In the 19th century, scientists knew that light was some kind of wave. And every kind of wave science had yet described needed a medium to travel through—the way that sound waves travel through matter. But scientists had never observed light's medium. As they searched for evidence of it, scientists decided to call light's theoretical medium the luminiferous ether.

Albert Michelson Edward Morley

In 1887, Albert A. Michelson and Edward W. Morley conducted an experiment to measure the speed at which the Earth moves through the ether. Their experiment was so refined that it should have finally detected the luminiferous ether. But it didn't. Repeated trials verified the surprising result that the ether did not exist.