Lunar Laser Ranging — A Comprehensive Probe of the Post-Newtonian Long Range Interaction

Abstract

30 years of lunar laser ranging (LLR) data has been modeled and fit with several millimeters precision using the general relativistic equations of motion for solar system dynamics. This produces several key tests of that tensor theory of gravity and strongly constrains presence of any supplementary interactions. Earth and Moon fall toward the Sun at rates equal to a couple parts in 1013, confirming both the universal coupling of gravity to matter's stress-energy tensor, and gravity's specific non-linear coupling to itself. The expected deSitter precession (with respect to the distant ‘fixed’ stars) of the local inertial frame moving with the Earth-Moon system is confirmed to 3.5 parts in 103 precision, and Newton’s constant indeed shows no cosmological time variation at the few parts in 1012 per year level. All the types of post-Newtonian terms in the N-body equation of motion—motional, gravito-magnetic, non-linear, inductive, etc.—contribute to the measured details of the lunar orbit, so LLR achieves ‘near- completeness’ as a gravity experiment and probe. The precision of these measurements, especially those connected with lunar orbit frequencies and their rates of change, should further improve as LLR observations continue into the future.