LIGO, the Laser Interferometer Gravitational-wave Observatory, has been called the most precise ruler in the world for its ability to measure motions more than 10,000 times smaller than the width of a proton. By making these extremely precise measurements, LIGO, which consists of two facilities—one in Washington and one in Louisiana—can detect undulations in space-time called gravitational waves that roll outward from colliding cosmic bodies such as black holes.