A researcher in lunar laser ranging and tests of gravity. With Slava G. Turyshev and Dale Boggs, he analyzed whether Earth and the Moon fall toward the Sun at the same rate. Timing laser pulses reflected by lunar retroreflectors turns the Earth–Moon system into an experiment on the equivalence principle.[ref]
Lunar Laser Ranging Tests of the Equivalence Principle by James G. Williams and others. Author affiliation; abstract and §2: ranging apparatus and equivalence-principle test
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