Box 1. Box 1: A matter of some gravity

From the following article:

Brane new world

Roland Pease

Nature 411, 986-988(28 June 2001)

doi:10.1038/35082684

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Eric Adelberger and Blayne Heckel of the University of Washington in Seattle are no strangers to difficult gravity experiments. In the 1980s, they led one of a number of groups that investigated the existence of a postulated fifth force, which would show up as a gravitational anomaly over distances of up to 100 metres. Their findings10 helped to kill the idea.

So when they heard about the possible existence of extra dimensions, and that these might reveal themselves as deviations from classical newtonian gravitation over distances of less than a millimetre, it was natural that they would pursue them.

The apparatus they use is a marvel of precision engineering. It consists of a pair of metal discs, one above the other (see right). The upper, aluminium disc is suspended on a fibre of tungsten; the lower, copper disc is spun by a precise motor. A series of holes cut around the perimeter of each disc means that when the lower disc is rotated slowly it exerts little effect on the upper disc with purely newtonian gravity, but any deviations resulting from extra dimensions will cause a just-detectable twist on the upper disc.

Brane new world MARY LEVIN/EOT-WASH GROUP

The angular movements needed to signal the presence of additional dimensions are incredibly small — just a millionth of a degree. In February, Adelberger and Heckel reported that they could find no evidence for extra dimensions over length scales down to 0.2 millimetres (ref. 11). But the quest goes on. The researchers are now designing an improved instrument to probe the existence of extra dimensions below 0.1 mm. Other physicists, such as John Price of the University of Colorado and Aharon Kapitulnik of Stanford University in California, are attempting to measure the gravitational influence on small test masses of tiny oscillating levers.

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