The discovery of gravitational waves was the crowning glory of Einstein's relativity. They might now have provided the first hint of something to scupper it. A LONG time ago, in a galaxy far away, two black holes collided. We know this because more than a billion years later, on the morning of 14 September 2015, we felt it: in the world's most exquisitely sensitive measuring device, laser beams shifted ever so slightly as ripples in space-time washed over Earth. This first detection of gravitational waves was the culmination of an epic scientific quest, and a stunning endorsement of general relativity, Einstein's landmark theory of gravity. Since then, our detectors have seen them five more times. But this is just the start – and although everything we have learned from the first waves is consistent with Einstein's masterpiece, the coming deluge of sightings could tear it apart. Gravitational waves carry us into uncharted waters, where the fabric of the universe is so