Michelson and Morley's experiment was devised to observe the influence of a hypothetical "ether" (that supposedly filled all space) on the speed of light, depending on the direction of the Earth's motion. A beam of light is shot toward a beam splitter that sends the two resultant beams to different mirrors. The signals are then reflected back and recombine at a detector. In the case of a stationary interferometer, both beams arrive at the same time. For a moving interferometer, one beam travels a greater distance and the combination of the two yields the characteristic dark fringes of an interference pattern. The result of the experiment was contrary to what Michelson and Morley had expected, in that no effect due to the ether was found, at least to second order in . This result can be considered a foundation of Einstein's special theory of relativity, which proposes that the speed of light is constant in any reference frame.