Stronger support for equivalence principle.

Stronger support for equivalence principle 

The equivalence principle -- a cornerstone of the general theory of relativity -- dictates that all forms of matter and energy fall with the same acceleration in a uniform gravitational field. Researchers can test this principle by comparing the accelerations of the Earth and moon toward the sun, but there's a catch. Such experimental tests are accurate only if no long-range, nongravitational force interferes with the measurements in a way that happens to mask any anomalous gravitational effects.

Scientists have now closed that loophole. Drawing on precise laboratory experiments designed to ferret out a nongravitational, "fifth" force, Eric G. Adelberger and his colleagues at the University of Washington in Seattle conclude that such a force -- if it exists -- would be too feeble to affect significantly the results of experiment designed to detect differences in the accelerations of the Earth and moon toward the sun.

Tests of the equivalence principle hinge on the idea that the moon's orbit would be distorted in a particular way if the Earth's gravitational binding energy, which contributes about 5 part in [10.sup.10] to the Earth's total mass, doesn't behave in the same way as othe forms of mass and energy. The effect should be large enough to appear in precise measurements of variations in the distance between the moon and Earth -- it no other, unknown force gets in the way.

The new analysis, reported in the Sept. 20 NATURE, shows that a nongravitational, fifth force would contribute no more than 1 part in [10.sup.12] to an acceleration difference. So far, lunar-ranging measurements -- which involve bouncing laser beams off a reflector on the moon's surface -- have established that these accelerations are identical to within 14 parts in [10.sup.12], in agreement with the equivalence principle and general relativity.

"We've been working in this whole business of trying to detect a long-range [fifth] force for a long time," Adelberger says. "It has to be very, very weak compared to gravity, and its's interesting that it has to be so weak that it wouldn't upset, for example, a test of the moon and Earth falling toward the sun."

Because any potential contribution from a long-range fifth force would be small at best, researchers can now try to improve the precision of lunar-ranging data without having to worry about some unknown force negating their efforts to test the equivalence principle with greater precision. The new findings also furnish encouraging news for researchers interested in pursuing high-precision, space-based test of the equivalence principle.

At the same time, laboratory experiments sensitive to the existence of a fifth force are bound to improve (SN: 10/1/88, p.214). "We anticipate about a factor of 10 improvement," Adelberger says, "and it will probably get better than that."