For all of history we studied the sky with light. In 2015, we heard it for the first time.

A bell made of spacetime

In 1916 Einstein spotted a loose thread in his own theory. If gravity is just the shape of spacetime, then violently moving masses should send ripples through that shape — waves of stretching and squeezing racing outward at the speed of light.

The catch: gravity is staggeringly weak, and the ripples are minuscule. It takes something monstrous to make a detectable one. The classic source is two black holes in their final dance, spiralling closer and faster until they merge in a fraction of a second. In that instant the system can radiate more power as gravitational waves than every star in the visible universe emits as light — all at once. And almost none of it is light. It's pure spacetime, ringing like a struck bell.

A wobble smaller than a proton

By the time a wave from a billion light-years away washes over Earth, that violence has faded to almost nothing. As it passes it stretches space one way and squeezes the perpendicular way, then reverses — but the effect is a fractional change of roughly one part in 10²¹.

To catch it, LIGO built two enormous L-shaped detectors — Michelson interferometers grown to monstrous scale — with arms four kilometres long, and fires a laser down each arm:

• The beams bounce off hanging mirrors and recombine.
• A passing wave lengthens one arm and shortens the other by a hair.
• That mismatch shows up as a shift in the recombined light.

How big is the hair? Across four kilometres, the arms change length by far less than the width of a single proton — thousands of times less. It is among the most delicate measurements humans have ever made, which means hushing the whole machine against passing trucks, distant ocean waves, and the faint patter of its own atoms.

The first sound from the dark

On 14 September 2015, both detectors — one in Washington, one in Louisiana — twitched the same way, seven thousandths of a second apart. Two black holes, each around thirty times the Sun's mass, had merged 1.3 billion years ago. About three Suns' worth of mass had been converted straight into the energy of the wave.

Played as audio, the signal is a brief rising swoop — a "chirp" — as the orbit tightened and sped up, then cut off at the merger. Two black holes give off no light; before LIGO, an event this colossal would have swept over Earth completely unnoticed.

Two years on, in 2017, the detectors caught a gentler, longer chirp — two neutron stars spiralling together — and this time telescopes swung round in time to catch the afterglow: the first cosmic collision ever heard and seen at once.

We didn't only confirm a century-old prediction. We grew a new sense — a way to listen for collisions that send out no light at all.