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THE INSPIRATION FOR THE MICHELSON-MORLEY EXPERIMENT. First edition, in the very rare original printed wrappers, of Maxwell?s last paper, ?published posthumously in the Proceedings of the Royal Society, [which] concerned a means of measuring the speed of the Earth through the hypothetical aether, and was the inspiration for Michelson and Morley?s famous experiment. The null result of that experiment was to lead to the Special Theory of Relativity? (Longair, Maxwell?s Enduring Legacy, p. 71). ?Maxwell?s words had stimulated Michelson to carry out one of the great experiments of physics? (Longair, CavMag, p. 13). ?The paper was in fact by George Stokes, reporting a letter which Maxwell sent to Mr. D[avid] P[eck] Todd of the Nautical Almanac Office in Washington dated 19 March 1879. The main body of the letter concerns the use of accurate timing of the eclipses of Jupiter?s satellites as a means of measuring the speed of light plus the Earth?s motion through the ether. This required an accurate knowledge of the orbits of Jupiter?s satellites. Maxwell writes, ?I have therefore taken the liberty of writing to you, as the matter is beyond the reach of anyone who has not made a special study of the satellites.? But, more germane is the remark in an earlier paragraph. ? in the terrestrial methods of determining the velocity of light, the light comes back along the same path again, so that the velocity of the earth with respect to the ether would alter the time of the double passage by a quantity depending on the square of the ratio of the earth?s velocity to that of light, and this is quite too small to be observed? ? Albert Michelson recognised that, contrary to Maxwell?s assertion, very small path differences could be detected by optical interferometry. In his paper on the first version of the experiment of 1881 [?The Relative Motion of the Earth and the Luminiferous Ether,? American Journal of Science, 22, 120-129], Michelson states explicitly that: ?The following is intended to show that, with a wave-length of yellow light as a standard, the quantity [the path difference between the light rays] ? if it exists ? is easily measurable? (ibid., p. 12). But Michelson made an error in his calculations in 1881 and realised that an improved version of the experiment was necessary; this he carried out with Morley in 1887 (?On the Relative Motion of the Earth and the Luminiferous Ether,? American Journal of Science, 34, 333-345). The offered paper was published simultaneously in the Royal Society?s Proceedings and in the journal Nature (Vol. 21, p. 315). ?Maxwell?s (1831-79) influence in suggesting the Michelson-Morley ether-drift experiment is widely acknowledged, but the story is a curiously tangled one. It originates in the problem of the aberration of starlight. During the course of a year the apparent positions of stars, as fixed by transit measurements, vary by ?20.5 arc-seconds. This effect was discovered in 1728 by [James] Bradley (1693-1762). He attributed it to the lateral motion of the telescope traveling at velocity v with the earth about the sun. On the corpuscular theory of light the motion causes a displacement of the image, while the particles travel from the objective to the focus, through an angular range v/c just equal to the observed displacement [c is the velocity of light]. An explanation of aberration on the wave theory of light is harder to come by. If the ether were a gas like the Earth?s atmosphere (as was first supposed), it would be carried along with the telescope and one scarcely would expect any displacement. [Thomas] Young (1773-1829) in 1804 therefore proposed the ether must pass between the atoms in the telescope wall ?as freely perhaps as the wind passes through a grove of trees.? The idea had promise, but in working it out other phenomena needed to be considered, many of which further illustrate the difficulties of classical ethers ? ?In 1859 [Hippolyte] Fizeau (1819-96) proved experimentally that the velo.
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