physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...

, extra dimensions are proposed additional

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...

time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

dimension In physics and mathematics, the dimension of a mathematical 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 (1D) because only one coord ...

s beyond the (3 + 1) typical of observed

spacetime In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differ ...

, such as the first attempts based on the Kaluza–Klein theory. Among theories proposing extra dimensions are: *

Large extra dimension In particle physics and string theory (M-theory), the ADD model, also known as the model with large extra dimensions (LED), is a model framework that attempts to solve the hierarchy problem. (''Why is the force of gravity so weak compared to the el ...

, mostly motivated by the ADD model, by

Nima Arkani-Hamed Nima Arkani-Hamed ( fa, نیما ارکانی حامد; born April 5, 1972) is an American-Canadian

Savas Dimopoulos Savas Dimopoulos (; el, Σάββας Δημόπουλος; born 1952) is a particle physicist at Stanford University. He worked at CERN from 1994 to 1997. Dimopoulos is well known for his work on constructing theories beyond the Standard Model. ...

, and

Gia Dvali Georgi (Gia) Dvali ( Georgian: გიორგი (გია) დვალი; born May 30, 1964) is a Georgian theoretical physicist and science communicator in Georgia. He is a professor of theoretical physics at the Ludwig Maximilian Universi ...

in 1998, in an attempt to solve the

hierarchy problem In theoretical physics, the hierarchy problem is the problem concerning the large discrepancy between aspects of the weak force and gravity. There is no scientific consensus on why, for example, the weak force is 1024 times stronger than grav ...

. This theory requires that the fields of the

Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces ( electromagnetic, weak and strong interactions - excluding gravity) in the universe and classifying all known elementary particles. It ...

are confined to a four-dimensional

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...

, while gravity propagates in several additional spatial dimensions that are large compared to the Planck scale.For a pedagogical introduction, see * Warped extra dimensions, such as those proposed by the

Randall–Sundrum model In physics, Randall–Sundrum models (also called 5-dimensional warped geometry theory) are models that describe the world in terms of a warped-geometry higher-dimensional universe, or more concretely as a 5-dimensional anti-de Sitter space whe ...

(RS), based on

warped geometry In mathematics and physics, in particular differential geometry and general relativity, a warped geometry is a Riemannian or Lorentzian manifold whose metric tensor can be written in form :ds^2 = g_(y) \, dy^a \, dy^b + f(y) g_(x) \, dx^i \, dx ...

where the

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the univers ...

is a five-dimensional anti-de Sitter space and the

elementary particles In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, anti ...

except for the

graviton In theories of quantum gravity, the graviton is the hypothetical quantum of gravity, an elementary particle that mediates the force of gravitational interaction. There is no complete quantum field theory of gravitons due to an outstanding mathem ...

are localized on a (3 + 1)-dimensional brane or branes. *

Universal extra dimension In particle physics, models with universal extra dimensions include one or more spatial dimensions beyond the three spatial and one temporal dimensions that are observed. Overview Models with universal extra dimensions, studied in 2001 assume that ...

, proposed and first studied in 2000, assume, at variance with the ADD and RS approaches, that all fields propagate universally in extra dimensions. *

Multiple time dimensions The possibility that there might be more than one dimension of time has occasionally been discussed in physics and philosophy. Similar ideas appear in folklore and fantasy literature. Physics Speculative theories with more than one time dimen ...

, i.e. the possibility that there might be more than one dimension of

, has occasionally been discussed in physics and philosophy, although those models have to deal with the problem of

causality Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...

. *

String theory In physics, string theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings. String theory describes how these strings propagate through space and intera ...

has one notable feature that requires extra dimensions for mathematical consistency. Spacetime is 26-dimensional in bosonic string theory, 10-dimensional in

superstring theory Superstring theory is an attempt to explain all of the particles and fundamental forces of nature in one theory by modeling them as vibrations of tiny supersymmetric strings. 'Superstring theory' is a shorthand for supersymmetric string t ...

, and 11-dimensional in

supergravity theory In theoretical physics, supergravity (supergravity theory; SUGRA for short) is a modern field theory that combines the principles of supersymmetry and general relativity; this is in contrast to non-gravitational supersymmetric theories such ...

and

M-theory M-theory is a theory in physics that unifies all consistent versions of superstring theory. Edward Witten first conjectured the existence of such a theory at a string theory conference at the University of Southern California in 1995. Witt ...

References

{{reflist Dimension Physical cosmology String theory Physics beyond the Standard Model