Talk:Fringe shift
Add topicFalw (talk) 02:40, 2 April 2025 (UTC)
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Wrong explanation of the Michelson–Morley experiment
[edit]In the MMX it was not expected that the arm length would be altered by the apparent aether wind; rather, that this wind – i.e. a wave medium moving relative to the observer – would change the observed speed of light, causing the phase shift, thus the fringe shift.
The original author’s contention that the arm length might be modified by motion through the luminiferous aether is a (common) misconception, either motivated by the length contraction postulated in Lorentz ether theory (posited in order to explain the null result of the MMX) or unrelated general-relativistic gravitational wave theory (GW observatories like LIGO also use a Michelson interferometer), both of which were posited later. I have corrected this (historian’s fallacy) and clarified the explanation.
Not sure the claim concerning speed versus lengths are correct
[edit]The author somehow states that the main purpose of the Michelson–Morley experiment is to measure changes in lengths covwered, and not variation of the speed of light with direction.
This is at best questionable. After all, Michelson and Morley in their paper set themselves the goal of measuring the speed of the ether with respect to Earth. That follows clearly from the second and third paragraphs of the paper's introduction[1]. It appears yet more clearly in the conclusion. There it is stated that "the relative velocity of the earth and the ether is probably less than one sixth the earth's orbital velocity, and certainly less than one fourth." and at the very end summarizing: "It appears from all that precedes reasonably certain that if there be any relative motion between the earth and the luminiferous aether, it must be small; quite small enough entirely to refute Fresnel's explanation of aberration."
These statements clearly give a central place to the velocity of the ether, and as a consequence to the speed of light varying as a consequence of angle.
Further, H.A. Lorentz in [2] discusses the earlier version of the Michelson experiment (the 1881 experiment in Potsdam) and states there (I translate from the French): "Mr. Michelson observes that, if the ether be not dragged by Earth, the time required for light to go from one point A to a point B, both linked to Earth, and to then return from B to A, must depend on the angle which the line AB makes with the direction of movement of our globe."
A statement uncannily similar to what the article appears explicitly to deny.