Gravitational waves are ripples in spacetime produced by accelerating massive objects. When black holes merge, they release a burst of gravitational radiation that stretches and squeezes space by an almost unimaginably small amount.
Laser interferometers detect these changes by measuring shifts in the relative length of long perpendicular arms. The observed waveform encodes the masses and spins of the merging objects.
As detectors improve, scientists expect to observe more events and potentially new kinds of sources, such as neutron star–black hole mergers or a background “hum” from the early universe.