**DENVER--**Galileo, the legend goes, dropped balls off the Leaning Tower of Pisa to study the effects of gravity. Nowadays, physicists are much more sophisticated: They drop neutrons. At a meeting here of the American Physical Society, physicists showed how these tiny particles are revealing the strength of gravity over vanishingly short distances.

Since Newton's time, scientists have known that the force of gravity between two bodies falls off as the square of the distance between them [Does what? Increases?]; this is the "r-squared" law. By observing planets moving around the sun and heavy masses here on Earth attracting other nearby masses, physicists are quite confident that the r-squared law holds from astronomical-length scales down to a few fractions of a meter. But recently, theorists have suggested that under certain circumstances, the r-squared law might not have complete jurisdiction over the nanoscale world.

It's extremely hard to do experiments that measure gravity on those scales. Gravity is a very weak force, so it's tough to measure the attraction between tiny masses when other forces, such as electrostatic repulsion, are so much stronger. Experiments with pendulums showed that the r-squared law holds down to scales of a tenth of a millimeter. But on 1 May at the meeting here, physicist Stefan Baessler of the University of Mainz, Germany, presented results on scales up to 100,000 times smaller still.

Baessler and his colleagues dropped very slow, very cold neutrons onto a surface. When a neutron hits, says Baessler, it bounces like a tennis ball. Unlike a tennis ball, a neutron is a quantum object, and a quantum object held down by a force can bounce only in fixed steps. Finding the height of the smallest of these steps reveals--with great precision--the gravitational force on the neutron. If there's even a tiny deviation from the r-squared law, the minimum bounce height should be different than expected. It wasn't, Baessler's team found.

Physicist Eric Adelberger of the University of Washington, Seattle, dubbed the neutron work a "very interesting experiment" and stresses the importance of probing the limits of the r-squared law on such tiny scales: "Questions about gravity are at the heart of physics." Further refinements to the technique should make it even more sensitive.

**Related sites**

Abstract of meeting presentation

Baessler's neutron experiment page