A Comprehensive Probe of Gravity via Lunar Laser Ranging

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APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) aims to test the foundations of our current theory of gravity--Einstein's General Relativity--by measuring the shape of the lunar orbit to one-millimeter precision. The technique involves bouncing laser pulses off reflectors placed on the moon by the Apollo astronauts. Measuring the round-trip time-of-flight of the laser pulses determines the distance to the moon, thus allowing the shape of the lunar orbit to be traced.Prior to APOLLO, this technique determined the lunar orbit shape to 2-3 cm precision, and verified the correctness of General Relativity to about the 0.1% level of precision. By using a 3.5 meter telescope and modern instrumentation, APOLLO is capable of one-millimeter range precision, and will thus allow 0.01% precision tests of General Relativity. APOLLO is now in routine operation collecting its first data. Under this award, APOLLO will refine its technique and produce its first scientific results. The simple elegance of the lunar ranging technique, coupled with its connection to our historically inspiring Apollo lunar program, make APOLLO an apt vehicle for communicating the excitement of science to the public. This award is co-funded by the Physics Division and by the Office of Multidisciplinary Activities in the Mathematical and Physical Sciences Directorate.