TLS signed “A. Einstein,” one page, 8.75 x 11, personal letterhead, June 27, 1931. Letter to physics student M. Frenkel in Vienna, correcting a scientific assumption made by his correspondent. In part (translated): "The assertion raised by you, that the Ritz hypothesis on the propagation of light cannot be tested in principle, is incorrect. De Sitter proved many years ago that the Doppler curve of double stars, depending on the distance, would be influenced by the dependance of the speed of light from the stars' speed of motion." In very good condition, with small areas of paper loss, and a tear to the lower right corner, all reinforced by complete silking on the reverse.
Einstein's theories created controversies for many years and in all levels of scholarship. The speed of a ball thrown at a velocity of 40 miles per hour, thrown forward from a train going 30 miles per hour, actually travels at 70 miles per hour with respect to someone standing on the station platform. Einstein's basic assumption is that the speed of light emitted from a flashlight on that train does not add on the 30 miles per hour of the train. Not all scientists agreed with him. Ritz's theory, developed by Swiss physicist Walther Ritz, insists among other things that the light does travel 30 miles faster from that train. In responding to the student's query, Einstein brings up the 'double star' experiment conducted by Willem de Sitter in 1913, which was used to support the special theory of relativity against Walther Ritz's competing 1908 emission theory.
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