Albert Einstein (1879-1955).

Autograph correspondence card signed ('A. Einstein') to [Roy J. Kennedy], California Institute of Technology, [postmarked 27 February 1932].

In German. 1½ pages, 87 x 135mm, on California Institute of Technology stationery (minor foxing). Envelope.

Einstein acknowledges the Kennedy-Thorndike experiment, validating special relativity's denial of absolute time. '[Richard] Tolman has just lectured in the seminar about your fine measurements with the interferometer. We all admired the ingenious methods by means of which you have observed such small shifts of the interference-fringes. It was also very commendable to search with such great care for an influence of electrical potential on the emission frequency'.

Kennedy and E.M. Thorndike described their 'Experimental Establishment of the Relativity of Time' as follows: 'The principle on which this experiment is based is the simple proposition that if a beam of homogeneous light is split […] into two beams which after traversing paths of different lengths are brought together again, then the relative phases […] will depend […] on the velocity of the apparatus unless the frequency of the light depends […] on the velocity in the way required by relativity' (Physical Review. 42 (3): 400–418). In effect, the experiment was an indirect demonstration of 'time dilation' – special relativity's remarkable proposition that an observer will measure a clock which is moving in relation to his reference frame as ticking slower than a clock that is at rest. The formal publication of the experiment was to occur only 9 months later, in November 1932.

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