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Part of the Einstein exhibition.
Most physicists of the time believed that light traveled through what they called the "luminiferous ether." In 1887, two American scientists, Albert Michelson and Edward Morley, built a device known as an interferometer, which they hoped would enable them to prove the existence of the ether.
Michelson and Morley ran their interferometer experiment numerous times but never saw any evidence of the ether. Other scientists, sure that the ether theory was correct, continued searching for it. It wasn't until 1905, when Einstein published his Special Theory of Relativity, that the physics community began to accept that the ether does not exist.
The Michelson-Morley interferometer worked by splitting a single beam of light in two. The two beams bounce off mirrors and arrive at a detector.
If the ether existed, it would remain still while the Earth moved through it. The ether would then change the speed of light depending on whether the light was moving in the direction of Earth's motion or at a right angle to that motion.
Michelson and Morley expected to find that two light beams arrived at the detector at different times. Instead they found that no matter which direction light traveled, it always moved at the same speed—indicating that the ether does not exist.