Einstein predicted gravitational waves in 1916. The math was clean: mass distorts spacetime, and accelerating mass should produce ripples in that distortion that propagate outward at the speed of light. The problem was that the predicted waves were absurdly weak. Even a catastrophic event — say, two black holes colliding — would produce ripples that, by the time they reached Earth, would stretch space by less than a thousandth the diameter of a proton.

For 99 years, no one detected one.

LIGO — the Laser Interferometer Gravitational-Wave Observatory — was built to do the impossible. It consists of two L-shaped detectors, one in Louisiana and one in Washington State. Each arm is four kilometers long. Lasers run down the arms, bounce off mirrors at the end, and recombine. If a gravitational wave passes through, it stretches one arm slightly while compressing the other. The recombined laser shifts by a fraction of a wavelength. Computers see the shift.

On September 14, 2015, both detectors registered a chirp. A specific signature. The signal lasted 0.2 seconds. Scientists analyzed it for months. The conclusion: 1.3 billion years ago, two black holes — 36 solar masses and 29 solar masses — had spiraled into each other and merged. In the final moments before the collision, they radiated more energy than every star in the observable universe combined. Almost all of it, gravitational waves.

That energy reached Earth as a tiny tremor in spacetime, smaller than the width of an atomic nucleus.

Three physicists shared the 2017 Nobel Prize. LIGO has since detected over 90 mergers. We hear black holes with lasers now.