We shall now investigate in what measure the graphical representation of classical science will have to be modified in order to harmonise with the empirical facts revealed by ultra-refined experiment. There is no need to modify our understanding of point-events and world-lines; these will remain undisturbed.
The bifurcation between the two graphs arises when we consider the principle of the invariant velocity of light. We saw that if a ray of light was sent along the embankment to the right, leaving the origin
at a time zero, its world-line was given by a line
bisecting the angle
([Fig. XIV]). On the other hand, for an observer travelling to the right and having a world-line
, the light-world-line