Prospects Grow Dim for a `Fifth Force' of Nature

AFTER nearly a decade of challenge, Newton's law of gravity remains intact. Some earth scientists have thought they found small deviations from that law when they measured gravity down mine shafts and deep bore holes or up high towers. Robert Parker and Mark Zumberge of Scripps Institution of Oceanography, who have restudied the data, now conclude ``that the present collection of geophysical experiments provides no compelling evidence for discrepancies in the inverse square law of gravity.''

This dims the prospect for a new natural force that some physicists speculate might be causing the alleged discrepancies. Indeed, Donald Eckhardt of the Air Force Geophysics Laboratory in Bedford, Mass., who led one of the tower experiments, says close examination of his own data shows they don't support the case for such a force.

That case is not yet closed, however. Reporting their analysis recently in Nature, Drs. Parker and Zumberge note: ``It is important to understand that we have not shown the inverse square law to be accurate on the scales of these experiments; we have only shown that the case for its failure has not been established.''

Actually, the gravity theories of both Newton and Einstein are under test.

In Newton's law, the gravitational force between two bodies depends on their respective masses and decreases as the square of the distance between them increases. Einstein's general theory of relativity extends Newton's ideas to include gravitational effects of energy and momentum. In many cases, it is the same as Newton's law.

One of the main principles of Einstein's theory is that gravitational mass - the mass that generates a body's gravity - is precisely the same as inertial mass - the mass that determines how a body responds to forces. Put another way, this ``equivalence'' principle says that a cannon ball and a feather dropped off a tall building would hit the ground at the same time if there were no air resistance to hold back the feather.

Both relativity theory and Newton's law (where applicable) have been shown to hold to a high degree of accuracy. But physicists have always wondered if this is the whole story.

Australian geophysicist Frank Stacey of the University of Queensland in Brisbane has suggested for many years that Newton's law may need adjusting over distances ranging from a few tens of meters to as much as a few hundred kilometers. He and various colleagues have measured gravity down mine shafts on several occasions. Their data seem to show small deviations from Newton's formula. Other such tests include measurements down a two-kilometer-deep bore hole in the Greenland ice cap in 1987, in which Parker and Zumberge participated, and Dr. Eckhardt's 1987 measurements of gravity along a 600-meter high television tower near Raleigh, N.C.

As Parker and Zumberge and, independently, Eckhardt now explain, anomalies due to local terrain better account for their ``discrepancies'' than any presumed deviation from Newton's law. James Thomas and associates at the Lawrence Livermore National Laboratory reinforce this skepticism with a report last month in Physical Review Letters. They measured gravity along a 465-meter tower at the Nevada (nuclear weapons) Test Site, where local gravity is very well known. They found Newton's law holding to an accuracy of over one part in 10 million.

Meanwhile, some physicists have wondered if the equivalence principle itself is entirely valid. They speculate about a force that would make gravity affect different materials slightly differently. A number of tests have been carried out to find such an effect - so far with largely negative results.