Autograph letter signed (‘A. Einstein‘) to [Wilhelm Wien], Prague, 17 May 1912.

In German, 4 pages, 173 x 111mm, on a bifolium. Provenance: by descent from the recipient.

‘It is a great pity that the theory of gravitation leads to so little that is observable’. Einstein apologises for forgetting to send the third page of a recent manuscript ('Supplement to my paper: "Thermodynamic proof of the law of photochemical equivalent"', Annalen der Physik 38 (1912), 881-884), of which he notes ‘I have come to the conclusion after many fruitless attempts that one will not be able to help the theory of radiation onto its feet purely by construction. That is why I attempted to come to new formulations of the question purely thermodynamically, without making use of a model’. He is sceptical about Planck’s ‘new theories’ on radiation (the ‘second quantum theory’ of 1911, which posited that absorption of radiation takes place continuously, whereas the emission process is quantized): ‘Planck’s new theories contain so many hypotheses that they seem almost worthless to me’, and he cannot understand how they can either be supported or applied. ‘The important question seems to me at the moment to be this: whether a non-monochromatically sensitive structure satisfies the hν-law for the ν of the effective radiation or for its mean proper frequency’: Einstein goes on to set out an important related question, in the scenario of a selective photokinetic effect, in which a metal is successively irradiated with two frequencies, leading to two contrasting scenarios. He also recounts conversations with Paul Ehrenfest during the latter’s recent visit to Prague: ‘Our conversation revolved mainly around fluctuations … One cannot seriously believe in the existence of countable quanta, since the interference properties of light emitted by a luminous point in different directions are not compatible with it’. The letter concludes with a reflection on his theory of gravitation (the germ of general relativity): ‘It is a great pity that the theory of gravitation leads to so little that is observable. But it must be taken seriously all the same, because the theory of relativity insistently demands such a further development, for the gravitation vector cannot be made to fit into the relativity theory with a constant c if one requires the gravitational mass of the energy’. Max Abraham has ‘converted’ to Einstein’s point of view. The Collected Papers of Albert Einstein, ed. Martin J. Klein, A. J. Kox, and Robert Schulmann, vol. 5, no. 395.

Wilhelm Wien (1864-1928) was editor of Annalen der Physik from 1907 to 1928: Einstein's paper would have been of particular interest to him, as it depended to a degree on the radiation formula which he had devised.

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