Magazine article Science News

Gravity's Force: Chasing an Elusive Element

Magazine article Science News

Gravity's Force: Chasing an Elusive Element

Article excerpt

Determining the values of fundamental physical constants has long served as a test of both physical theory and measurement technology. Now, experiments by three independent groups have produced values for the strength of the gravitational force (G) that disagree significantly with the currently accepted number and with each other (see table).

The teams involved in these experiments reported their results at last week's American Physical Society meeting, held in Washington, D.C.

"Each of these groups has done a careful job, but G is an extremely hard number to nail down," says George T. Gillies of the University of Virginia in Charlottesville.

The problem stems from the fact that gravity is much weaker than the other forces of nature. Moreover, it's always attractive, and there's no way to shield an experiment from external gravitational influences.

Mark P. Fitzgerald and his coworkers at the Measurement Standards Laboratory in Lower Hutt, New Zealand, used a torsion balance, which consists of a copper bar dangling horizontally from a thin, tungsten wire fastened to the bar's midpoint (SN: 3/12/94, p.376).

Normally, the gravitational force of two large masses brought near the bar's ends causes it to rotate, slightly twisting the wire. To avoid complications due to wire twisting and bar oscillations, the researchers applied an electric field to attract the copper bar and balance the

gravitational force, keeping the bar still at all times. By carefully measuring the compensating electric force, they could deduce the gravitational force and calculate G. …

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