LONG BEACH, CALIFORNIA--Scientists have been scrutinizing gravity since the time of Newton, but they've had difficulty measuring the power of its pull. Now, thanks to a clever device, physicists have the most precise measurement yet.
The strength of gravity, represented by the numerical constant "big G," is quite puny--it takes a huge amount of mass to exert a palpable pull. Until now, big-G experiments drew on a technique developed by physicist Henry Cavendish at the end of the 18th century. He dangled a dumbbell-shaped pendulum from a thread and placed heavy masses nearby. By measuring how much the dumbbell twists due to its ends' attractions to the masses, Cavendish obtained a fairly good measurement of big G. But in 1995, physicists realized that there was a subtle bias in Cavendish-style torsion experiments: The string or wire that suspends the pendulum is not perfectly elastic, thus distorting the measurement.
Enter the big-G whizzes at the University of Washington, Seattle. Physicist Jens Gundlach and his colleagues eliminated the string-twisting bias by mounting a flat pendulum--eliminating some geometrical modeling problems of the dumbbell--on a turntable that slowly rotates about once every 20 minutes, bringing the pendulum's ends toward and away from four 8-kilogram steel balls. As the pendulum tips get close to the balls, they feel the increased gravitational force and the pendulum begins to twist. Immediately, a laser sensor triggers a switch that accelerates the turntable, counteracting the torque. Instead of the string twisting in response to the gravitational acceleration, the turntable turns. The speed of the turntable yielded a measurement of big G with an error of a mere 14 parts per million--about 10 times more precise than ever before.
"[It] should have been obvious" that previous measures of big G were off, says physicist Randy Newman of the University of California, Irvine. The new result, announced this week at the American Physical Society meeting, sets big G tentatively at 6.67423 ± 0.00009 x 10-11 m3/(kg s2). "It's one of the fundamental constants," Gundlach says. "Mankind should just know it. It's a philosophical thing."