More than four billion years ago a pair of black holes engaged in a dramatic dance. With gas and dust whirling around them, the voids spiraled closer and closer together, orbiting perilously near to a supermassive black hole. Then the two smaller black holes merged in a collision so powerful that it shook the fabric of space itself. As the resulting gravitational waves rippled toward Earth, the merged black hole recoiled like a rocket, heating the surrounding gas and generating a blinding flare of light that lasted for weeks.