Newton's law of gravity: scientists have some nagging doubts

Newton's law of gravity is on of the gems of modern science. Even Einstein's relatively theory, which subsumes it, has not displaced it for many practical purposes.

Now some scientists wonder if this useful law is as reliable as it is thought to be.

Newton's gravitational theory has never been firmly grounded in laboratory experiments. Gravity is a very weak force compared to, say, electro-magnetic attraction. IT is difficult to measure in the laboratory. The so-called gravitational constant. "G," which figures in the law is known only with moderate precision from laboratory studies. There is no way to determine it astronomically.

This uncertain foundation for the famous law has nagged in the background of physicists' thinking for centuries. However, over the past five years, new doubts have arisen. In particular, there are indications in some laboratory tests and geophysical data that the constant G may not be so constant after all.

This inconstancy is different from the gradual weakening of G over time as the universe evolves which some cosmologists have suggested. It is a variation of G over short distances -- a few millimeters to a few thousand meters. Over large distances, G would settle down to a constant value. But over distances within a laboratory, between the top and bottom of deep mine shafts, or between the surface and floor of the sea, some data suggest that the value of G depends upon the distance over which the gravitational is measured.

So far, nothing conclusive has emerged. However, as F. D. Stacey and G. J. Tuck of the University of Queensland in Australia observe in reporting the most recent such study in Nature, "strong circumstantial evidence suggests that well controlled large-scale experiments on [Newton's law] are urgently needed."

In 1976, Daniel R. Long of Eastern Washington State College reported laboratory experiments which suggest the law fails over short ranges. Subsequent experiments by others using different techniques have not always supported this. For example, H. Hirakawa at the University of Tokyo last year reported laboratory verification of Newton's law within an experimental accuracy of 3 percent.

However, 3 percent is still rather rough. It is imprecise enough to conceal the effect Long suggests. Now Stacey and Tuck find new evidence of short range failure in four sets of geophysical gravity data. Three of the sets were taken at the top and bottom of deep mine shafts. A fourth was taken at the surface and sea bed in the Gulf of Mexico. All of these data sets yield larger values for G than has been traditionally determined by laboratory measurements.

As Tuck and Stacey note, it is time physicists paid serious attention to their long-standing qualms and took a hard look at one of the best known laws of natural science.

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