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WHAT happens to gravity over tiny distances? Using delicate apparatus to measure gravitational forces over just a tenth of a millimetre, the shortest distance ever, a team of physicists has found that, as far as they can tell, the forces are roughly as Newton鈥檚 laws predict. The result narrows down the possible nature of hidden extra dimensions, which would boost gravity over small scales.

It is extraordinarily difficult to measure gravity on these scales because weights that are small enough to be manipulated and held so close together only exert very weak gravitational forces. The most sensitive experiment until now tested the attraction between two masses 0.2 millimetres apart and found that gravity was no stronger than expected. What it might be like over smaller scales remained a mystery.

Now Joshua Long and his colleagues at the University of Colorado, Boulder, have cut that distance in half. Their source of gravity is a metal strip about 20 millimetres long and 0.3 millimetres thick. 鈥淚t鈥檚 like a tungsten diving board that vibrates up and down,鈥� says Long, now at the Los Alamos National Laboratory in New Mexico. Just 0.1 millimetres below the strip is a second wafer-thin tungsten spring, the 鈥渢est mass鈥�. The test mass is tuned to vibrate at the same frequency as the source mass above, so even the faint pull of gravity between the two is enough to set it vibrating in sympathy.

When the Colorado team set the upper strip vibrating, they found that the size of the resulting vibrations in the test mass showed the attraction between the two to be roughly what you would expect from ordinary gravity, just as Newton would have predicted (Nature, vol 421, p 922).

This rules out theories that say gravity should be much stronger over such small scales. In string theory, which is physicists鈥� best attempt so far to unify the forces of nature, space has six or seven extra dimensions 鈥� like the familiar three except curled up, perhaps as small as 10^鈭�35 metres.

As these dimensions vibrate, they should transmit powerful 鈥渕odulus forces鈥� over a very short range, which would feel like gravity only tens of thousands of times stronger. The new results show that, if they exist, these modulus forces must have a range less than 0.1 millimetres, ruling out versions of the theory that require the forces to be longer.

Another set of theories suggest that two of the hidden dimensions could be as large as a 0.1 millimetres across, although they would be invisible to us because only the force of gravity can penetrate them. These larger dimensions would not produce modulus forces, but they would make gravity鈥檚 effects several times stronger over scales comparable to the dimensions themselves. The latest result doesn鈥檛 narrow down the possible size of these dimensions any further than previous experiments, but according to theorist Savas Dimopoulos of Stanford University in California it does rule out the possibility that one hidden dimension is larger than the other.

Gravity could still be stronger than expected over distances of less than 0.1 millimetres. So the Colorado group are now refining their experiment to measure it on even smaller scales. To do this they have removed a gold-plated sapphire shield from between the two tungsten strips, which blocks any electromagnetic forces, and replaced it with a thinner beryllium-copper foil, allowing the masses to be moved closer together. They also plan to cool the experiment to cut down thermal vibrations.