Fifth force sunk in ocean experiments.

After creating a splash among physicists in the mid-1980s, the hypothesized "fifth force" now appears washed up. A set of extremely accurate gravity measurements made in the Pacific Ocean shows no evidence of the extra force, confirming the results of laboratory experiments that have also failed to detect this would-be addition to the familty of four universal forces.

Scientists raised the idea of a fifth force in 1986 after finding hints that the gravity inside an Australian mine shaft did not follow Newton's inverse square law. They theorized that a previously unrecognized force -- acting over distances of tens to thousands of meters -- had altered the attraction between the gravity meter and the rock around the shaft.

Geophysicists from the Scripps Institution of Oceanography in La Jolla, Calif., and elsewhere have now tested this hypothesis by measuring gravity while ascending in a submersible through 5 kilometers of water off the coast of California. Carefully surveying the seafloor, the sea level and other variables, the scientists determined the gravitational constant G to an accuracy of 2 parts in 1,000. These are the most precise gravity measurements made in large-scale field experiment, says John A. Hildebrand of Scripps.

Laboratory tests have found no sign of a fifth force but these experiments could not test for the force acting over great distances. If a fifth force did manifest itself over 1,000 meters, the recent gravity measurements near the seafloor should have differed from those several kilometers above the seafloor -- a distance presumably out of the range of the hypothetical force. But the scientists found no appreciable variation.