Open Questions: Higher Dimensions of Spacetime
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See also: Supersymmetry 
Superstring theory
The notion that there may be dimensions of space beyond the three
that are apparent in everyday experience has exerted a powerful
fascination on popular (and scientific) imagination for over a
century.
It probably began with mathematicians like G. F. B. Riemann in the
1860s who first explored geometries that are alternatives to the
3dimensional Euclidean geometry that had been studied since the
Greeks, more than two millennia ago. The trend gathered considerable
momentum from Einstein's special theory of relativity (1905) and
general theory of relativity (1915), in which spacetime was conceived
as a 4dimensional geometry with a decidedly nonEuclidean metric.
(Einstein was heavily indebted to Riemann for the proper mathematics
with which to develop his theories.)
Site indexes
Sites with general resources
Surveys, overviews, tutorials

Why 10 dimensions
 Article from
Wikipedia.

Extra Dimensions, Quantum Gravity, and Superstrings
 Slides from a talk by
Sean Carroll at an October 2001
conference
on cosmology.

Brane new world
 June 28, 2001 news feature article in
Nature about the
"braneworld" proposal of ArkaniHamed, Dimopoulos, and Davali.

Testing the gravitational inversesquare law
 April 2005 article form
Physics World, by
Eric Adelberger, Blayne Heckel and C D Hoyle.
"If the universe contains more than three spatial dimensions,
as many physicists believe, our current laws of gravity should
break down at small distances."

The hunt for new dimensions
 February 2001 news article from
PhysicsWeb.
"For decades physicists have toyed with the idea that the
universe may contain extra dimensions beyond the familiar
four dimensions of space and time. This idea has been
proposed to account for the exceptional weakness of gravity.
But remarkably, nobody has measured the strength of gravity
on scales much less than a centimetre  and that is exactly
where theorists believe the extra dimensions could be hiding.
Now Eric Adelberger and coworkers at the University of
Washington in the US have measured for the first time
the gravitational attraction between objects just 0.2 mm
apart  and concluded that any new dimensions must be
concealed on even smaller scales."

The search for extra dimensions
 November 2000 article from
Physics World, by
Steven Abel and
John MarchRussell. "The idea that the universe is trapped on a
membrane in some highdimensional spacetime may explain why gravity
is so weak, and could be tested at highenergy particle accelerators."

Extra dimensions round the corner?
 Summary of a June 1999 article in
Physics World.

(De)Constructing Dimensions
 April 2001 technical paper by Nima ArkaniHamad, Andrew G.
Cohen, and Howard Georgi.
"We construct renormalizable, asymptotically free, four
dimensional gauge theories that dynamically generate a
fifth dimension."
 Large Extra Dimensions: A New Arena for Particle Physics
Nima ArkaniHamed; Savas Dimopoulos; Georgi Dvali
Physics Today, February 2002, pp. 3540
 In the "desert picture" of the unification of fundamental forces,
there is an enormous range of energies in which no essentially new
physics occurs. This situation gives rise to the hierarchy problem,
among other things. A new proposal suggests that all forces,
including gravity, can be unified at a much lower energy, with the
extreme weakness of gravity being explained by the existence of
"large" extra dimensions of spacetime.
 The Universe's Unseen Dimensions
Nima ArkaniHamed; Savas Dimopoulos; Georgi Dvali
Scientific American, August 2000, pp. 6269
 Although superstring theories have stimulated consideration
of physics in more than three spatial dimensions, it now seems
that such extra dimensions could be much larger than supposed.
The observable universe could line on a 3dimensional membrane
in a higherdimensional space, which might help clear up several
current important puzzles such as the comparative weakness of
gravity and the nature of "dark matter".

Hunting for Higher Dimensions
Peter Weiss
Science News, February 19, 2000, pp. 122124
 Various extensions to the standard model, including unified
threories and string theories, imply that space has 6 or 7
additional dimensions which occur in a "compactified" form.
This means that the additional dimensions would have extents
far too small to be detectable. Newer considerations
suggest the extents might not necessarily be so small that
they couldn't be observed with current technology.
 The Hidden Dimensions of Spacetime
Daniel Z. Freedman; Peter van Nieuwenhuizen
Scientific American, March 1985, pp. 7481
 Shortly after Einstein's general theory of relativity was
published, Theodor Kaluza and Oskar Klein discovered how forces
other than gravity might arise by extending Einstein's equation
to higher dimensions. Several distinct modern theories utilize
higher dimensions to unify the four known forces.
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