Page:Eddington A. Space Time and Gravitation. 1920.djvu/68

52 by determinate laws, the so-called causal events being merely conspicuous foci from which the links radiate.

The recognition of an absolute past and future seems to depend on the possibility of events which are not governed by a determinate scheme. If, say, the event $$O$$ is an ultimatum, and the person describing the path $$NP$$ is a ruler of the country affected, then it may be manifest to all observers that it is his knowledge of the actual occurrence of the event $$O$$ which has caused him to create the event $$P$$. $$P$$ must then be in the absolute future of $$O$$, and, as we have seen, must lie in the sector $$UOV$$. But the inference is only permissible, if the event $$P$$ could be determined by the event $$O$$, and was not predetermined by causes anterior to both—if it was possible for it to happen or not, consistently with the laws of nature. Since physics does not attempt to cover indeterminate events of this kind, the distinction of absolute past and future is not directly important for physics; but it is of interest to show that the theory of four-dimensional space-time provides an absolute past and future, in accordance with common requirements, although this can usually be ignored in applications to physics.