A space-time oscillation created by the motion of matter, as predicted by Einstein's general relativity. When an object accelerates, it creates ripples in space-time, just like a boat causes ripples in a lake. Gravitational waves are extremely weak even for the most massive objects like supermassive black holes. They had been inferred from observing a binary pulsar in which the components slow down, due to losing energy from emitting gravitational waves. Gravitational waves were directly detected for the first time on September 14, 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO). Since then several other events have been detected by LIGO and Laser Interferometer Space Antenna (LISA).