What happens when black holes collide?
Colliding black holes emit a powerful burst of gravitational waves – ripples in the geometry of space-time. Black holes are disconnected regions of the universe, defined by an invisible one-way membrane called the event horizon: you can enter, but never escape. Black holes form when massive stars die; they grow by swallowing up stars, other black holes or interstellar gas.
Like the stars from which they formed, they often live in binary pairs. Over time these binaries get closer, and their orbits strongly perturb spacetime, emitting gravitational waves like a moving finger makes waves in a pool. As these waves remove potential energy from the binary, the black holes spiral closer and orbit faster.
Eventually they touch – the event horizons merge like two liquid drops, releasing a final powerful burst of gravitational-wave energy and momentum. A new, more massive black hole remains – but not as massive as the black holes at the start, the mass difference being converted to gravitational-wave energy. Like the sound from a struck bell, the black hole remnant ‘rings’ away the distortions of its new horizon with a few more waves. The momentum carried away by the waves makes the remnant recoil – like a high-powered rifle – possibly ejecting the new black hole from the star cluster or galaxy where it formed. If not it may sit around for a time, eventually encountering a new, probably lighter black hole, with another
collision in its future.