In 2015 we caught a ripple in spacetime a fraction of a proton wide

Einstein predicted in 1916 that violent cosmic events should send ripples through spacetime itself, travelling outward at the speed of light. It took a century to catch one — and for decades many physicists doubted any instrument could measure a distortion as small as 10⁻¹⁹ metres, a fraction of a proton’s width.

On 14 September 2015, the twin LIGO detectors registered a signal from two black holes — about 29 and 36 times the Sun’s mass — spiralling together and merging roughly 1.3 billion years ago. The waveform rose in frequency and amplitude over a fraction of a second, the characteristic “chirp” of two bodies whirling ever faster before they collide. Crucially, both observatories saw it: the detectors at Hanford and Livingston, about 3,000 km apart, recorded the same chirp within milliseconds of each other, ruling out local noise or a fluke at a single site.

LIGO measures these ripples by bouncing lasers along two four-kilometre arms and watching for tiny changes in their length. The merger stretched and squeezed those arms by less than one ten-thousandth the diameter of a proton — a distortion smaller than an atomic nucleus.

The achievement earned Rainer Weiss, Barry Barish, and Kip Thorne the 2017 Nobel Prize in Physics. That same year, the merger of two neutron stars, GW170817, was seen in both gravitational waves and light, launching multi-messenger astronomy — and letting us, at last, hear the universe rather than only see it.