GW150914

On 14 September 2015, the two LIGO detectors in the United States twitched in unison for two-tenths of a second, registering a tiny rising chirp. The signal, named GW150914 for the date it arrived, was the first direct detection of gravitational waves — a century after Einstein predicted them and decades after the hunt began. It is one of the landmark measurements in the history of physics.

Reading the chirp's shape, physicists reconstructed what made it: two black holes, of roughly 36 and 29 times the Sun's mass, spiraling together about 1.3 billion light-years away and merging into a single black hole of about 62 solar masses. The missing few solar masses had been converted directly into the energy of gravitational waves in a fraction of a second. The entire signal that crossed more than a billion light-years lasted about 0.2 seconds in LIGO's data and shifted the detectors' mirrors by far less than the width of a proton — yet matched the predictions of Einstein's theory for merging black holes with stunning precision.

GW150914 matters as the opening of gravitational-wave astronomy: proof that the ripples are real, that black holes truly exist and can merge, and that we can now sense the universe through spacetime as well as light. It confirmed that pairs of heavy black holes form and collide, launched a flood of later detections, and earned a Nobel Prize. It is the moment a brand-new sense switched on for humanity.