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 ]:

Figure 1.10: The views of space and time that I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself and time by itself are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality. Hermann Minkowski

Figure 1.11:ict space

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 ].