GW150914

Two black holes become one

GW150914 was produced when two black holes spiralled together and merged. During the final fraction of a second, they radiated more power in gravitational waves than the combined light of every star in the observable universe.

The merged remnant contained about 62 solar masses. Roughly three solar masses were converted into gravitational-wave energy, briefly distorting spacetime around Earth by less than the width of a proton.

How LIGO heard it

On 14 September 2015, the two LIGO detectors, one in Hanford, Washington, and one in Livingston, Louisiana, recorded the same growing chirp within milliseconds of each other. The signal matched the predicted waveform of two black holes merging.

This was the first direct detection of gravitational waves, a century after Einstein's general theory of relativity predicted them, and the first direct evidence of a binary black-hole merger.

Why it matters

GW150914 opened a new way of observing the universe. Gravitational waves carry information about black holes that emit almost no light, allowing astronomers to measure masses, spins and distances in a way that electromagnetic telescopes cannot.