TLS signed “A. Einstein,” one page, 8.5 x 11, blindstamped personal Princeton letterhead, August 6, 1947. Letter to H. B. Whetsel, in full: "Historically the development of the matter you are interested in was as follows: The negative result of the Michelson experiment caused an embarrassing situation. Fitz Gerald and H. A. Lorentz, independently from each other, have remarked that the discrepancy between experiment and theory vanishes when the hypothesis of contraction was introduced. Later on it was found that this contraction is a consequence of the special theory of relativity, so that no arbitrary hypothesis had to be made with respect to moving solid bodies." In fine condition, with some light soiling and toning along the edges. Accompanied by the original mailing envelope.
In this significant letter, Einstein refers to the 'Michelson–Morley experiment,' conducted in 1887, which was an attempt to measure the motion of the Earth relative to the 'luminiferous aether,' a permeating space then thought to carry light waves. Using a system of mirrors and a device called an interferometer, Albert Michelson and Edward Morley were able to measure the speed of light by looking for interference fringes between the light which had passed through the two perpendicular arms of their apparatus. They proposed that these would occur since the light would travel faster or slower in the 'same' direction as the Earth was moving through the ether than in the perpendicular direction. Although Michelson and Morley were expecting to measure different speeds of light in various directions, they found no such differences. This result was the first strong evidence against some aether theories, and initiated a line of research that eventually led to Einstein's development of special relativity in 1905.
George FitzGerald and Hendrik Lorentz sought to explain the null result of the Michelson-Morley experiment by suggesting that objects moving through the 'luminiferous ether' would contract in length along the direction of their motion, effectively canceling out any expected difference in the measured speed of light. Lorentz introduced a 'length contraction equation,' developing it further into Lorentz transformations, in connection with his theory of electrons.
Although this interpretation by Lorentz was incorrect, the equation was correct and formed the basis of Einstein's Theory of Relativity. In fact, the contraction of length and time in objects traveling near the speed of light completely changed the common ideas of space and time. In the present letter, Einstein summarizes the Michelson/Lorentz story and concludes that the contradiction is a consequence of the Special Theory of Relativity.
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