The event was catastrophic on a cosmic scale—a merger of black holes that violently shook the surrounding fabric of space and time and sent a blast of space-time vibrations known as gravitational waves rippling across the universe at the speed of light. But it was the kind of calamity that physicists on Earth had been waiting for. On September 14, 2015, when those ripples swept across the freshly upgraded Laser Interferometer Gravitational-wave Observatory (Advanced LIGO), they showed up as spikes in the readings from its two L-shaped detectors in Louisiana and Washington State. For the first time ever, scientists had recorded a gravitational-wave signal.