In 1880 Albert Michelson came up with an experiment that he hoped would prove the existence of the ether. He reasoned that, though it was assumed to be very thin, the ether had to have some substance. As the Earth moved through the ether, there would be an “ether wind,” like the wind you feel when a car moves through still air. Even though there is no wind in the sense of air movement relative to the Earth, the movement of the car gives the appearance of wind. Michelson thought that something similar would occur as the Earth moves through the ether and that this wind would affect the speed of light if the light was traveling against the wind rather than at an angle to the wind.
Michelson set up mirrors at right angles to each other so as to split a single beam of light in two. One beam traveled at a right angle to the movement of the Earth, and therefore at a right angle to the ether wind. The other beam traveled into the ether wind. Both beams then reflected back and recombined into one beam. Upon recombining the two beams underwent a process called interference. In this way Michelson could measure any difference in speed of the two beams. If there was an ether wind the beam that traveled against and then parallel to the wind would take longer than the beam that traveled perpendicular to the wind. What Michelson found was absolutely no difference whatsoever.
Michelson was stunned. He thought there must be an error in his experiment so he recruited Edward Morley, a chemist. They rebuilt the apparatus and refined the equipment. After seven years of experimentation they published their findings along with their conclusion: there is no ether. That conclusion created more problems. Not only was science still in the dark about how light was propagated but the notion of absolute space was challenged. If there was no ether then there was no reference frame against which all motion could be measured. All motion was relative to the observer.