Editing Kennedy–Thorndike experiment (section)

{{Short description|Modified form of the Michelson–Morley experiment, testing special relativity}}
[[File:Kennedy-Thorndike experiment DE.svg|thumb|250px|Figure 1. The Kennedy&ndash;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 experiment]]al procedure, testing [[special relativity]].<ref name=KT>{{cite journal |last=Kennedy |first=R. J. |author2=Thorndike, E. M.  |year=1932 |title=Experimental Establishment of the Relativity of Time |journal=Physical Review |volume=42 |issue=3 |pages=400–418 |doi=10.1103/PhysRev.42.400  |bibcode = 1932PhRv...42..400K }}</ref>
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 (waves)|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.<ref name=rob>{{cite journal |author=Robertson, H. P.|year=1949|title=Postulate versus Observation in the Special Theory of Relativity |journal=Reviews of Modern Physics |volume=21 |issue=3 |pages=378–382 |doi=10.1103/RevModPhys.21.378 |bibcode=1949RvMP...21..378R|url=https://cds.cern.ch/record/1061896/files/RevModPhys.21.378.pdf|doi-access=free }}</ref>

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