where w ^ ^ is the unit vector along the new w axis. The transformation mapping p to η can be uniquely inverted; for example, the x component of the momentum equals p x = p 0 η η x 1 − − η η w , and similarly for p y and p z. In other words, the three-dimensional vector p is a stereographic projection of the four-dimensional η η vector, scaled by p 0 (Figure 8).