Was Sir Isaac All Wet?

By John Langone

As every physics student learns, there are four known forces of nature: gravity, electromagnetism, a "strong" force that binds atomic nuclei and a "weak" one that governs certain types of radioactive decay. Last week researchers at the Los Alamos National Laboratory announced that they may have found the best evidence yet for a hypothetical, elusive "fifth force." If confirmed, their findings could mean that Sir Isaac Newton's famous inverse- square law of gravity* is in danger of losing the exalted position it has held for three centuries. "It's like saying Mom and apple pie's no good anymore," admits the leader of the gravity project, Geophysicist Mark Ander. "You just don't do that lightly."

The physicists reached their conclusion as the result of an experiment conducted in Greenland last summer. They lowered a supersensitive gravity meter into a mile-deep hole bored in glacial ice -- chosen because its density is more uniform than that of rock -- and monitored the gravitational pull as the meter descended. What occurred was startling: the expected increase in gravitational force predicted by Newton was there, but it got stronger faster than expected. Either something was enhancing the force of gravity or the researchers had come upon a heretofore unknown, far more complex working of gravity itself. Or, just possibly, they had made a mistake.

The fifth force, if that is what it is, has been a source of debate among physicists since its existence was suggested in 1981 by Australian mineshaft experiments. Five years later, Purdue University Physics Professor Ephraim Fischbach measured a weak force he called "hypercharge" and theorized that it caused objects of different composition to fall at different rates. Since Fischbach's finding, as many as 45 experiments have sprung up in search of the mystery force, and so far each has served only to confound rather than clarify the issue.

In some, for example, gravity appears to be enhanced, while in others it seems to be counteracted. Moreover, findings from a U.S. Air Force gravity study were even interpreted by some scientists as evidence of a "sixth force." But if the existence of an additional force was proved, scientists would have to readjust their calculations of gravitational force. "It's like something completely out of left field," notes Los Alamos Physicist Terry Goldman. "You don't know quite what to do with it."

Jim Thomas, a physicist at the California Institute of Technology, praises the technical precision of Ander's experiment, but cautions that measuring gravity in holes is inexact at best. He points out, for example, that an aberration in the earth's crust might have caused the unusual measurements. "What we're really talking about is the possible modification of gravity, which is the fourth force," adds Thomas. Even Ander stresses that rigorous confirmation is needed before he accepts the results of his Greenland experiment. Says he: "You keep saying to yourself, 'Gee, I've gotta be wrong -- Newton certainly can't be wrong.' "

FOOTNOTE: *Newton's law holds that gravitational pull increases in inverse proportion to the square of the distance between two bodies.

With reporting by J. Madeleine Nash/Chicago and Dennis Wyss/San Francisco