A team of theoretical physicists and astronomers has calculated that any hidden extra dimension beyond our familiar three-dimensional space, a world known in physics parlance as a 3-brane, must be less than 3 micrometers. The researchers base their findings on the recent discovery of one of the smallest and oldest black holes ever found.
The new limit is less than half that of previous limits on the length of an extra dimension, Oleg Gnedin of the University of Michigan in Ann Arbor and his colleagues report in a study posted online June 30 (http://arxiv.org/abs/0906.5351).
Physicists since the 1920s have postulated the existence of dimensions beyond the three of space and one of time. Extra dimensions might explain, for example, why the strong nuclear force is roughly 1040 times stronger than gravity. If the gravitational force spreads or leaks out along an extra dimension, as some versions of string theory suggest, it would be weaker in the observable three-dimensional space.
In basic string theory, which describes subatomic particles as tiny vibrating loops or strands of energy, extra dimensions are far too small to be directly detected by any conceivable experiment. But some versions of string theory allow the possibility of larger dimensions whose presence could be detected by measuring the force of gravity at small distances or from the results of atom-smasher experiments or astrophysical observations.
The results further constrain string theory, the leading versions of which require ten, eleven or twenty-six dimensions, all but four of which are rendered invisible in some way. It adds to the limitations on string theory posed by the fact that no supersymmetric particles (predicted to exist by most versions of string theory) have been discovered, and other criticisms of string theory.