To explain the fundamental nature of gluons, quarks, and all the other elementary particles, physicists dreamed up a theory that has long defied their ability to test it. A novel experiment provides the best measurements yet on the strength and range of forces that might emerge from string theory, the leading candidate for unifying all of particle physics. For the moment, the forces still have room to hide.
String theory supposes that elementary particles are vibrations in minuscule one-dimensional loops or squiggles. For consistency, the theory requires at least six extra dimensions of space, which would be curled up, or "compactified," into spaces so small that we can't perceive them directly. At these distances, around 10-33 meters, string theory predicts that the force of gravity is much stronger than usual and that it exists alongside other fundamental forces never seen before.
String theorists weren't always sanguine about testing the theory, as it seemed impossible to probe the forces operating over such short distances. In the second half of the 1990s, some theorists began thinking there might be short-range forces just within experimenters' grasp. Sensitive torsion balance tests, in which one mass is suspended over another mass by a wire, have already begun seeking out new forces and stronger gravity at short distances. But such experiments are vulnerable to thermal and seismic noise and are difficult to position precisely.
A new device devised by John Price and co-workers at the University of Colorado, Boulder, circumvents these difficulties. Their apparatus consists of a stiff shield sandwiched between two tungsten strips, which jut out in opposite directions. One strip is set vibrating, like a diving board, and the experimenters look for a corresponding twisting of the second mass--an indication that the two masses feel some new force. Price compares it to singing at a bell and listening for its ring. In this case, however, the bell was silent. That means that no new force more than a few times stronger than gravity can propagate farther than the 108 micrometers separating the two masses, the researchers report in the 27 February issue of Nature. It also means that compactified dimensions must be smaller than about 100 micrometers.
The result is impressive because it has already probed shorter distances than the older torsion balance approach, says C. D. Hoyle, an experimental physicist at the University of Trento in Povo, Italy. String theorist Savas Dimopoulos of Stanford University, who helped discover the possibility of new millimeter-scale forces in 1996, says, "I never imagined that 7 short years later I would be talking about testing these ideas at 10 to 100 micrometers." He's not discouraged about his theory just yet, though. He says experimenters would have to find no new forces down to about a tenth of a micrometer before the prospects turned grim.