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Taking the measure of Newton's gravity law.

More than 300 years ago, Isaac Newton devised a remarkably simple mathematical relationship to encapsulate how the force of gravity depends on the separation of two objects and their masses. Since then, researchers have sought possible deviations from Newton's gravitational law, but have generally failed to produce any compelling experimental evidence of such discrepancies. Instead, these efforts -- especially over the last decade -- have substantially increased the precision with which experiment agrees with theory.

Using what they describe as the world's most sensitive "gravity gradiometer," Ho Jung Paik and his co-workers at the University of Maryland in College Park have now improved that precision by a factor of 10. Their experiment sets a new upper limit on any potential deviations from Newtonian gravitation, when measured over a range of a few meters between gravitationally interacting objects.

To obtain this result, Paik and his colleagues used a new instrument designed to detect minute variations in a gravitational field. Each of the instrument's three cylindrical arms contains a pair of motion-detecting accelerometers. Each accelerometer, in turn, consists of a disk of niobium connected to a special spring.

Moving an object closer to the instrument slightly shifts the positions of the niobium masses by different amounts, depending on the distance of each of the masses from the object. The researchers can readily detect differences between the tiny displacements of pairs of accelerometers by using superconducting circuitry and taking advantage of the fact that niobium itself becomes a superconductor at liquid-helium temperatures.

To check Newton's gravitational law, Paik and his colleagues observed how their gravity gradiometer responded to the motion of a nearby, 1,500-kilogram pendulum about 3.3 meters long. Although they applied an ingenious strategy to circumvent the need for precise measurements of the instrument-pendulum separation, they still had to find ways to correct for the pendulum-induced shaking, tilting, and twisting of the building in which the pendulum was mounted.

"That small amount of [extraneous] motion bothered us," Paik says. It took nearly two years to work out ways to eliminate these effects from the measurements.

The final result confirmed that Newton's law of gravity holds to within two parts in 10,000 for masses a few meters apart. "We have plans to improve this further," Paik says.

The researchers are also developing new instruments, based on the same technology, for detecting gravitational waves and for observing variations in Earth's gravitational field from an aircraft or an orbiting satellite.
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Title Annotation:Isaac Newton's theory of gravity's dependence on separation of two objects and their masses
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Oct 3, 1992
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