At 09:50 Greenwich time on 14 September 2015, a signal swept through a detector in Livingston, Louisiana, and seven-thousandths of a second later through an identical instrument in Hanford, Washington, nearly 3,000 kilometres away. It lasted a fifth of a second. On a loudspeaker it would have sounded like a faint bird-like "chirp", a note sliding up in pitch and then abruptly stopping. It was the first time humanity had ever directly detected a gravitational wave — a ripple in the fabric of spacetime itself — and it had come from the collision of two black holes 1.3 billion light years away.
The physics behind that chirp had been sitting on paper for a century. In 1916, working from his newly completed general theory of relativity, Albert Einstein predicted that violent movements of mass should send waves rippling outward through spacetime at the speed of light, stretching and squeezing space as they passed. He also thought them so vanishingly weak that no one would ever measure them — and for decades he was nearly right. The problem was one of scale. By the time a wave from a distant catastrophe reached Earth, it would stretch a four-kilometre ruler by less than a thousandth of the width of a proton.
Listening with lasers
Detecting a change that small demanded one of the most sensitive machines ever built. The Laser Interferometer Gravitational-Wave Observatory, or LIGO, consists of two L-shaped detectors, each arm four kilometres long. A laser beam is split and sent down both arms, bounced off mirrors, and recombined. Normally the returning beams cancel out. But when a gravitational wave passes, it lengthens one arm and shortens the other by a minuscule amount, and the beams fall very slightly out of step. Two widely separated sites are used so that a real astrophysical signal — arriving at both within milliseconds — can be told apart from a passing lorry or a local tremor.
The event of 14 September, catalogued as GW150914, was almost too good. Analysis showed two black holes, roughly 29 and 36 times the mass of the Sun, spiralling together and merging into one. In the final instant they were whirling around each other hundreds of times a second before colliding. The collision converted about three solar masses of material into pure energy, radiated away as gravitational waves in a fraction of a second — briefly outshining, in this invisible form, all the stars in the observable universe combined.
Remarkably, the detection came almost as soon as the upgraded "Advanced LIGO" instruments switched on for their first observing run. So clean was the waveform that some physicists initially suspected a "blind injection" — a fake signal secretly slipped in to test the team. It was not. After months of checking, the discovery was announced to the world on 11 February 2016, one hundred years almost to the season after Einstein's prediction.
A new sense
The achievement was the culmination of decades of work and the persistence of a handful of physicists who kept the field alive when it was widely thought a fool's errand. In 2017 the Nobel Prize in Physics was awarded to three of them: Rainer Weiss, who devised the laser-interferometer design, and Kip Thorne and Barry Barish, who led the theoretical and organisational effort to make LIGO real. Thorne, a Caltech theorist, had also served as scientific consultant on the film Interstellar; Barish is widely credited with turning LIGO from a promising experiment into a functioning big-science collaboration.
What made 2015 a true turning point is that gravitational waves gave astronomers an entirely new sense. For all of history the sky had been studied with light — visible, radio, X-ray, infrared. Black-hole mergers emit almost no light at all, so they had been effectively invisible. Gravitational waves let us "hear" events that telescopes can never see. In 2017 the detectors, now joined by the European instrument Virgo, caught the collision of two neutron stars, an event also seen by conventional telescopes across the spectrum — the opening of what is now called multi-messenger astronomy.
The first chirp was a distortion smaller than an atomic nucleus, teased out of the noise by machines exquisite enough to notice a passing wave from the far side of the universe. A century after a man with a pen decided the effect was too faint ever to find, we had learned to listen to the shape of space itself.
Quiz nuggets
- The first direct detection of gravitational waves, GW150914, was made by LIGO on 14 September 2015 and announced on 11 February 2016.
- Albert Einstein predicted gravitational waves in 1916 from his general theory of relativity, but thought them too weak ever to detect.
- The signal came from two black holes, about 29 and 36 solar masses, merging 1.3 billion light years away.
- LIGO's L-shaped detectors have arms four kilometres long and can measure a change smaller than one-thousandth the width of a proton.
- The 2017 Nobel Prize in Physics went to Rainer Weiss, Barry Barish and Kip Thorne for the detection.