Study sizes up extra dimensions: small, old black hole places new limit on hidden spaces.

The size of any hidden extra dimension beyond the familiar three must be less than 3 micrometers, a new analysis based on an old black hole has found.

That new size limit is less than half that of previous such limits, Oleg Gnedin of the University of Michigan in Ann Arbor and his colleagues report in a study posted online June 30 at arXiv.org.

Dimensions beyond the common three of space and one of time might explain why the strong nuclear force is roughly [10.sup.40] times stronger than gravity. If the gravitational force leaks out along an extra dimension, as some versions of string theory suggest, it would be weaker in the observable 3-D space.

In basic string theory, which describes subatomic particles as tiny vibrating loops or strands of energy, extra dimensions are too small to be directly detected. But some versions of string theory allow larger extra dimensions, detectable by measuring the force of gravity at small distances or from the results of atom-smasher experiments or astrophysical observations (SN: 2/19/2000, p. 122).

"The existence of large extra dimensions seems like an attractive idea in theoretical physics, but they have not revealed themselves in any experiment so far" Gnedin notes.

Enter small, old black holes. All black holes radiate energy, known as Hawking radiation. As it radiates, the black hole shrinks, and the shrinking proceeds more rapidly as the black hole gets smaller. In some models, extra dimensions dramatically speed up that rate, hastening the black hole's demise, notes theorist Igor Klebanov of Princeton University. The larger the extra dimension, the faster the black hole evaporates.

Two years ago, astronomers reported evidence for a black hole, only about 10 times as heavy as the sun, in the galaxy NGC 4472, some 50 million light-years from Earth. The cluster containing the black hole is about 10 billion years old, researchers say. The very existence of a black hole this small and old suggests that any extra dimension cannot exceed 3 micrometers, the team calculates.

But Paul Steinhardt of Princeton University cautions that the details of the new limit depend on exactly which model for extra dimensions scientists rely on.