Michelson interferometer

A Michelson interferometer is a device that combines light that has traveled in two (often perpendicular) directions aiming to detect small speed or distance differences by noting the recombined light's interference. Albert Michelson developed this type of device to measure differences in the speed of light according to the direction of its travel and famously found there were none, a step toward the development of relativity theory. Much larger Michelson interferometers are used to detect gravitational waves, by detecting otherwise-unexplained variations in the distance between two objects.