Speake and Quinn reply: The effects of local gravity gradients in G experiments are almost always negligible because the experiments are designed so that only changes in the gravity gradients that are in phase with the experiment’s sequence of operations would have any effect. Thus, unless the lab walls, nearby elevators, heavy vehicles, or any other large mass is moving in phase with the experiment, there will be no effect. Changes that aren’t precisely in phase with the experiment would increase gravitational noise but would not alter the measured signal. (Because our torsion balance was carefully designed, even that gravitational noise falls off as the fifth power of the distance.)
There could be an effect, however, if the source masses—the repositionable masses that couple with the smaller, test masses on a torsion balance or pendulum—were sufficiently large to move the torsion balance or pendulum enough to place it in a significantly different part of the local background gravitational field. In laboratory-sized experiments, however, the movements in torsion balances and pendulums are many orders of magnitude below the level at which such effects could be significant. In our experiment, for example, the rotation of the torsion balance was, at most, only some 150 microradians, with the test masses on the balance moving only about 15 micrometers. The change in local gravity field over that distance is negligible.
