Ever since the proverbial apple fell on Sir Isaac Newton's head, scientists have been able to calculate the force of gravity over a variety of distances. The first measurement of the gravitational constant came more than 100 years later, but testing gravity over very short distances has proved difficult. Now scientists have examined the gravitational attraction between two objects just a tenth of a millimeter apart--the smallest gap yet for such trials. The findings, published today in the journal Nature, set upper limits for some of the forces predicted by string theory.

String theory has emerged as the most promising approach to unifying quantum mechanics--the laws governing very, very small things such as atoms, nuclei and quarks--with general relativity, which describes the world on a scale as large as that of stars and galaxies. It holds that what appear to be pointlike elementary particles are instead tiny one-dimensional strings whose vibrations give rise to fundamental particles. The hypothesis calls for six or more spatial dimensions (on top of the three that we can observe) that are curled up into tiny spaces. This so-called compactification generates a number of "modulus" forces, some of which would be comparable to gravity at distances approaching a tenth of a millimeter under certain string theory scenarios.

To investigate the forces at work over such small distances, Joshua C. Long and his colleagues at the University of Colorado at Boulder designed a new device containing at its core a tungsten metal strip. The diving board-like plank can vibrate up and down and a second metal strip lies 0.1 millimeter below it. The researchers found that gravity performed pretty much as predicted by Newton. Furthermore, they did not observe any new forces at work. String theory's modulus forces must therefore have a range shorter than 0.1 millimeter. The next step, the authors say, will be to further narrow the gap between the objects in such gravity tests, perhaps to 0.01 millimeter.