Berlin, 3 January 1916

'In terms of physics, nothing is real except for the entirety of the coincidence of points in space-time'.


'The great success of gravitation pleases me extraordinarily. I have the serious intention of writing a book soon about special and general relativity, and yet have difficulty in getting started, as with all things that don't spring from a burning desire. And yet if I don't do it, the theory will not be understood, no matter how simple its basis'. Discussing the movement of the perihelion of Mercury (one of the key experimental demonstrations of General Relativity), Einstein notes that this movement is guaranteed only for Mercury: 'If the orbits of Venus and the Earth were more eccentric, one could detect the effect there. The strong increase of the effect compared with our calculations comes from the fact that, in line with the new theory, the g₁₁ – g₃₃ appear also in the magnitudes of the first order and thus contribute to the movement of the perihelion. The value is very precisely determined through the transits of the sun'. Responding to a suggested elaboration of his theory by Besso, Einstein lays down an essential rule: 'In terms of physics, nothing is real except for the entirety of the coincidence of points in space-time. If for example one had to construct the physical phenomenon on the basis solely of the movement of material points, then the meeting of points – i.e. the points of intersections of their lines in the universe – would be the only reality, meaning the only really observable one ... It is therefore entirely natural to expect that laws will determine nothing more than the totality of spacio-temporal coincidences. As I have said, this is already achieved by the equations of general covariance'. Einstein goes on to point out that a crucial term is missing from the first two of his papers on General Relativity: this is however corrected in the last of the four papers. Einstein also discusses at the outset of the letter his attempts to correspond with his eldest son, Hans Albert (then 15 years old: he had scarcely seen his father for more than a year). Einstein has been discouraged from visiting Zurich by the difficulties and costs of wartime travel, but hopes to come at Easter. Personally he is fine, and his relatives still meet him cordially, in spite of the fact that he has declined to contemplate marriage [to his cousin Elsa, whom he would eventually marry in 1919: this passage is omitted in the published text].

Einstein's projected book on relativity theory was to be published later that year as Die Grundlage der allgemeinen Relativitaetstheorie.