The Michelson-Morley experiment was based on a special experimental technique called interferometry. A beam of light was sent to a silvered mirror set at an angle; some of the light would travel through the mirror, and the rest would bounce off the mirror. Each partial beam of light would then bounce off other mirrors, recombine at the silvered mirror, and then return to the original location of the light source. If the partial beams of light were altered during their travel, the recombined light beam would show a measurable interference pattern.

Since the two light paths had different directions of travel, Michelson and Morley hypothesized that they would interact differently with the luminiferous ether, and thus produce an interference pattern. To their surprise, the recombined beam showed no measurable interference. This null result implied that, despite traveling in different directions for a time, the speed of both beams had remained exactly the same. If any sort of luminiferous ether existed in the universe, this result would not be possible.