Berlin, 26 July 1920

On the Einstein shift: 'You will see that in the end a dazzling demonstration of the theory will emerge'.

Research by the Dutch physicist Willem Julius which argues against the red-shift of spectral lines is interesting, but demonstrates nothing: 'Until the terrestrial sources of light have been systematically researched and both spectra analysed with a spectrophotometer, it serves no purpose to pile up materials. One makes the comparison of earthly lines with those of the solar spectrum under the assumption that the lines which correspond are those which have the small possible shift in relation to each other, i.e. on the premise that there exists no gravitational effect. In the case of the solar spectrum, which is immensely rich in lines, this can lead to the identification of lines which do not in fact correspond... You will see that in the end a dazzling demonstration of the theory will emerge: I have not doubted it for a second'; the theory of Hermann Weyl does not apply in this instance, in spite of Einstein's admiration for him. Einstein goes on to discuss Weyl's theory of electricity in some detail, countering an interpretation by Besso and invoking amongst other arguments Newton's law and 'the law of gravitation in combination with the invariability of the speed of light', and illustrating the discussion with a diagram of a rotating electrical conductor. Einstein agrees with a remark of Besso's that mankind is led with remarkable docility by 'God knows what – in any case not by insightful men, free of selfish motivations': enlightened despots like Marcus Aurelius or [Woodrow] Wilson appear only exceptionally. As for himself, he is 'plagued by an exaggerated glorification' and a mass of correspondence and other unofficial duties.

Einstein had proposed three tests of his theory of general relativity in 1916: the perihelion precession of Mercury's orbit, the deflection of light by the Sun, and the gravitational redshift of light. The publication of Arthur Eddington's experimental demonstration of the second test in November 1919 had made front-page news around the world, and propelled Einstein to the degree of international celebrity which was to dog him for the rest of his life. The present letter concentrates on discussion of the third test.