EINSTEIN, Albert (1879-1955). Autograph letter signed ('Albert') to Michele Besso, [Berlin], 31 October 1916.
In German, 5 pages, on a bifolium, 222 x 142mm, and a single leaf of graph paper, 176 x 125mm (short tear to upper margin of singleton, approx. 14mm). Provenance: by descent from Michele Besso.
'The objective significance of space and time': a long letter of dense scientific content during the first flush of acceptance of the general theory of relativity.
Einstein has just spent some 'wonderful days' ('wunderschöne Tage') in Holland, where 'general relativity is already flourishing ... In England too the theory has put down roots. I spent unforgettable hours with Ehrenfest and especially with Lorentz'. As a side-note, Einstein is pleased to hear news of the improved health of his wife (Mileva, from whom he had been separated since 1914, and with whom Besso was in close contact), and promises that she will not be bothered by him, and that he has given up on the idea of a divorce.
With some relief, he then moves on to scientific matters ('Nun zum Wissenschaftlichen!'): 'The objective significance of space and time lies in the first instance in the fact that the four-dimensional continuum is hyperbolic; so that "spatial" and "temporal" linear elements start out from each point ...': he sets out the equations which demonstrate that 'the "spatial" or "temporal" character is real. But it is not the case that one coordinate is "naturally" temporal, the others spatial'. Einstein goes on to refute propositions made by Walter Dällenbach and Marcel Grossman: 'As for Grossman, he is mistaken. The case of ordinary relativity is the case of disappearing curvature...'. He provides a definition of tensors and an explanation of their operation in his calculations, before explaining a case in which 'in the framework of special relativity, there is no difference between covariant and contravariant'. Moving on to a further remark of Besso's, Einstein observes: 'Your observation about the equivalence between scales or clocks which are physically different (and have undergone different previous states) is quite correct. But this supposition also figures tacitly in the Galileo-Newtonian theory ... It would be permissible, as regards the generalisation of relativity, to argue as you do ... But this manner of looking at things has the disagreeable consequence that one must start our from the universe as a whole. It is more agreeable to start from a part and to leave the boundary conditions unspecified, as I have done in the equivalence hypothesis'. On a paper by Dolder, Einstein remarks 'The necessity of the premise of the principle of the constancy of the speed of light can only be attained if one calls upon all the material of experience'. A postscript refers to his famous 1916 paper Die grundlage der allgemeinen Relativitätstheorie: 'You will soon receive a little work of mine about the basis of the general theory of relativity, in which it is demonstrated how the requirements of relativity relate to the principle of energy. It is very amusing'.
Published (in French and German) in Pierre Speziali (ed. and tr.). Albert Einstein. Michele Besso. Correspondance 1903-1955. Paris: Hermann, 1972. No. 27.