In Chap. 14 we have discussed the relation between the 4-momentum of a point particle and its instantaneous energy and 3-momentum relative to an arbitrary observer at any event in an arbitrary spacetime. In Chap. 27 we outlined how, in 1923, A. H. Compton provided compelling experimental evidence for the existence of Einstein’s photon which he interpreted as a particle with zero rest mass. Using the notation of Chap. 14 we demonstrate in Chap. 29 how Compton’s deduction follows from the relativistic kinematics that model an instantaneous (impulsive) multi-particle collision in spacetime based on the conservation of total 4-momentum. We discuss how the Compton formula can be generalised to instantaneous collision processes that include electrons, muons, photons and massive neutrinos.