On his renewed acceptance of the necessity of the cosmological constant (λ), the potential reversibility of the elementary processes of the universe, and challenging a theory of Hermann Weyl.
Although Einstein had hoped to have dealt with the 'blemish' of the cosmological constant in his previous letter (19 July 1918), he now once again accepts its importance: 'What I wrote to you about the λ is worthless. The grounds remain the following: // Either the universe has a centre point, is overall of insignificant density, empty at infinity, whither all thermal energy dissipates eventually as radiation. // Or: All points are on average equivalent, the average density is overall the same. Then one needs a hypothetical constant λ, which indicates at what average density of matter this can be in equilibrium. // One absolutely feels that the second possibility is the more satisfactory, especially as it implies a finite magnitude for the universe. Since the universe only exists in one example, it is essentially all the same whether one gives to a constant the form of a constant springing from a natural law or of an "integration constant"'. Einstein writes also of the potential reversibility of the elementary processes of the universe: 'A priori, one can certainly expect irreversible elementary laws. But detailed observations up to now do not argue for it (especially the quantum laws), even less the fact of a thermal equilibrium. I believe, in accordance with everything I know, in a reversibility of elementary events. All temporal bias seems to be based on "order". You will counter with radioactivity. But I am persuaded that the inverse process is impossible only in practical terms'.
The first half of this richly scientific letter discusses the offer of a post at the University and Polytechnic of Zurich, which Einstein feels he cannot accept in view of all he has been offered in Berlin (he remarks 'How happy I would have been 18 years ago with a paltry assistantship...'); he has proposed instead to come to Zurich twice a year to give lectures, which he thinks his health is now sufficiently recovered to withstand. He also discusses in detail some propositions of Hermann Weyl on relativity, speaking highly of Weyl himself ('not only an outstanding but also personally a very delightful fellow'), and expressing optimism that he will see the error of his ways: 'He will soon come back out of the relativistic dead end. His theoretical endeavour does not agree with the fact that two originally congruent rigid bodies also remain congruent, independently of what destinies they pursue. ... If the relative size of rigid bodies is independent of their past history, then there is a measurable distance between two (neighbouring) points in the universe...'. As regards the question of energy, Besso's letter has implied that 'you too are of the opinion that one can do without an energy tensor for gravitation. But then the energy law loses any value...'.
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