Ten years later, LIGO is a black-hole hunting machine

On September 14, 2015, a signal arrived on Earth, carrying information about a pair of remote black holes that had spiraled together and merged. The signal had traveled about 1.3 billion years to reach us at the speed of light-but it was not made of light. It was a different kind of signal: a quivering of space-time called gravitational waves first predicted by Albert Einstein 100 years prior.