Funding Source: Jointly funded by the National Science Foundation (PHY-1404491) and the National Aeronautics and Space Administration (NNX-15AC51G) – Quantum mechanics and General Relativity are immensely successful frameworks that agree with experiment with unprecedented precision. But they are incompatible with each other. New ideas in physics that unite quantum mechanics and general relativity predict tiny but detectable deviations from General Relativity. Using a technique called Lunar Laser Ranging, we can measure the Earth-Moon distance with a precision of one millimeter (that’s a part in a trillion of the distance). We pulse green laser light from a telescope on Earth to retro-reflectors placed on the Moon by Apollo astronauts and Russian rovers. By monitoring the shape of the lunar orbit over time, we achieve leading constraints on relativistic gravity. Although our observations thus far are consistent with General Relativity, ongoing enhancements to our experimental sensitivity could reveal the first cracks in Einstein’s gravitational edifice, and would provide a vital clue toward a grand, unified theory.