BEST MEASUREMENT OF THE GRAVITATIONAL CONSTANT. At this week's American Physical Society Meeting in Long Beach, Jens H. Gundlach of the University of Washington (paper P11.3) reported a long-awaited higher precision measurement of the gravitational constant, usually denoted by the letter G. Although G has been of fundamental importance to physics and astronomy ever since it was introduced by Isaac Newton in the seventeenth century (the gravitational force between two objects equals G times the masses of the two objects and divided by their distance apart squared), it has been relatively hard to measure, owing to the weakness of gravity. Now a group at the University of Washington has reduced the uncertainty in the value of G by almost a factor of ten. Their preliminary value is G=6.67390 x 10-11 m3/kg/s2 with an uncertainty of 0.0014%. Combining this new value of G with measurements made with the Lageos satellite (which uses laser ranging to keep track of its orbital position to within a millimeter) permits the calculation of a brand new, highest precision mass for the earth: 5.97223 (+/- .00008) x 1024 kg. Similarly the new mass of the sun becomes 1.98843 (+/- .00003) x 1030 kg. Gundlach's (206-543-4080, jens@phys.washington.edu)
The setup is not unlike Cavendish's venerable torsion balance of two hundred years ago: a hanging pendulum is obliged to twist under the influence of some nearby test weights. But in the Washington experiment measurement uncertainties are greatly reduced by using a feedback mechanism to move the test weights, keeping pendulum twisting to a minimum. (See Gundlach's written summary; figures at Physics News Graphics.)
