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Debate over fifth force revived

I don't know what to do with this. If it's already down on the boards, just send it to done-pro. I have no more edits to make on it. -- Reuven

By Jayant Kumar

A recent experiment has reopened the controversy surrounding the existence of a "fifth force." This force, if it exists, would account for disagreement between predicted and observed values of G, the gravitational constant.

Results of a three-year study in the Pacific Ocean found no variation of G from the value predicted by Newton's inverse square law for gravitational force. The study suggests that the fifth force does not act over distances from a few feet to a few miles.

The experiment leaves open the question of whether the force acts at distances less than one inch or greater than a few dozen miles. Furthermore, variations smaller than 0.1% of G would not have been detected during the study, conducted at the University of California at San Diego's Scripps Institution of Oceanography. These openings have left physicists around the world, including those at MIT, in disagreement over the existence of a fifth force.

Debate began in earnest five years ago, when several groups of physicists claimed to have found a force distinct from the four fundamental forces of nature: weak, strong, gravitational and electromagnetic.

One of the first experiments, conducted in 1987, measured G at different depths in a 500-meter mine shaft in Australia. A team of researchers from the University of Queensland measured G to be 1% greater than the value of 6.672 x 10[el-3] newton-meters per kilogram squared predicted by Newton's inverse square law.

Their results were questioned one year later at a physics colloquium on New and Exotic Phenomena held in the French Alps. The colloquium found that the Australian group had not fully taken into account the local density distribution of the rocks in the mine shaft. This oversight resulted in an overestimate of the size of the gravitation anomaly present in the rock layer.

In response to this difficulty, Donald Eckhardt and his colleagues at the Air Force Geophysics Laboratory in Massachusetts measured G in the air as they descended a 600-foot tower located on flat North Carolina terrain. Eckhardt's measurements of G also showed a discrepancy from Newton's inverse square law, but his value of G was 2% less than the expected value. These results further added to the fifth force confusion, leaving physicists unsure whether this new force is attractive or repulsive.

Another group of researchers claimed that the fifth force is caused by a chemical property called isotopic spin. Isotopic spin is a quantum number related to the number of different values of electric charge a subatomic particle can have. The fifth force would thus differentiate between protons and neutrons, which have the same mass but different quark constituents, and thus different charges.

This theory was discredited by Eric Adelberger and Christopher Stubbs of the University of Washington at Seattle. Using a shielded and freely oscillating torsion pendulum with four hanging test masses, they found no significant variation from Newton's gravitational constant attributable to isotopic spin.

Although no fifth force experiments have been carried out at MIT, the physics faculty has followed the topic closely, and many disagree on the issue. Stephen G. Steadman, senior research scientist in the Laboratory for Nuclear Science, is skeptical. He believes "the Adelberger experiments were extremely well done and showed no indication of a fifth force."

George S. F. Stephans, who works in Steadman's research group, said "the fifth force is a very interesting field which can't be ruled out conclusively." Stephans has avoided forming a final opinion on the fifth force. He is awaiting the results of experiments underway at the Lawrence Livermore National Laboratory before making his decision.

Professor Alan J. Lazarus said the consensus of the physics community is that the fifth force does not exist. He added, however, that more accurate instruments could reveal effects too small to be detected at present.