The Michelson-Morley Experiment Albert Michelson and Edward Morley tried to measure the speed of light in different directions using light interferometers. An interferometer is an apparatus that splits light at right angles, using a half mirror, and then measures the interference of the beam of light with itself. Using this apparatus they thought they could measure the difference in the speed of light in each direction and thus measure the relative motion of the Earth and the aether.  An interferometer works by splitting a beam of light into two perpendicular beams, and then combining them back together. By looking at the interference pattern of the combined beam you can calculate the different speed of each half beam. With this, they could, theoretically, measure the difference in speed of each path and thus measure the relative velocity of the Earth and the aether. This idea can be seen in the following animation: When the aether is stationary relative to the experiment there is no interference and the two waves (blue and green) combine back into one (pink). When the aether is moving relative to the experiment the two waves are out of phase and create an interference pattern (the green reaches the detector before the blue). A sketch of the Michelson-Morley interferometer. The light bounces off multiple mirrors making it more sensitive. From their 1887 paper: On the Relative Motion of the Earth and the Luminiferous Ether. In order to make the apparatus more precise and sensitive they used multiple mirrors to make the light path longer. They also took different measurements when the Earth was at different places in its orbit. They published their findings in the 1887 paper named On the Relative Motion of the Earth and the Luminiferous Ether in the American Journal of Science. In this paper they reported an insignificant result, and bounded the relative speed of the Earth and the aether to be less than 1/6 of Earth's orbital speed. This result puzzled many physicists, as a relative speed of less than the Earth's orbital speed, at any time during its orbit, meant that the aether moves with the Earth. This result was unexplainable. This experiment, and others like it ,put aether theory into a problematic place. Though some physicists, most notably Hendrik Lorentz, tried to explain this result, it is clear that the lack of empirical data signaled the beginning of the end of the luminiferous aether.