Alessandra Buonanno

Two dead stars circle in a final dance. Their approach creates ripples in space — gravitational waves. But there may be a third, invisible partner in this dance. Its gravity accelerates the pair, and the signal changes pitch slightly, like the siren of a passing car.

Drop two stones of different shapes into a pond — the ripples will tell you which one was smooth and which was jagged. Gravitational waves work the same way: when two ultra-dense objects merge, the trembling of spacetime carries details about their inner workings.