Kennedy–Thorndike experiment

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The Kennedy-Thorndike experiment ('Experimental Establishment of the Relativity of Time'), first conducted in 1932, is a modified form of the Michelson-Morley experimental procedure. The modification is to make one arm of the classical Michelson-Morley (MM) apperatus very short. It served as a test for Special relativity to verify time dilation: according to special relativity, no phase shifts will be detected while the earth moves around the sun, while such would result from length contraction alone.

In their own words: ”The principle on which this experiment is based is the simple proposition that if a beam of homogeneous light is split [...] into two beams which after traversing paths of different lengths are brought together again, then the relative phases […] will depend [] on the velocity of the apparatus unless the frequency of the light depends […] on the velocity in the way required by relativity."

The original Michelson-Morley experiment was useful for testing the Lorentz-FitzGerald contraction hypothesis only, on the assumption that the dragged aether theory had been abandoned. Kennedy had already made several increasingly sophisticated versions of the MM experiment through the 1920s when he struck upon a way to test time dilation as well.

By making one arm of the experiment much shorter than the other, a change in speed of the earth would cause changes in the travel times of the light rays, from which a fringe shift would result except if the frequency of the light source would change to the same degree. In order to determine if such a fringe shift took place, the interferometer was made extremely stable and the interference patterns were photographed for later comparison. The tests were done over a period of many months.

As no significant fringe shift was found, the experimenters concluded that time dilation occurs as predicted by Special relativity:

"the conclusion to be drawn is that the frequency of a spectral line varies in the way required by relativity."