Kennedy–Thorndike experiment

The Kennedy–Thorndike experiment, first conducted in 1932 by Roy J. Kennedy and Edward M. Thorndike, 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.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 Kennedy–Thorndike experiment, first conducted in 1932 by Roy J. Kennedy and Edward M. Thorndike, 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: Referring to Fig. 1, key optical components were mounted within vacuum chamber V on a fused quartz base of extremely low coefficient of thermal expansion. A water jacket W kept the temperature regulated to within 0.001 °C. Monochromatic green light from a mercury source Hg passed through a Nicol polarizing prism N before entering the vacuum chamber, and was split by a beam splitter B set at Brewster's angle to prevent unwanted rear surface reflections. The two beams were directed towards two mirrors M1 and M2 which were set at distances as divergent as possible given the coherence length of the 5461 Å mercury line (≈32 cm, allowing a difference in arm length ΔL ≈ 16 cm). The reflected beams recombined to form circular interference fringes which were photographed at P. A slit S allowed multiple exposures across the diameter of the rings to be recorded on a single photographic plate at different times of day. By making one arm of the experiment much shorter than the other, a change in velocity of the Earth would cause changes in the travel times of the light rays, from which a fringe shift would result unless the frequency of the light source changed 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 (corresponding to a velocity of 10±10 km/s within the margin of error), the experimenters concluded that time dilation occurs as predicted by Special relativity.