Newton's gravity law may take a fall.

Newton's gravity law make take a fall

Preliminary results from a gravity experiment conducted deep within the Greenland ice cap may lend support to the existence of a much-disputed fifth force of nature.

In the summer of 1987, investigators lowered a sensitive meter into a 2-kilometer-deep borehole in the ice and found gravity to be about 3 percent stronger than expected, says experiment coordinator Mark E. Ander of the Los Alamos (N.M.) National Laboratory (LANL), who collaborated with colleagues from several U.S. and British universities. Ander reported his findings to colleagues this week at Los Alamos and will discuss them next week at a conference in Australia.

In the experiment, the researchers compared their measurements with predictions based on the standard Newtonian law of gravity. The standard theory is called the inverse square law, because gravitational attraction is thought to depend on the square of the distance separating any objects. However, the results indicate "there is an apparent violation of the inverse square law," says Ander, who is still analyzing the measurements.

If the measurements are correct, says LANL theorist Richard Hughes, "it is telling us either gravity is more complicated than we ever thought before, or there is a new force of nature."

The Greenland experiment is the latest in a series of sensitive tests over the last two years that have reportedly found violations of Newton's law of gravity (SN: 12/19&26/87, p.388). Theorists have proposed that the minute departures from standard gravity may be manifestations of a fifth force -- one that works over distances ranging from a few meters to several kilometers. Of the four traditional fundamental forces, gravity and electromagnetism act over infinite distances, while the strong and weak forces operate on the atomic and subatomic scales.

In design, Ander's project resembles a test conducted in Australian mine shafts two years ago. But the earlier experiment found gravity slightly weaker than predicted, an effect completely opposite that seen in the recent test. Ander says the Greenland borehole allows for greater accuracy because gravitational measurements depend on density and ice's density is more uniform than that of rock. He and others are planning further gravity tests in the Antartic and in the ocean.

For now, physicists are not rushing to amend the inverse square law or declare the existence of a new force. Says geophysicist David Rubincam from NASA Goddard in Greenbelt, Md., "I think we're all waiting for more definitie results since the Earth is a very dirty laboratory."