Albert Einstein (1879-1955).

Autograph letter signed ('A.E.') to 'Herr Strauss' [sic: i.e. Ernst Gabor Straus], n.p., 'Sonntag', n.d. [summer 1945].

In German. One page, 280 x 216mm.

'Feeling our way': a densely scientific letter to his assistant discussing their work on gravitational fields, including an equation. Einstein opens by noting an equation which he had always assumed was true only for a specific choice of coordinates: 'It is thus clear that the special approach I have suggested leads to nothing. If this equation holds true generally, it is natural to try the choice of coordinates g = -1, as it leads to the simple equation d + 2rf = 0'. He cites a second formula which he had always felt was of little use as an introduction if one does not seek to define Λ specifically at the outset: 'But this may not be correct. I am therefore delighted that you are trying the approach of leaving Λ unconstrained'. Meanwhile, the proofs of 'our cosmological paper' ('A Generalization of the Relativistic Theory of Gravitation', Annals of Mathematics, second series, vol.46, no. 4, pp. 578-584) are ready, and Einstein has suggested they be sent to Straus, 'as any printed matter which is addressed to me hangs around in Princeton'. Returning to their scientific work, Einstein has now discovered 'a quite natural possibility of constructing equations which are covariant in relation to any complex transformations and represent a transfer of the equations Rik = 0 to complex space with a hermetic metric. But I am still far from having sussed it out enough to feel whether this possibility really needs to be taken seriously'. He looks forward to being able to resume regular meetings with Straus: 'Letter-writing is too cumbersome a thing in this situation of feeling our way'. He will be returning home (from his summer holiday in Saranac Lake, upstate New York) around 15 September.

The German-American mathematician Ernst Gabor Straus (1922-1983) was Einstein's assistant at Princeton for four years between 1944 and 1948, during which period they produced three joint papers. The scientific discussion in the present paper may relate to the early development of ideas for the second of these, 'The Influence of the Expansion of Space on the Gravitation Fields surrounding the Individual Stars' (Reviews of Modern Physics, 17 (April-July 1945), pp.120-124). Einstein never quite seems to have grasped the correct spelling of Straus's second name.

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