The Michelson–Morley experiment was performed over the spring and summer of 1887 by Albert A.
 Michelson and Edward W. Morley at what is now Case Western Reserve University in Cleveland, Ohio, and published in November of the same year. 
It compared the speed of light in perpendicular directions, in an attempt to detect the relative motion of matter through the stationary luminiferous aether ("aether wind").
 The result was negative, in that the expected difference between the speed of light in the direction of movement through the presumed aether, and the speed at right angles, was found not to exist; this result is generally considered to be the first strong evidence against the then-prevalent aether theory, and initiated a line of research that eventually led to special relativity, which rules out a stationary aether.
 The experiment has been referred to as "the moving-off point for the theoretical aspects of the Second Scientific Revolution".

Michelson–Morley type experiments have been repeated many times with steadily increasing sensitivity.
 These include experiments from 1902 to 1905, and a series of experiments in the 1920s. More recent optical resonator experiments confirmed the absence of any aether wind at the 10−17 level.
Together with the Ives–Stilwell and Kennedy–Thorndike experiments, Michelson–Morley type experiments form one of the fundamental tests of special relativity theory.