Princeton, 10 September 1952

On the nature of time, including a diagram of a point in space-time.

'In special relativity one can satisfy the specific character of space-time through Minkowski's imaginary time-coordinate. In general relativity, however, the fourth coordinate has nothing to do with adjacency in time ... If one now considers in one point two opposed temporal directions, one might well say that one is directed towards the future, the other towards the past. The field laws however show no dissimilarity between these two directions. And this is as in classical theory with the positive and negative directions of time. Such a distinction only has sense on the basis of the second principle, so is based not on the form of the elementary laws but only on the conditions at the limits (improbability with respect to order grows in the negative direction of time)'.

Einstein points out that Besso's proposal (that a field of probability originates from each realisation) is completely at odds with contemporary quantum theory: 'A real state absolutely cannot be described in the present quantum theory ... "Orthodox" quantum theorists absolutely proscribe the notion of the real state. One thus gets into a situation which corresponds fairly accurately with the one described by the good Bishop Berkeley'. [The 18th-century philospher George Berkeley famously posed the question: 'If a tree falls in a forest and no one is around to hear it, does it make a sound?']. Evidently this is an uncomfortable intellectual status, even if it is, for the time being, the only way of reconciling quantum theory with experience. Einstein is however convinced that the truth is far removed from current teaching, and remains hopeful that his approach, of a 'general relativistic theory of the unsymmetrical field' may still be the right one, even if the mathematical difficulties for comparing it with reality remain insurmountable. 'However that may be, in any case we are still as far removed from a truly rational theory (of the double nature of light quanta and particles) as we were more than 50 years ago!'.