Kennedy–Thorndike experiment

The Kennedy–Thorndike experiment, first conducted in 1932, is a modified form of the Michelson–Morley experimental procedure, testing special relativity. The modification is to make one arm of the classical Michelson–Morley (MM) apparatus shorter than the other one. While the Michelson–Morley experiment showed that the speed of light is independent of the orientation of the apparatus, the Kennedy–Thorndike experiment showed that it is also independent of the velocity of the apparatus in different inertial frames. It also served as a test to indirectly verify time dilation – while the negative result of the Michelson–Morley experiment can be explained by length contraction alone, the negative result of the Kennedy–Thorndike experiment requires time dilation in addition to length contraction to explain why no phase shifts will be detected while the earth moves around the sun. The first direct confirmation of time dilation was achieved by the Ives–Stilwell experiment. Combining the results of those three experiments, the complete Lorentz transformation can be derived.

Improved variants of the Kennedy–Thorndike experiment have been conducted using optical cavities or Lunar Laser Ranging. For a general overview of tests of Lorentz invariance, see Tests of special relativity.

The original Michelson–Morley experiment was useful for testing the Lorentz–FitzGerald contraction hypothesis only. 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. 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.

...
Wikipedia