Two black holes circle each other, faster and closer, until they merge. The collision shakes space itself. A ripple crosses a billion light-years and stretches everything it passes, Earth included, by less than the width of a proton.
Load the GW150914 preset and press Play the chirp: that rising "whoop" is the sound of the first gravitational wave ever detected, as heard in LIGO's data on 14 September 2015.
On the left, two black holes orbit their shared centre of mass. Their separation is drawn to scale, and so are their event horizons. Every orbit sends out two crests of gravitational wave, which carry energy away, so the pair sinks closer and orbits faster.
On the right, a ring of free-floating particles feels the wave go by, face-on. Space stretches one way while it squeezes the other, then swaps, twice per orbit. Below, the strain h(t) is the fractional stretch a detector records: the "chirp" of rising pitch and loudness, then a fast ringdown as the merged black hole settles.
Nothing physical moves between the black holes and the ring. Space itself changes length, and rulers stretch with it, so LIGO measures the wave with light: a laser crossing each 4 km arm takes a slightly different time. For GW150914 the arms changed length by about 4 × 10⁻¹⁸ m, a few thousandths of a proton.
In a fifth of a second, three Suns' worth of mass vanished into waves. For that moment the merger outshone all the stars in the observable universe combined, yet it emitted no light at all.
Einstein predicted the waves in 1916 and doubted they could ever be measured.
Since 2015 gravitational waves have become an astronomy of their own. They pass through dust and stars untouched and come from objects that give off no light.
Laser interferometers in the USA, Italy and Japan run as one network. Through their fourth observing run, which ended in 2025, they have caught hundreds of merging black holes and neutron stars. Comparing arrival times between sites locates each source on the sky.
In the demo: the ring's horizontal and vertical diameters are the two arms of a detector.On 17 August 2017 detectors caught GW170817, two neutron stars merging. A gamma-ray burst arrived 1.7 seconds after the merger signal, and telescopes then watched a kilonova glow for weeks. Its light showed heavy elements being made, including gold and platinum.
In the demo: lighter objects chirp longer; GW170817 stayed in LIGO's band for about 100 seconds.The chirp's frequency sweep gives the chirp mass, and its loudness then gives the distance directly, with no cosmic distance ladder. GW170817 plus its host galaxy's redshift gave an independent measurement of the Hubble constant.
In the demo: peak strain is set by chirp mass and distance alone.Millisecond pulsars tick with clock-like regularity. In 2023 NANOGrav, EPTA, PPTA and others reported evidence for a background hum of waves with periods of years, probably from supermassive black hole pairs across the universe.
In the demo: the same stretch and squeeze, but on a galaxy-sized ring.ESA's Laser Interferometer Space Antenna, adopted in 2024 and planned for launch around 2035, will fly three spacecraft 2.5 million km apart. It will hear millihertz waves from merging supermassive black holes that ground detectors cannot.
In the demo: heavier pairs sing lower; millions of solar masses sing far below any sound.GW170817 showed gravity travels at the speed of light to about one part in 10¹⁵. Ringdowns check that merged black holes have exactly the "Kerr" shape relativity demands, set only by mass and spin.
In the demo: the ringdown frequency comes from the final mass and spin alone.