In the 1905 paper about mass-energy equivalence, Einstein's final equation (original german) is:

K_{0}-K_{1} = L/V^{2} v^{2}/2

Where V is the speed of light, L the energy of both opposite light beams, and (K_{0}-K_{1}) the energy change at the system composed by

both frames of reference (considering one at rest with respect to the second one moving at v speed over x axis).

In this "newtonian" formula for Kinetic Energy, the mass came from the energy of both beams of light, each one having L/2 energy.

Einstein, as far as I know, never was able either to probe this approximation to the final statement (which leads to E_{0}=m_{0}c^{2}), nor he could give a satisfactory explanation between the relationship between kinetic and electromagnetic energy in the same formula.

Does it mean that he was actually satisfied thinking that mass has an electromagnetic nature, as PoincarĂ¨ thought with his cannon of light shots that had a kinetic recoil, or the true original nature of this (questionable) formula has been buried into oblivion by those who adopted his expression?

**Edited by rhertz, 18 June 2019 - 08:47 PM.**