It’s hard to think of gravity as something that travels. Instead, we think of it as a force that pulls objects together, and that’s true enough. But it moves through space as waves, just as light does. And a recent discovery has allowed scientists to make the most accurate measurement of how fast those waves are moving.

Back in August, scientists detected gravitational waves produced by the violent merger of two neutron stars — the ultra-dense cores of exploded stars. As the stars got closer and closer, they whipped up the space around them, producing ripples in space and time — gravitational waves. Scientists converted the waves to sound.

And less than two seconds after the waves reached Earth, telescopes in space detected gamma rays from the merger. The fact that the two types of waves arrived at Earth at almost the same instant — from an event that happened 130 million light-years away — tells us that they traveled at the same speed: the speed of light. In fact, the difference in their speed is no more than one part in a million billion.

Albert Einstein’s theory of gravity had predicted that gravity should move at the speed of light. It’s been impossible to measure that speed, though, because it’s been impossible to capture gravitational waves. But over the last couple of years, detectors in the United States and Europe finally detected them — a discovery that won last year’s Nobel Prize in Physics.

Script by Damond Benningfield