Editing Kennedy–Thorndike experiment (section)

==The experiment==
The original Michelson–Morley experiment was useful for testing the [[length contraction|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:<ref name=KT />

{{quote|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.}}

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&nbsp;°C. Monochromatic green light from a mercury source ''Hg'' passed through a [[Nicol prism|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 ''M<sub>1</sub>'' and ''M<sub>2</sub>'' which were set at distances as divergent as possible given the [[coherence length]] of the 5461&nbsp;Å mercury line (≈32&nbsp;cm, allowing a difference in arm length Δ''L''&nbsp;≈&nbsp;16&nbsp;cm). The reflected beams recombined to form circular [[interference fringe]]s 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&nbsp;km/s within the margin of error), the experimenters concluded that time dilation occurs as predicted by Special relativity.