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Superstring theory
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In theoretical physics, a membrane, brane, or p-brane is a spatially extended, mathematical concept that appears in string theory and its relatives (M-theory and brane cosmology). The variable p refers to the spatial dimension of the brane. That is, a 0-brane is a zero-dimensional pointlike particle, a 1-brane is a string, a 2-brane is a "membrane", etc. Every p-brane sweeps out a (p+1)-dimensional world volume as it propagates through spacetime.
Description
Originally string theory was a theory of 1-branes called strings. By the mid-1990s it became apparent that the theory could be extended to also include higher dimensional objects. Typically these objects are non-perturbative features of the theory (meaning they do not appear in perturbation theory) which is partially why early string theorists were unaware of them. Besides the fundamental string (or F-string) of string theory and its magnetic dual, the NS5-brane, the most important type of branes that appear are the D-branes. Different types of D-branes appear in different theories. Even Dn-branes for n appear in type IIA string theory where as the odd Dp-branes appear in type IIB string theory. With the development of M-theory, an extra dimension appeared and the fundamental string of string theory became a 2-dimensional membrane called an M2-brane (or supermembrane). Its magnetical dual is an M5-brane. The various branes of string theory are thought to be related to these higher dimensional M5-branes wrapped on various cycles.
