Michelson-Morley Experiment

During the 1800s scientists had become convinced that light was composed of waves, as opposed to a theory that light was made up of particles proposed more than a century earlier by Isaac Newton. They based their belief on experiments that demonstrated phenomena such as interference—the change in intensity caused by mixing two or more beams of light; and diffraction—the fact that beams of light do not always travel in straight lines.

But if light was a wave, what medium did it travel through? Earthquakes produce seismic waves that are transmitted by Earth's crust and a clanging bell makes sound waves carried by air. Scientists were certain that light had to be transmitted by something, so it was hypothesized that there existed a luminiferous ether. The term luminiferous means light-bearing, but the word ether was not so specific. No substance could be associated with the ether, especially in space where sunlight and starlight travel in what otherwise appears to be a vacuum. The ether was predicted, but had not been observed.

Scientists thought that the ether should be everywhere and that it must be stationary, at rest with respect to absolute space which, following Newton, was believed to exist independently of the objects in it. It was thought that by measuring the motion of Earth relative to the ether it would be possible to observe the latter.