When black holes collide, the mass of entire suns vanishes in a split second. The merger releases more energy than every star in the observable universe combined, yet emits no light.
The process begins long before the actual impact. Black holes that find themselves trapped in each other’s gravitational pull enter a phase called the inspiral. As they orbit, their immense mass churns the fabric of spacetime, creating ripples known as gravitational waves. These waves carry energy away from the binary system. As the system loses energy, the black holes are drawn closer together, orbiting faster and faster. By the time they are within a few hundred miles of each other, they are moving at a significant fraction of the speed of light.
When the two event horizons finally touch, they fuse to create a single, larger black hole. This new object is not immediately stable. For a brief moment, it is deformed and asymmetrical. To settle into a stable shape—a perfect sphere, or an oblate spheroid if it is spinning rapidly—the newly formed black hole must shed its imperfections. It undergoes a "ringdown" phase, vibrating like a struck bell and radiating the last of its distortions away as a final burst of gravitational waves.
The resulting single black hole is surprisingly lighter than its two parent objects. The missing mass is converted entirely into the energy of those spacetime ripples, following Einstein's . When the LIGO observatory detected the first black hole merger in 2015, the measurements revealed that two black holes weighing 36 and 29 solar masses combined to form a single 62-solar-mass black hole. In that fraction of a second, three solar masses were converted directly into pure wave energy.
Sometimes, the waves are not emitted equally in all directions. If the original black holes were different sizes or spinning at odd angles, the resulting gravitational waves can shoot out asymmetrically. This imparts a massive recoil velocity—a "kick"—to the final black hole. In extreme cases, this kick can reach thousands of miles per second, enough to completely eject the newly formed black hole from its host galaxy to wander intergalactic space alone.