Albert Einstein (1879-1955).

Autograph letter signed ('A. Einstein') to [Wilhelm Wien], Bern, 23 'August' [actually July] 1907.

In German, four pages, 215 x 134mm, bifolium, on graph paper (split on horizontal fold on f.1). Provenance: by descent from the recipient.

'A contradiction with the principle of relativity in connection with the principle of the constancy of the velocity of light in a vacuum'. Wien has raised 'a highly interesting question', to which Einstein has immediately applied himself. He sets out his reasoning in four numbered paragraphs: first 'I defined as "group velocity" U the velocity with which a (slow) variation of amplitude is propagated, and he sets out the equation which results; he goes on to note that 'There is in my opinion a contradiction with the principle of relativity in connection with the principle of the constancy of the velocity of light in a vacuum, if for a specific metal and a specific colour U > L (the velocity of light in a vacuum)'; furthermore 'The diffusion of an electromagnetic signal with velocity greater than the speed of light is also not compatible with Maxwell's theory of electricity and light'. Einstein goes on to explore in detail the consequences of a study by Emil Wiechert which shows that 'one obtains an equivalent for the Maxwell equations if one introduces certain actions-at-a-distance, which are propagated with the velocity of light L in the vacuum and act from one electrical mass to another'. Einstein concludes 'It can be concluded in general that no process in the optics of stationary bodies that is consonant with the Maxwell-Lorentz theory can be in contradiction with the principle of relativity: because the foundation of the Maxwell-Lorentz theory is consistent with the principle of relativity. // All in all, it seems to me of the greatest interest whether the case U > L really occurs'.

Two years after his 'annus mirabilis' of 1905, Einstein was at the time of this letter in the unique position of being a theoretical physicist of international importance whilst still being a full-time employee of the Federal Patent Office in Bern. On 17 June he had made his first attempt to resolve this situation by applying for a teaching position at the University of Bern, but this was not to be approved until February 1908. This is the earliest surviving letter from Einstein to Wilhelm Wien (1864-1928), who was at this time professor of physics at the University of Würzburg: he was the author of Wien's displacement law, and of arguments on adiabatic invariance which were crucial in the development of quantum mechanics; Wien was awarded the Nobel Prize in 1911 for his work on heat radiation. The redating of the letter from August to July is by the editors of the Collected Papers. See The Collected Papers of Albert Einstein, ed. Martin J. Klein, A.J. Kox and Robert Schulmann, vol. 5 (1993), no.49.

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