Newtonian physics falls short in studies of gravitational law
IT'S been a tantalizing year for physicists on the trail of possible new natural forces. These would be small corrections to Newton's law of gravity that are effective only over a relative short range - somewhere between a few tens of feet to a few miles. Experiments reported during the year have been ambiguous. Yet, as Albert T. Hsui of the University of Illinois has noted, they do ``provide some support for the scientific merits of the studies.''
Frank Stacey of the University of Queensland in Brisbane, Australia, started this round of speculation in the early 1980s with gravity studies in mine shafts. The change in gravitational attraction with increasing depth suggested that a small short-range repulsive force may counter the gravitational attraction calculated from Newton's classical law.
Hsui has repeated this kind of measurement in a borehole in Michigan. Last August, he reported that his results reinforce the suspicion that Newton's law needs correction. But, as with Stacey's data, they are too uncertain to be definitive.
The correction would do more than modify Newtonian gravity at short ranges. It also would introduce a factor that, unlike ordinary gravity, depends on the composition of material objects as well as on the amount of material they contain. Several experiments in different laboratories have tried to test whether earth's gravity acts slightly more or less strongly on such different materials as copper and aluminum. Some of these experiments suggest there may be a difference while others show no such effect. But in the speculations of some of those hunting a new force, this is not as contradictory as it may seem.
Certain unifying theories that represent all forces, including gravity, as different aspects of one underlying force treat Newtonian gravity in a way that requires not one but two corrective factors. One of these would be a short-range repulsive force. The other would be a short-range force that is either attractive or repulsive depending on the composition of the bodies involved. Thus there could be cases in which these two corrective factors would cancel out.
Stacey and his colleagues recently reported that analysis of different types of gravitational data, including observations on the scale of the Solar System, are compatible with such a pair of new forces.
Earlier this month, a research team at the US Air Force Geophysical Laboratory reported gravity data taken along the height of a 2,000-foot tower. These show signs of a small attractive force over and above that expected from normal gravity. The team notes that this is compatible with the theory that three kinds of gravitational force exist.
All of this remains speculative. In fact, most physicists, including those pursuing this research, are skeptical of these extra gravitational forces. But there are strong enough hints of their reality to whet the researchers' enthusiasm for moving ahead with even more sensitive experiments in 1988.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.
