new measurement of G

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed May 17 2000 - 10:08:09 PDT


Message-Id: <l0311070bb548825be239@[192.174.2.173]>
Date: Wed, 17 May 2000 09:08:09 -0800
From: Paul Doherty <pauld@exploratorium.edu>
Subject: new measurement of G


  5 May 2000 2:00 PM

 Gravity Turntable Sets New Record

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."

--CHARLES SEIFE

ScienceNOW -- 2000 (505): 2
Science Online
2000 by the American Association for the Advancement of Science.

Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld


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