The Michelson-Morley Experiment

The Michelson-Morley experiment was an attempt to detect the theorized "ether." This ether was proposed because there did not seem to be any medium for the waves of light to propagate through-light behaved as though it was composed of waves, but waves of what? Thus, scientists postulated an ether which would carry light waves, just as air carries sound waves.

If there was an ether, then the speed of light should be dependent on the ether. If the speed of light were constant with respect to the ether, then the measured speed of light in perpendicular directions should be different due to the movement of the earth through the ether. In 1887, A.A. Michelson and E.W. Morley refined an experiment that Michelson had performed a few years earlier. They split a beam of light in two, sending the two halves in different directions. Each ray of light was reflected back and the two beams combined again. By observing the interference patterns, they were able to determine the difference in the speed of light in the two perpendicular directions. It was zero. This negative result served to discredit the idea of ether and eventually led to Einstein's 1905 special relativity, stating that the speed of light is a universal constant.

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