Two black holes spiral faster and faster until they merge into one. This dance shakes space itself: waves ripple outward from the collision site, like ripples from a stone thrown into a pond. Sometimes the merger also produces a flash of light. If you catch both the waves and the flash, you can measure the distance to the event twice, like timing the same path by sound and by light.

Gravitational waves and light should travel at the same speed. If they don't, our understanding of gravity is incomplete. To look for such discrepancies, scientists usually take a ready-made theory and fit it to data. The new work takes a different approach: simply compare the distances, without committing to any model.

They tested the method on the GWTC-3 catalog of black hole mergers. The distances obtained matched the predictions of general relativity spacetime curvature. So far, so good: light and gravity keep pace, as expected.