The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics (Oxford Landmark Science)

by Roger Penrose

This will provide a very good picture, and it should not be hard to accept that in principle the ideas will extend, without much change, to the full four-dimensional situation. The thing to bear in mind about a space-time diagram is that each point in the picture represents an event – that is, a point in space at a single moment, a point having only an instantaneous existence. The entire diagram represents the whole of history, past, present, and future. A particle, since it persists with time, is represented not by a point but by a line, called the world-line of the particle. This world-line – straight if the particle moves uniformly and curved if it accelerates (i.e. non-uniformly) – describes the entire history of the particle’s existence. Fig. 5.16. A light cone in Minkowski’s space–time (with just two spatial dimensions), describing the history of the light flash of an explosion that takes place at the event O, the space–time origin.

How time is represented on a space time graph