Michelson–Morley experiment
Since waves in water need something to move in (water) and sound waves do as well (air), it was believed that light also needed something to move in. Scientists in the 18th century named this substance "aether", after the Greek god of light. They believed that aether was all around us and that it also filled the vacuum of space. Michelson and Morley created this experiment to try and prove the theory that aether existed. They did this with a device called an interferometer. This experiment is a test of Einstein's special relativity theory.