Magazine article Science News

Gravity Gets Measured to Greater Certainty

Magazine article Science News

Gravity Gets Measured to Greater Certainty

Article excerpt

Despite more than 200 years of experiments, the best laboratory determinations of the force of gravity have yielded no better than a ballpark figure. Adding to the haziness, meteorologists have scarcely probed the force's strength at distances of 1 millimeter or less. There, gravity may vary drastically from Newton's law, possibly signaling the presence of unseen extra dimensions, some theories suggest (SN: 2/19/00, p. 122).

Penetrating the fog, researchers in Seattle now report preliminary results from two separate gravity experiments. One set of findings offers the most precise value thus far for the gravitational constant, so-called Big G, which relates mass and distance to the strength of gravity in Newton's law.

The other experiment provides the first measures of gravity between objects separated by as little as 0.2 mm. No deviation from Newton's law turned up. Both University of Washington teams unveiled results at the American Physical Society meeting last week in Long Beach, Calif.

The new precision comes at a time when determinations of G disagree notoriously (SN: 4/29/95, p. 263). In particular, a 1994 German figure lies far out of line with most other recent values, including this latest one. Consequently, in a just-completed reevaluation of fundamental physical constants, the Committee on Data for Science and Technology (CODATA) of the International Council for Science in Paris has increased G's uncertainty by a factor of 12.

In typical experiments designed to find G, researchers hang a dumbbell horizontally from a fiber and measure the weight's twist when it is near objects of accurately known mass. In 1995, however, Kazuaki Kuroda of the University of Tokyo pointed out that mechanical stresses in the. twisting fibers can introduce errors.

Washington researchers Jens H. …

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