Before string theory won the full attention of the theoretical
physics community, the most popular unified theory was an eleven
dimensional theory of supergravity, which is supersymmetry combined
with gravity. The (11-dimensional spacetime) was to be compactified
on a small (7-dimensional) sphere, for example, leaving (4 spacetime
dimensions) visible to observers at large distances.
This theory didn't work as a unified theory of particle physics,
because it doesn't have a sensible quantum limit as a point particle
theory. But this (11 dimensional) theory would not die. It eventually
came back to life in the strong coupling limit of superstring theory
in (10 dimensions). How could a superstring theory with (10 spacetime
dimensions) turn into a supergravity theory with (11 spacetime
dimensions)?
Theoretical physicists already learned that duality relations between
superstring theories relate very different theories, equate large
distance with small distance, and exchange strong coupling with weak
coupling. So there must be some duality relation that can explain how
a superstring theory that requires (10 spacetime dimensions) for
quantum consistency can really be a theory in (11 spacetime
dimensions) after all. Since we know that all string theories are
related, and we suspect that they are but different limits of some
more fundamental theory,
then perhaps that more fundamental theory exists in (11 spacetime
dimensions)? These question bring us to the topic of (M-theory).
Technically speaking, (M-theory) is the unknown (11-dimensional)
theory whose low energy limit is the supergravity theory in (11
dimensions) discussed above. However, many people have taken to also
using (M theory) to label the unknown theory believed to be the
fundamental theory from which the known superstring theories emerge
as special limits. Physicists still don't know the fundamental M
theory, but a lot has been learned about the (11-dimensional M-theory)
and how it relates
to superstrings in (10 spacetime dimensions).
In M-theory, there are also extended objects, but they are called (M
branes) rather than (D branes).One class of the M branes in this
theory has (2 space dimensions), and this is called an (M2 brane). Now
consider (M-theory) with the tenth space dimension compactified into a
circle of radius R. If one of the 2 space dimensions that make up the
(M2 brane) is wound around that circle, then we can equate the
resulting object with the fundamental string (one-brane) of type IIA
superstring theory. The type IIA theory appears to be a (10
dimensional theory) in the normal perturbative limit, but reveals an
extra space dimension, and an equivalence to (M-theory), in the limit
of very strong coupling.Theoretical physicists still don't know what
the fundamental theory behind string theory is, but judging from all
of these relationships, it must be a very interesting and rich theory,
one where distance scales, coupling strengths and even the number of
dimensions in
spacetime are not fixed concepts but fluid entities that shift with
our point of view."
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For more information about M-theory and related topics you can see:
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http://groups.yahoo.com/group/kiarashniknejad/
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http://groups.yahoo.com/group/kiarashniknejad/links
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