Newton's law still rules
WHEN a team of earth scientists reported last summer that Newton's hallowed law of gravity may need correction, they thought they had a case for ``an apparent new force of nature.'' They've had to think again. As Robert Parker, a team member from th Scripps Institution of Oceanography, told the recent fall meeting of the American Geophysical Union, Newton's traditional law can account for their data after all. ``Our results do not need new forces of nature,'' he said.
It's a useful lesson in the skepticism needed when enthusiasm for a potentially sensational discovery colors the announcement of preliminary findings.
Scientists have been probing for flaws in Newtonian gravity for the past seven years. Showing that the law needs correction would be a significant discovery in itself. But many of the searchers are after a bigger prize. They think an inaccuracy in the law would reflect the action of a previously unknown natural force with an effective range of a few tens of meters to as much as a few kilometers. The unambiguous discovery of such a force would be an achievement of Nobel Prize-winning quality.
Some critics, especially Richard Hughes of Los Alamos National Laboratory, warn against the concept of a new force. He has repeatedly insisted that, if medium-range deviations from Newton's law do exist, they will be due to an unknown aspect of gravity itself, not a new type of force.
Frank Stacey of the University of Queensland in Brisbane, Australia, and colleagues inaugurated the present search with careful measurements of gravity in two Australian mines, beginning in 1981. They have what they consider evidence of a repulsive force that, at maximum, causes a 0.7 percent deviation from Newtonian gravity.
But there is enough uncertainty in their work to leave many other scientists unconvinced. Irregularity in the mass of surrounding rocks or deep within the earth could cause errors in their data larger than the claimed effect. Several other measurements of how gravity varies with height or depth have yielded similarly ambiguous results.
Some scientists think the action of a medium-range force might depend on the materials involved. Newton's law assumes that gravity acts equally on all masses regardless of their composition. A medium-range effect that depends on composition would cause deviations from the classical law. So far, experiments looking for such an effect have also yielded unclear answers.
The team that reported last summer was trying to get around some of the uncertainties that have plagued Stacey's work and similar research. In 1987, scientists from AT&T Bell Laboratories, Los Alamos, and Scripps recorded gravity variations down a hole in the Greenland icecap. They expected that the ice-covered surface would have few density anomalies to muddy their data. So when a preliminary analysis of those data seemed to show the effect of a ``new force,'' they went public. They have learned since that density uncertainties surrounding and beneath their site confuse their data enough so they have no clear evidence of a non-Newtonian effect at all.
Now the team members are looking forward to making new measurements at sea. They explain that the high accuracy with which water density can be measured in three dimensions, the ability to make measurements at diverse locations, and better knowledge of the sea floor compared with land should greatly reduce the old uncertainties.
If they do this and come up with a startling discovery - or if any other research team makes such an announcement - we should remember Richard Hughes's advice at the recent American Geophysical Union meeting. ``Read my lips,'' he said. ``No new forces.''
A Tuesday column