On September 14, 2015, at 09:50 UTC, the two detectors of the Laser Interferometer Gravitational-Wave Observatory recorded the same signal within seven milliseconds of each other: a rising chirp lasting about a fifth of a second. One detector is at Hanford, Washington, and the other at Livingston, Louisiana, 3,000 kilometres apart. The event is called GW150914. It was the first direct detection of gravitational waves.
The signal came from the merger of two black holes, of about 36 and 29 solar masses, roughly 1.3 billion light-years away. In the final fraction of a second they released about three solar masses' worth of energy as ripples in spacetime itself. By Earth, it was tiny. It changed the length of LIGO's four-kilometre arms by about one ten-thousandth of the width of a proton, which is the measurement that had to be made, and it was made with laser interferometry on mirrors suspended as pendulums in vacuum, isolated from ground motion by a factor of a hundred million.
Einstein predicted gravitational waves in 1916, a year after publishing general relativity. Then he doubted them. For part of two decades he was unsure whether they were real or an artefact of the mathematics. Indirect evidence arrived in 1974, when Hulse and Taylor found a binary pulsar whose orbit was decaying at exactly the rate that energy loss by gravitational radiation predicted, and they received the Nobel Prize for it in 1993. Direct detection took another forty years of instrument building.
What made LIGO different from every previous attempt was that it was two detectors and not one. A single detector cannot distinguish a passing gravitational wave from a truck on a nearby road, and the entire detection claim rests on the coincidence between two sites, plus the fact that the measured waveform matches the prediction for a binary black hole merger to a precision that left no room for anything else.
They checked for five months. The result was announced on February 11, 2016. Rainer Weiss, Kip Thorne and Barry Barish received the 2017 Nobel Prize in Physics. Since then LIGO, with Virgo in Italy and later KAGRA in Japan, has detected dozens of mergers, including a neutron star collision in August 2017 that was also seen by telescopes and settled several questions about where heavy elements come from.