LIGO, or the Laser Interferometer Gravitational-Wave Observatory, is a set of two detectors located in Livingston, Louisiana and Hanford, Washington. Last year, LIGO announced it had detected gravitational waves for the first time, a major breakthrough that vindicated the predictions of Albert Einstein. In his theory of general relativity, he described spacetime as a physical substance whose shape could be altered by massive objects like black holes. When these objects undergo energetic changes—say two of them collide, a black hole consumes another massive object, or a star goes supernova—they create ripples in spacetime like the ridges you see when you toss a stone into a pond. To learn more about how black holes form, and how well the theory of general relativity holds up, we have to observe these waves—because black holes do not emit light, so we have no other way to view their behavior.