Lunar ranging confirms equivalence principle

Accumulated ranging data from reflector packages placed on the Moon by the Apollo astronauts have verified a previously untestable aspect of the equivalence principle. According to two simultaneously reported but essentially independent analyses of the same laser ranging data, Earth’s gravitational self energy contributes equally to its inertial and passive gravitational mass. This equivalence would rule out Brans–Dicke–Jordan theory for small values of the coupling constant. Both analyses were published in the 15 March issue of Physical Review Letters. One, covering six years of data, was reported by the “LURE” group (for Lunar Ranging Experiment): This group was established before the first Apollo mission to carry out the laser ranging experiment. Among its members are James Williams (Jet Propulsion Laboratory), Robert Dicke (Princeton University) and Peter Bender (Joint Institute for Laboratory Astrophysics), Carroll Alley (University of Maryland), Douglas Currie and James Faller (JILA), Derral Mulholland and Eric Silverberg (McDonald Observatory, University of Texas). The second report, covering essentially the same data, is by Irwin Shapiro and Charles Counselman (Massachusetts Institute of Technology) and Robert King (Air Force Cambridge Research Laboratories).

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