In September 2015, scientists witnessed something never seen before: two black holes colliding. Both about 30 times as big as our Sun, they had been orbiting each other for millions of years. A fraction of a second before the crash, they sent a vibration across the universe at the speed of light that was picked up by the LIGO detector. So what are these ripples in space? Amber L. Stuver explains.

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Lesson by Amber L. Stuver, animation by Eoin Duffy.