Past exam of the short options course of the University of Oxford / 2017 / 8 / a / ii / Solution

For a massless particle of energy $E>0$ moving in the unit direction $\mathbf{n}$,Its spatial momentum is $(E/c)\mathbf{n}$, and its four-momentum has Minkowski norm zero. A rest-frame expression $m\gamma (c,\mathbf{v})$ is not used for a massless particle.