Theorists believe that gravity may seem weak because it is spread out among up to eleven dimensions, unlike the other fundamental forces, which are thought to be confined to the familiar three spatial dimensions. If this is the case, the gravitational attraction between two objects may deviate from the well-known 'inverse square law' when they are close together. 'String theory' suggests that extra dimensions may be curled up or 'compactified' on short length scales - not least because there is no evidence that they exist on the larger scale of our everyday experience. If there are extra dimensions in which only gravity exists, we can only probe them using the interactions of gravity. "No one has ever detected gravity at distances less than a millimetre", says team member Blayne Heckel.

Adelberger and colleagues set up a 'torsion pendulum' consisting of a horizontal aluminium ring 2 mm thick and 55 mm in diameter, suspended on a fine wire and free to spin. Beneath it - separated by 0.2 mm - is a slowly turning horizontal copper disk called the 'attractor'. The pendulum and the attractor each have a ring of ten equally spaced holes bored through them. As the attractor slowly turns, it twists the pendulum ten times - corresponding to the ten copper 'bridges' between the holes - for each complete revolution. A laser reflected off the pendulum measures the degree of twist, from which the attraction between the two components is calculated. The experiment was also shielded from spurious electrical and gravitational effects.

The Washington team found that gravity between the just-separated objects does indeed exist - and that it acts exactly as it does at larger separations. This implies that any dimensions that might be revealed by the strange behaviour of gravity must be tucked away in smaller regions of space. "This doesn't mean the idea of extra dimensions is crazy", says Adelberger. "It's just not as straightforward as the simplest picture". The team is currently designing a more sensitive experiment to measure gravity at even smaller scales.