On the morning of 14 September 2015, at 09:50:45 UTC, the two LIGO detectors in Louisiana and Washington recorded the same signal 7 milliseconds apart: a 35-millisecond 'chirp' — a vibration climbing from 35 Hz to 250 Hz and then dying away. The instrument's mirrors had moved toward and away from each other by about a thousandth the diameter of an atomic nucleus. The signal was named GW150914. Source: the merger of two black holes 1.3 billion light-years away, weighing 29 and 36 solar masses. In the last tenth of a second, energy equivalent to 3 solar masses poured into space as pure gravitational waves.

In 1916 Einstein had predicted, as a natural consequence of general relativity, that gravitational waves must exist: as mass moves, the curvature of spacetime changes, and that change spreads at the speed of light. But the effect is so weak that even Einstein doubted it could ever be measured. LIGO worked for half a century to make it possible: two L-shaped vacuum tubes 4 km on each arm, mirrors with laser light bouncing back and forth, and active suspension systems that filter out every vibration on Earth — from truck traffic to ocean waves. The instrument's sensitivity is equivalent to measuring the Earth-Sun distance to the precision of a hydrogen-atom diameter.

When the announcement came on 11 February 2016, the scientific community read it as a double confirmation: general relativity had passed its hardest test, and the inspiral-merger of two black holes — a phenomenon that for years had lived only as equations — had been seen to actually occur. The 2017 Nobel Prize in Physics went to Rainer Weiss, Barry Barish, and Kip Thorne; LIGO's inventors and decades-long stewards.

GW150914 was a beginning. In 2017 the merger of two neutron stars (GW170817) was observed both as a gravitational wave and as electromagnetic light — 'multi-messenger astronomy' was born, and the question of where heavy elements (gold, platinum) come from finally had an answer. By 2024 more than 90 mergers had been catalogued. Humanity now reads the universe through two distinct senses: light and the vibration of spacetime. Since Galileo's telescope, no methodological expansion in astronomy has been larger.