(device to measure small changes in distance or light speed)
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.