Experiments In Gravity, Dimensions, Diving Boards.

Gravity has been tested over a shorter distance than ever before. Using a delicate apparatus to measure gravitational forces over just a tenth of a millimetre, a team of physicists has found that they are roughly as Newton's laws predict.

The result narrows down the possible nature of hidden extra dimensions, which would boost gravity over small scales.

The most sensitive previous experiment tested the attraction between two masses 0.2 millimetres apart, and found that gravity was no stronger than expected. But 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.

"It's like a tungsten diving board that vibrates up and down," 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 "test mass."

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.

The Colorado group found that the resulting vibrations in the test mass were roughly what you would expect from ordinary gravity, just as Newton would have predicted.

This rules out theories that say gravity should be much stronger over such small scales.
New Scientist