Testing Gravity via Next-Generation Lunar Laser-Ranging

Lunar laser ranging has a long history of performing the most precise tests of various gravitational phenomena, dating from the placement of retroreflector arrays on the lunar surface by the Apollo astronauts. Gravity is the least well understood of the fundamental forces of nature, our current model being incompatible with quantum mechanics. We must therefore push our tests of gravity further, with the expectation that general relativity will fail to reproduce exactly measurements of sufficient precision. We aim to improve lunar ranging performance by an order-of-magnitude, resulting in comparable sensitivity gains in various tests of the basic nature of gravity. Here we describe the scientific motivation and techniques behind this next-generation lunar ranging effort, APOLLO.