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The Lorentz-Fitzgerald Contraction Hypothesis and the Combined Rod Contraction-Clock Retardation Hypothesis

In a recent paper in this journal which was part of a panel discussion of Grunbaum's philosophy of science, M. G. Evans discusses the Lorentz-Fitzgerald contraction hypothesis and related matters [2]. The purpose of this note is to clarify and correct some of the points in Evans' paper. 1. The Length Ratio of the Interferometer Arms in the Michelson-Morley and Kennedy-Thorndike Experiments. In his paper Evans stated: It has been shown that certain aspects of Grunbaum's argument are false to the facts, and that he has misconstrued what is essential to the difference between the Michelson-Morley and the Kennedy-Thorndike experiments. Actually, Griinbaum has mistaken a convenient, but greatly oversimplified, model of the Michelson-Morley experiment for that experiment as actually carried out. Hence, he has centered attention on a feature which in fact never existed, and is in theory strictly irrelevant, namely, the supposed equality in the length of the Michelson-Morley interferometer arms ([2], p. 361). Evans is correct in stating that the approximate equality of the interferometer arms in the Michelson-Morley experiment is not needed in theory because the effect which was looked for depends on the sum of the interferometer arm lengths. Evans is incorrect, however, in saying that the (approximate) interferometer arm equality was not present in the actual experiments,' and that the (approximate) arm equality in the Michelson-Morley experiment, and the arm inequality in the KennedyThorndike experiment, was not one of the important differences between these two experiments. In the actual Michelson-Morley experiments the interferometer arms were (very closely) equal, and in the Kennedy-Thorndike experiment the interferometer arms were made as unequal as possible. This lack of equality of interferometer arms in the Kennedy-Thorndike experiment is required because here the effect sought depends on the difference of the arm lengths. Thus, the (approximate) equality of the interferometer arm lengths in the Michelson-Morley experiment (as