Impossibility of isolated massive-particle decay into one photon
= Impossibility of isolated massive-particle decay into one photon
Conservation of <four-momentum> forbids an isolated massive initial particle from decaying into a single <photon>: the initial <Minkowski norm> is $m^2c^2>0$, while the final norm would vanish. This argument is independent of <inertial frame>. A second final particle or external recoil changes the problem because the total final <four-momentum> need not be null.