Consider a null ray [ for example a light ray propagating
in the x direction: then , so the null
ray represents a particle moving at the speed of light. If we
consider 3D Minkowski spacetime then the null rays lie on the
surface of a cone [ see Figure 1.9 ]:
Before we look at some of the wonderful consequences of the
Einstein postulates, let us return briefly to the Lorentz
transformations. If we define we
can write them in the following way:
Formally this corresponds to a rotation through an angle
in (ict,x) space [ see Figure 1.11 ].