In fact, neglecting the photon, for it is Massless, the electron is not even the lightest
particle we have yet discovered;
The argument in his 1905 paper can be carried out with the emission of any Massless particles, but the
Maxwell equations are implicitly used to make it obvious that the emission of light in particular can be achieved only by doing work.
In this case if the particles are Massless, or highly relativistic(
E>> m{\displaystyle E>>m},) then the invariant mass becomes: The rest energy E 0{\displaystyle E_{0}} of a particle is defined as: E 0 m 0 c 2{\displaystyle\ E{ 0} m{ 0} c{ 2}}, where c{\displaystyle c} is the speed of light in vacuum.