Introduction to M-theory
Encyclopedia
In non-technical terms, M-theory
M-theory
In theoretical physics, M-theory is an extension of string theory in which 11 dimensions are identified. Because the dimensionality exceeds that of superstring theories in 10 dimensions, proponents believe that the 11-dimensional theory unites all five string theories...

presents an idea about the basic substance of the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

.

Background

In the early years of the 20th century, the atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

 – long believed to be the smallest building-block of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

 – was proven to consist of even smaller components called protons, neutrons and electrons, which are known as subatomic particles. Beginning in the 1960s, other subatomic particles were discovered. In the 1970s, it was discovered that protons and neutrons (and other hadron
Hadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...

s) are themselves made up of smaller particles called quarks. Quantum theory
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

 is the set of rules that describes the interactions of these particles.

In the 1980s, a new mathematical model of theoretical physics
Theoretical physics
Theoretical physics is a branch of physics which employs mathematical models and abstractions of physics to rationalize, explain and predict natural phenomena...

 called string theory
String theory
String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

 emerged. It showed how all the particles, and all of the forms of energy in the universe, could be constructed by hypothetical one-dimensional "strings", infinitely small building-blocks that have only the dimension of length, but not height nor width. Further, string theory suggested that the universe is made up of multiple dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

s. Height, width, and length constitute three-dimensional space, and time gives a total of four observable dimensions; however, string theories initially supported the possibility of ten dimensions – the remaining six of which we cannot detect directly. This was later increased to 11 dimensions based on various interpretations of the ten dimensional theory that led to five partial theories as described below. Super-gravity theory also played a significant part in establishing the existence 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 which 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 mentioned above, ran into a problem: another version of the equations was discovered, then another, and then another. Eventually, there were five major string theories. The main differences between each theory 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 correct. Scientists were not comfortable with five seemingly contradictory sets of equations to describe the same thing.

In 1994, a string theorist named Edward Witten
Edward Witten
Edward Witten is an American theoretical physicist with a focus on mathematical physics who is currently a professor of Mathematical Physics at the Institute for Advanced Study....

 of the Institute for Advanced Study
Institute for Advanced Study
The Institute for Advanced Study, located in Princeton, New Jersey, United States, is an independent postgraduate center for theoretical research and intellectual inquiry. It was founded in 1930 by Abraham Flexner...

 and other important researchers considered that the five different versions of string theory might be describing the same thing seen from different perspectives. They 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", "mother", "monster", "mystery", "magic" have also been claimed. M-theory brought all of the string theories together. It did this by asserting that strings are really 1-dimensional slices of a 2-dimensional membrane vibrating in 11-dimensional space.

Status

M-theory is not yet complete, but the underlying structure of the mathematics has been established and is in agreement with not only all the string theories, but with all of our scientific observations of the universe. Furthermore, it has passed many tests of internal mathematical consistency that many other attempts to combine quantum mechanics and gravity had failed.

Unfortunately, until we can find some way to observe higher dimensions (impossible with our current level of technology) M-theory has a very difficult time making predictions that can be tested in a laboratory. Technologically, it may never be possible for it to be "proven".

Some cosmologists are drawn to M-theory because of its mathematical elegance and relative simplicity. Physicist and author Michio Kaku
Michio Kaku
is an American theoretical physicist, the Henry Semat Professor of Theoretical Physics in the City College of New York of City University of New York, the co-founder of string field theory, and a "communicator" and "popularizer" of science...

 has remarked that M-theory may present us with a "Theory of Everything
Theory of everything
A theory of everything is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle....

" which is so concise that its underlying formula would fit on a t-shirt. Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

 originally believed that M-theory may be the ultimate theory but later suggested that the search for understanding of mathematics and physics will never be complete
Gödel's incompleteness theorems
Gödel's incompleteness theorems are two theorems of mathematical logic that establish inherent limitations of all but the most trivial axiomatic systems capable of doing arithmetic. The theorems, proven by Kurt Gödel in 1931, are important both in mathematical logic and in the philosophy of...

.

Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

 and Leonard Mlodinow
Leonard Mlodinow
Leonard Mlodinow is a physicist and author.Mlodinow was born in Chicago, Illinois, of parents who were both Holocaust survivors. His father, who spent more than a year in the Buchenwald concentration camp, had been a leader in the Jewish resistance under Nazi rule in his hometown of Częstochowa,...

, in The Grand Design
The Grand Design (book)
The Grand Design is a popular-science book written by physicists Stephen Hawking and Leonard Mlodinow and published by Bantam Books in 2010. It argues that invoking God is not necessary to explain the origins of the universe, and that the Big Bang is a consequence of the laws of physics alone...

, take a philosophical position to support a view of the universe as a multiverse
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

, and define it in the book as model-dependent realism
Model-dependent realism
Model-dependent realism is a controversial philosophical approach to scientific inquiry, which accepts that reality can always be interpreted in a number of different ways, and focuses on how well our models of the world do at describing the observed phenomena. It claims that it is meaningless to...

 which along with a sum over histories approach (see Path integral formulation
Path integral formulation
The path integral formulation of quantum mechanics is a description of quantum theory which generalizes the action principle of classical mechanics...

 of Quantum Mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

) to the universe as a whole, is used to claim that M-theory is the only candidate for a complete theory of the universe.

See also

  • String theory
    String theory
    String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

  • Superstring theory
    Superstring theory
    Superstring theory is an attempt to explain all of the particles and fundamental forces of nature in one theory by modelling them as vibrations of tiny supersymmetric strings...

  • Theory of everything
    Theory of everything
    A theory of everything is a putative theory of theoretical physics that fully explains and links together all known physical phenomena, and predicts the outcome of any experiment that could be carried out in principle....

  • Quantum mechanics
    Quantum mechanics
    Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...


Books

  • Brian Greene
    Brian Greene
    Brian Greene is an American theoretical physicist and string theorist. He has been a professor at Columbia University since 1996. Greene has worked on mirror symmetry, relating two different Calabi-Yau manifolds...

     has written books explaining string theory and M-theory for the layperson in 1999, The Elegant Universe
    The Elegant Universe
    The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory is a book by Brian Greene published in 1999, which introduces string and superstring theory, and provides a comprehensive though non-technical assessment of the theory and some of its shortcomings...

    , ISBN 0-375-70811-1 and in 2004, The Fabric of the Cosmos
    The Fabric of the Cosmos
    The Fabric of the Cosmos: Space, Time, and the Texture of Reality is the second book on theoretical physics, cosmology, and string theory written by Brian Greene, professor and co-director of Columbia's Institute for Strings, Cosmology, and Astroparticle Physics .- Introduction :Greene begins with...

    , ISBN 0-375-41288-3.
  • Smolin, Lee. The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next (2006), Houghton Mifflin. ISBN 978-0-618-55105-7.
  • Woit, Peter. Not Even Wrong: The Failure of String Theory & the Continuing Challenge to Unify the Laws of Physics, 2006. ISBN 0-224-07605-1 (Jonathan Cape), ISBN 0-465-09275-6 (Basic Books)

External links

  • The Elegant Universe - A Three-Hour miniseries with Brian Greene
    Brian Greene
    Brian Greene is an American theoretical physicist and string theorist. He has been a professor at Columbia University since 1996. Greene has worked on mirror symmetry, relating two different Calabi-Yau manifolds...

    by 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. Excellent references on string theory and M-theory for the layperson and expert.
  • Basics of M-Theory by A. Miemiec and I. Schnakenburg is a lecture note on M-Theory published in Fortsch.Phys.54:5-72,2006.
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