Now, what happens when two black holes enter each other’s sphere of influence and begin to circle each other? General relativity predicts that this interaction generates gravitational waves, oscillating disturbances of spacetime that propagate across the universe at the speed of light. This is just the Einstein equation at work: Two black holes orbiting each other form a periodically changing configuration of masses to which, the Einstein equation says, spacetime responds with its own periodic disturbances. These ripples are gravitational waves.
