Geophysics on the fifth force's trail.

Geophysics on the fifth force's trail

Isaac Newton made one of the greatestdiscoveries in classical physics when he realized that the force that draws an apple toward the ground is the same one that keeps the moon in orbit around the earth. For hundreds of years, his formula for the gravitational attraction between two bodies has helped scientists and engineers calculate everything from the orbits of planets to the trajectories of rockets.

But gravity has given modern theoristsa problem. As physicists have tried to combine all four known forces of nature -- the gravitational, electromagnetic, weak and strong forces -- into one unified field theory, they have been unable to incorporate gravity without postulating the existence of other, as-yet-undiscovered forces (SN: 7/26/86, p.55). In particular, the unified field theories say that the force of attraction between two bodies is given by Newton's formula plus a much smaller "fifth force" that seems to come into play at distances of about 100 to 1,000 meters.

Since this range is outside the scale ofgravity measurements in the laboratory, theoretical physicists have turned to earth scientists to hunt for the fifth force. Indeed, a number of geophysical experiments have indicated such an effect, but because of experimental uncertainties they have yet to convince most scientists. A new generation of experiments is now in the works, and scientists reported on two of these in San Francisco at the recent meeting of the American Geophysical Union.

The basic purpose of the experimentsis to measure the force of gravity at different elevations over several hundred meters and to compare these measurements with what Newton's formula predicts, based on the measured masses of nearby bodies. In practice, scientists measure an effective gravitational constant, which, in Newtonhs expression, relates the force of gravity to the masses and the distance between them.

One of the most comprehensivegeophysical experiments so far has been conducted by Frank Stacey at University of Queensland in Brisbane, Australia, and his colleagues. Working in two metal mines, the researchers have measured a gravitational constant that is 0.7 percent greater than that measured in the laboratory -- suggesting the presence of a fifth force. Other scientists, however, worry that Stacey's group has been unable to determine the density of surrounding rocks with sufficient precision. After making 14,000 boreholes into mine rocks, Stacey maintains that the mine density is well known, but he is worried that there may be some unknown, deeply buried bodies that are affecting his group's results.

So Stacey and others have been searchingfor geologic settings in which the densities and distribution of nearby masses are as uniform and well known as possible. Mark E. Ander at Los Alamos (N.M.) National Laboratory and Mark A. Zumberge, George E. Backus, Alan D. Chave, John Hildebrand and Fred N. Spiess of Scripps Institution of Oceanography in La Jolla, Calif., report that they are planning to do an experiment next summer in a 2,000-meter-deep borehole in the Greenland ice sheet. The researchers believe they will be able to measure the gravitational constant to better than 1 part in 1,000 over depths of about 100 to 1,500 meters down the borehole. To achieve this accuracy, they are taking painstaking care in their measurements. For example, they will "season" or stretch the wire line, which will hold the gravity meter, many times before the experiment and will calibrate its absolute length in an Idaho silver mine both before and after their experiment.

Ander's group would like to repeat theice experiment in a Soviet borehole in Antarctica. This group and, independently, Stacey's group are also planning experiments in the ocean from about 100 to 3,000 meters' depth. Measuring depths is easier in the ocean than in a borehole, and the seafloor topography is more even than the rocky terrain underlying ice. However, no one has measured gravity through the water column to such depths before, so there are technical problems still to be worked out.

Another experiment was proposed atthe meeting by Christopher Jekeli and Andrew R. Lazarewicz at the Air Force Geophysical Laboratory at Hanscom Air Force Base, Mass. They want to measure gravity up along one of two several-hundred-meter-tall television towers in Houston or in Raleigh, N.C. The advantage of this approach, says Jekeli, is that the effect of the air density is far less important than that of water, ice or rock. Stacey's group has tried a similar experiment, but it did not succeed because the tower was shaking too much, Jekeli and Lazarewicz, however, think their tower will be stable enough.

Jekeli and Ander say their experimentsare not likely to be definitive: If they don't find evidence for a fifth force, the theorists can respond that the force must act over greater distances than the experimental ranges, and if they do measure a gravitational constant different from the Newtonian value, skeptics can always say this result was due to experimental errors.

In this respect, these are high-risk experiments,says Ander. But over time, such experiments may yield a big payoff. What's most exciting about doing this work, he says, is that "there are very few opportunities for geophysicists to make an impact on fundamental physics, and this is clearly one of them."