Gravitational waves

Gravitational waves were first directly detected on September 14, 2015, by the Laser Interferometer Gravitational-Wave Observatory (LIGO), nearly a century after Einstein predicted them in 1916. The signal, designated GW150914, was produced by the merger of two black holes roughly 1.3 billion light-years away.

LIGO operates two L-shaped interferometers—one in Hanford, Washington, and one in Livingston, Louisiana—each with arms 4 kilometers long that use laser beams to measure spacetime distortions smaller than the diameter of a proton. The dual-site setup helps confirm signals and roughly triangulate the cosmic source of each event.

The first event, GW150914, was produced by the collision of two stellar-mass black holes of approximately 29 and 36 solar masses. The merger converted about three solar masses of energy into gravitational radiation in a fraction of a second.

Einstein predicted gravitational waves in his 1916 and 1918 papers on general relativity but doubted they would ever be observed because the distortions reaching Earth are unimaginably small. He even briefly published a 1936 paper arguing the waves might not be real, though he later retracted that claim.