Ahrenshoop, 29 July 1918

Dealing with the 'blemish' of the cosmological constant (λ), and also mentioning his early publications, his relations with Mileva and Elsa, and his work on the quanta of radiation.

'In a paper on the energy principle in the gen[eral] theory of rel[ativity] [i.e. Einstein's own 'The law of energy conservation in the general theory of relativity', 1918] it has been shown that the total energy of a system is completely independent of the choice of coordinates (an integral invariant, which corresponds with no differential invariant). The energy of the whole universe, assuming that the latter is closed, proves itself in a uniform distribution of matter to be solely determined by this alone: gravitational field energy and energy contribution of the λ-term cancel each other out. Further it is not uninteresting that one can easily formulate the theory so that λ appears not as a universal constant in the customary sense but as an integration constant or alternatively as a Lagrangian factor'. Einstein provides the corresponding formulae and goes on 'I think I should publish this at some point, for it cleanses the theory of a blemish. What if other universal constants might not also lose their painful character in the same way?'.

This characteristic outburst of enthusiasm for a new idea comes as the postscript to a letter in which a discussion of personal matters is sandwiched between references to science: Einstein is touched that Besso and two friends have been discussing some of his early papers on Brownian motion: 'It is really touching of you that you have devoted so much time and care to my old publications. But still I have to say that a considerable amount has been superseded so that it's not worth the trouble. Particularly the opalescence paper, which is burdened with the superfluous Fourier expansion'. Einstein mentions that his mother is moving to live with his uncle in Zurich, though he will keep her separate from his estranged wife and children; the latter are writing 'nice letters' to him. Besso has been giving Einstein advice about remarrying, which he is going to ignore, even if he is concerned that Elsa will be impoverished by the effects of the war: 'Besides, she is very good, so that I am really happy with her. I would also have remained faithful to Mileva if it had been bearable with her ... If she's not my wife, I can tolerate her quite well'. Einstein has been pondering for 'countless hours on the quantum question, naturally without really getting anywhere. But I no longer doubt the reality of quanta in radiation, even if I still stand quite alone in this conviction. Thus it will remain as long as no mathematical theory succeeds'.