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Newton derived theories about gravity by studying stationary or very slowly moving objects. Laboratory measures of G performed since share this fundamental approach. While this may accurately measure G, any possible relative velocity dependence to this constant or to the force of gravity goes undetected. Determining that force involves only the masses, the separation, and G, representing a static gravitational acceleration. Most cases of interest involve objects in motion, including confirmations of general relativity in high-velocity binary pairs and around possible black holes. Laboratory research involving masses with high relative velocities, though strongly needed, is absent from the literature.

Curt Renshaw
Alpharetta, Ga.

Major obstacles prevent scientists from measuring G in the laboratory using objects moving at near-light speed. In the case of subatomic particles at such relativistic speeds, Heisenberg’s uncertainty principle and other problems bar measuring the particles’ positions accurately enough, says Douglas S. Robertson of the National Geodetic Survey in Boulder, Colo. Accelerating larger objects to such speeds requires prohibitive amounts of energy and raises safety concerns, he says. Such objects “would attain nuclear-weapon-scale kinetic energies,” he notes .–P. Weiss Measuring G continues to be important, and it is interesting to realize how far back the study goes. I recently visited the site in the Scottish Highlands where astronomer Nevil Maskelyne measured G in 1774. A plaque set up by the Royal Society deep into the mountains near the peak of Schiehallion describes how this symmetric mountain was used to test the difference in a plumbline from one side to another and so determine “the attraction of mountains.” As part of the task, Charles Hutton invented the idea of surface-contour lines.

Jay Pasachoff
Williams College
Williamstown, Mass.