Philosopher Harvey R Brown wrote in 2001 about how FitzGerald and Lorentz brilliantly and correctly analyzed the Michelson-Morley experiment in The origins of length contraction: I. The FitzGerald-Lorentz deformation hypothesis:
In a letter to Einstein written in 1915 and unearthed many years later by A.J. Kox 64, Lorentz admitted that he had arrived at the deformation hypothesis shortly before he developed the plausibility argument based on molecular forces. But he expressed regret that he had not emphasised the dynamical argument more from the beginning: had he done so “the hypothesis would have made less an impression of having been devised ad hoc.”The most startling part of relativity is that motion affects space and time. Brown traces the origin of this concept to 1889, long before Einstein. As Brown shows, both FitzGerald and Lorentz arrived at the contraction hypothesis as a logical consequence of the MM experiment showing that the speed of light is the same in all frames, together with other experiments rejecting the aether drift theory.
Brown notes that the MM experiment only showed that the length is contracted relative to the width. FitzGerald and Lorentz were not sure whether the length is contracted, or the width is expanded, or some combination. Lorentz eventually concluded that the deformation was purely a length contraction, and published that in 1904. FitzGerald died in 1901, without knowing that his 1889 letter was published in AAAS Science, or that it contained what would later be considered one of the greatest insights in the history of science.
Einstein wrote his first relativity paper in 1905. He relied on Lorentz's analysis of the MM experiment, but did not mention the experiment and later denied that he even knew about it. His main argument was that the Lorentz transformations can be deduced from the speed of light being the same in all frames, without mentioning experiments. Anti-positivist philosophers have praised Einstein largely for ignoring the experiments. He did not mention the dynamically argument because he could not get it to work, and he explained many years later.
Thus FitzGerald and Lorentz discovered (independently) the length contraction using the speed-of-light argument, and then tried to support it with a dynamical argument. 13+ years later, Einstein published a paper presenting the length contraction using the speed-of-light argument, and did not mention the dynamical argument.
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