The Michelson-Morley Experiment

In 1881, Albert A. Michelson did an experiment to try to detect a difference in the speed of light in two different directions: parallel to, and perpendicular to, the motion of the Earth around the Sun. Much to his dismay, he found no difference. According to the ether hypothesis, light the speed of light depends on the velocity of the apparatus relative to the ether.

In 1887, he repeated the measurement with Edward Morley. The results of their measurements are shown below. As they turned their apparatus, there was no measurable difference between the speed of light in the two directions: at most 1/40 of the expected amount. Later workers improved the accuracy of the measurement by about another factor of ten.

Michelson and Morley's results from A. A. Michelson, Studies in Optics
(Univ. of Chicago Press, Chicago, 1962)

Interference "fringes" showing no change as the interferometer is rotated.
(G. Joos, Lehrbuch der Theoretischen Physik, Akademische Verlags., Leipzig, 1930)

The table shows a selection of measurements over a fifty year span. The observed fringe shift is the upper limit estimated by the experimenter. The ratio to the calculated value is the ratio of the observed shift to the value predicted by the ether hypothesis, assuming a motion of the Earth through the ether equal to the orbital speed of the Earth.

observed fringe shift ratio to calculated value
Michelson 1881 0.02 0.5
Michelson & Morley 1887 0.01 0.02
Morley & Miller 1904 0.015 0.01
Miller 1926 0.08 0.07
Kennedy 1926 0.002 0.03
Illingsworth 1927 0.0004 0.006
Joos 1930 0.002 0.003

The null results obtained by Michelson and subsequent experimenters showed that the ether hypothesis was incorrect. The speed of light did not depend on the motion of the source, as had been widely assumed. The special theory of relativity, with its counter-intuitive hypothesis that the speed of light is the same in all inertial frames, stepped in to reconcile the results of the Michelson-Morley experiment with the rest of physics.

Michelson received the Nobel Prize in 1907 for his work, the first American to receive the Prize in science. Find out more about the Michelson-Morley experiment at the American Institute of Physics website. 

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