These properties must hold generally at all points of the space–time. There is nothing special about the origin; the point O is no different from any other point. Thus there must be a light cone at every point of the space–time, with a significance the same as that for the light cone at the origin. The history of any light flash – or the world-lines of photons, if we prefer to use a particle description of the light – is always along the light cone at each point, whereas the history of any material particle must always be inside the light cone at each point. This is illustrated in Fig. 5.17.
...more
