups (G2, F4, E6, E7, E8) ADE classification Dirac string p-form electrodynamics


Kaluza–Klein theory Compactification Why 10 dimensions? Kähler manifold Ricci-flat manifold
Calabi–Yau manifold Hyperkähler manifold
K3 surface G2 manifold Spin(7)-manifold Generalized complex manifold Orbifold Conifold Orientifold Moduli space Hořava–Witten domain wall K-theory (physics) Twisted K-theory


Supergravity Superspace Lie superalgebra Lie supergroup


Holographic principle AdS/CFT correspondence


Matrix theory Introduction to M-theory

String theorists

Aganagić Arkani-Hamed Atiyah Banks Berenstein Bousso Cleaver Curtright Dijkgraaf Distler Douglas Duff Ferrara Fischler Friedan Gates Gliozzi Gopakumar Green Greene Gross Gubser Gukov Guth Hanson Harvey Hořava Gibbons Kachru Kaku Kallosh Kaluza Kapustin Klebanov Knizhnik Kontsevich Klein Linde Maldacena Mandelstam Marolf Martinec Minwalla Moore Motl Mukhi Myers Nanopoulos Năstase Nekrasov Neveu Nielsen van Nieuwenhuizen Novikov Olive Ooguri Ovrut Polchinski Polyakov Rajaraman Ramond Randall Randjbar-Daemi Roček Rohm Scherk Schwarz Seiberg Sen Shenker Siegel Silverstein Sơn Staudacher Steinhardt Strominger Sundrum Susskind 't Hooft Townsend Trivedi Turok Vafa Veneziano Verlinde Verlinde Wess Witten Yau Yoneya Zamolodchikov Zamolodchikov Zaslow Zumino Zwiebach

Physics Encyclopedia



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