Two black holes collided and stretched Earth by a fraction of a proton.
On 14 September 2015 a tremor in spacetime, sent out by two black holes merging over a billion light-years away, passed through Earth. Two detectors in the US caught it, a change in length of about a thousandth of a proton's width. Einstein had predicted such waves a century earlier but doubted anyone would ever detect them.
Einstein's general theory of relativity describes gravity as the warping of spacetime by mass. In 1916 he predicted that accelerating masses should send ripples through that fabric at the speed of light, but he thought them far too faint ever to measure. Indirect evidence came in 1974, when Russell Hulse and Joseph Taylor found a pair of neutron stars spiralling together at exactly the rate expected if they were losing energy to gravitational waves, work that earned them the 1993 Nobel Prize.
Catching a wave directly required extraordinary precision. LIGO operates two observatories, in Hanford, Washington, and Livingston, Louisiana, about 3,000 kilometres apart. Each splits a laser beam down two arms about four kilometres long and recombines it; a passing wave stretches one arm and squeezes the other, shifting how the beams interfere. Two sites are needed to rule out local vibrations.
The signal, named GW150914, arrived while the detectors were still in an engineering phase, days before formal observations began, and at first the team had to check it was not a test injection. It hit Livingston seven milliseconds before Hanford. It matched the predicted pattern of two black holes of about 36 and 29 solar masses spiralling in and merging into one of about 62. The missing three solar masses were converted into gravitational waves, and in its final moments the merger briefly radiated more power than all the stars in the observable universe combined. Converted to sound, the signal rises in a fraction of a second like the chirp of a bird.
The discovery, announced on 11 February 2016, was the first direct detection of gravitational waves and the first observation of two black holes merging. Rainer Weiss, Barry Barish and Kip Thorne shared the 2017 Nobel Prize in Physics. Many more mergers have been detected since, opening a new way to observe the universe.
Source: Wikipedia — First observation of gravitational waves · Text summarised from Wikipedia (CC BY-SA 4.0)
