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Einstein on relativity and the speed of light

EINSTEIN, ALBERT. Typed letter signed to Sam Moskowitz

Princeton: Institute for Advanced Study, April 4, 1951

One page. One word (“local”) underlined by Einstein. Original folds, light wear, very short tear at right margin. Original envelope.

Einstein on the speed of light in the special and general theory of relativity.

In this fascinating letter Einstein writes that “According to the special theory of relativity the velocity of light relatively to an inertial system is something clearly defined and this speed of light is a limit for the speed of any object.” In the general theory, he explains, the picture is more complex: “there is still the well-defined velocity of light with respect to a local coordinate system,” but the theory does not allow for any “privileged” coordinate systems “so that it makes no sense in general to speak about the speed of light with respect to the coordinate system.”

Einstein goes on to consider “special cases where natural choice of the coordinate system exists” which would allow for discussion of the velocity of light, which would however no longer be a constant “but something which depends from the coordinates … from time.”

The recipient of this letter from Einstein was science fiction writer and historian of science fiction Sam Moskowitz. Moskowitz wrote to Einstein to clarify “a vital point in the writing of science fiction.” He observed that “one of the biggest obstacles science fiction writers have had to overcome in carrying their characters to the far galaxies is the general acceptance of the idea that a material object cannot exceed the speed of light.” Noting that in a recent paper English astronomer Fred Hoyle had said that the general theory of relativity allows such a possibility, Moskowitz asked Einstein for clarification.

Einstein’s letter is accompanied by a carbon copy of Moskowitz’s letter to Einstein, 28 March 1951, and a 3pp letter by English astronomer Fred Hoyle to Moskowitz explaining Einstein’s letter.