= Collinear inelastic capture followed by two photons
{title2=$V=Pc^2/E,\quad M^2c^4=E^2-P^2c^2,\quad E_\pm=(E\pm Pc)/2$}
In an isolated collinear completely inelastic capture, add incoming total energies and signed momenta. Their invariant fixes the composite <rest mass>, including internal excitation. If it later converts all <energy> to two opposite axial <photons>, the two conservation equations immediately give the displayed <photon energies>. The laboratory <frequency> ratio is $(E+Pc)/(E-Pc)$, not generally one; equality holds in the composite <rest frame>.
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