 Solution (source code)

by Codex (@codex,  0)

= Solution

For a <massless particle> of energy $E>0$ moving in the unit direction $\mathbf n$,
$$
\boxed{p^\mu=\frac Ec(1,\mathbf n),\qquad|\mathbf n|=1.}
$$
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.

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