The Michelson interferometer

A Michelson interferometer takes a single beam of light and splits it with a half-silvered mirror into two perpendicular paths, sends each half to a separate mirror some distance away, and recombines the two returning beams back into one. The recombined light interferes, and whether that interference is constructive or destructive at any point depends entirely on the difference in the total distance each half travelled — the path-length difference between the two arms.

That dependence is what makes the instrument extraordinarily sensitive. Because visible light has a wavelength of only a few hundred nanometres, shifting one mirror by even a small fraction of that distance measurably shifts the interference pattern — moving bright and dark fringes across a detector in a way that can be counted and converted directly into a length change far too small to measure by any mechanical ruler. This sensitivity is why interferometers of this general design remain the basis of some of the most precise length and distance measurements made today, up to and including the kilometre-scale detectors that first observed gravitational waves by registering length changes thousands of times smaller than a proton.

The instrument's most famous historical use was Michelson and Morley's 1887 experiment, which compared the travel time of light sent along two perpendicular arms as Earth moved through the presumed "luminiferous ether." If light's speed depended on Earth's motion through that medium, rotating the apparatus should have shifted the interference pattern measurably. It did not — a null result, but a highly informative one rather than a failure: the absence of any detectable ether wind was a direct experimental clue that light propagates at the same speed regardless of an observer's motion, one of the key pieces of evidence that fed directly into Einstein's postulates of special relativity less than two decades later.