During Einstein's day, there were many who believed in the aether theory, the theory that Earth must be travelling through some medium in order for light to travel from the Sun to the Earth. At the time, scientists believed that waves had to have some medium to travel through. The medium that Earth was travelling through was called the luminiferous aether or aether wind. In came Albert Michelson and Edward Morley. They wanted to prove that the aether existed and set up an experiment to do just that.

In this experiment, Michelson and Morley set out to measure the movement of the Earth through the aether. They created what was later called an interferometer where light traveled from the source to a semi-transparent mirror. Some of the light reflected and some transmitted through the mirror. They hypothesized that the beam that traveled parallel to the aether would take longer than the beam that traveled perpendicularly to the aether. Michelson thought that the beam that was received by the detector would have shifted toward another color, but they did not see these results. Repeated trials with more and more accuracy had the same results—there was no evidence for a liminiferous aether.