TLS, signed “A. Einstein,” one page, 8 x 11, Institute for Advanced Study letterhead, January 5, 1951. Einstein writes to Murray B. Loss in State College, Pennsylvania. In full [spellings and punctuation retained]: “I am, of course, tired of explaining the same thing to hundreds of people separately. The essential point is this that the stationary clock U1 and the travelling clock U2 are not physically equivalent. For U1 has not been accelerated but U2 has. Treated according to the special theory of relativity there is no inertial system relatively to which U2 is continually [handwritten above a stricken phrase: ‘at rest’]. This may suffice to give you the possibility to analyze the case yourself.” The clocks that Einstein refers to, one stationary and one moving, are a classic illustration of the principle of time dilation, one of the consequences of his theory. In simplified summary, Einstein posited that from the point of view of a stationary observer, a moving clock would appear to run more slowly as it approached the speed of light. Accompanied by the original mailing envelope. Intersecting mailing folds and subtle toning with a few small, faint marginal stains, otherwise fine condition. Auction LOA John Reznikoff/PSA/DNA and R&R COA.
