Past exam of the physics 1 paper of the University of Oxford / 2019 / 10 / a

For a boost along $+x$ at speed $v$, the energy-momentum four-vector transforms asThe total four-vector is the sum over the particles. Write $E={\sum }_i{E}_i$ and $\mathbf{P}={\sum }_i{\mathbf{p}}_i$. A centre-of-momentum rest frame exists when this total is timelike, with ${\mathbf{V}}_{\mathrm{CM}}=c^2\mathbf{P}/E$. Boost along $\mathbf{P}$ and use $P=E{V}_{\mathrm{CM}}/c^2$:ThereforeThe timelike qualification is needed: exactly parallel photons alone have null total four-momentum and no such rest frame.