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.
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.
In their own words:[1] 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.
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.
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.
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.
But in the Kennedy–Thorndike experiment, the lengths LL and LT are different from the outset, so it is also capable of measuring the dependence of the speed of light on the velocity of the apparatus.
[2] According to the previous formula, the travel length difference ΔLA−ΔLB and consequently the expected fringe shift ΔN are given by (λ being the wavelength): Neglecting magnitudes higher than second order in v/c: For constant ΔN, i.e. for the fringe shift to be independent of velocity or orientation of the apparatus, it is necessary that the frequency and thus the wavelength λ be modified by the Lorentz factor.
But this is not strictly correct, since length contraction and time dilation having their exact relativistic values are sufficient but not necessary for the explanation of both experiments.
The bounds on velocity dependence according to the Robertson-Mansouri-Sexl test theory (RMS), which indicates the relation between time dilation and length contraction, have been significantly improved.
On the right, the 532 nm absorbance line of a low pressure iodine reference is used as a time standard to stabilize the (doubled) frequency of a second Nd:YAG laser.
