The most famous experiment designed to detect small changes in the speed of light was performed in 1887 by A. A. Michelson and E. W. Morley. It should be noted from the outset that the result of the experiment was negative, thus contradicting the then popular belief that Earth moved through an ether which served as the propagating medium for light. Prior to the Michelson-Morley experiment it was widely believed that light propagated through an hypothetical ether much like water waves propagate through water. The experiment was designed to determine the velocity of Earth with respect to this hypothetical ether. To test this hypothesis, Michelson devised the famed Michelson interferometer.

Based on the wave nature of light, the Michelson interferometer was designed with two mirrors, a light source, and a detector, all placed at right angles to each other with a third half-reflecting mirror positioned at an...