I'm using here a 1923 English translation of his original 1905 paper: "Ist die Tragheit eines Korpers von seinem Energieinhalt
abhanging? (13, 639, Annalen der Physik)."
To explain it briefly: There are two reference frames E(x,y,z) and H(x',y',z'). There is a point-like mass located at E(0,0,0) which is
at rest relative to the frame H, which is moving at constant velocity "v" towards the positive side of the x' axis.
There are two virtual observers: Observer 1 at E(x,y,z), who isn't moving with reference to E(0,0,0), and Observer 2 at H(x',y',z')
who also doesn't move with reference to H(0,0,0). A third virtual observer (Einstein) collects information about what both observers
perceive with respect to energies at E and its perception at H.
There is an initial state, ZERO, at which energy values from E and H are collected.
At a given instant, and without any further explanation about the technology used, the mass at the frame E (at rest), emits two
opposite beams of light (to cancel momentum), each one with energy L/2.
The light beams form an angle PHI with the x axis, which is arbitrary. Due to this fact, and to simplify, I selected PHI = 0.
The effect of the angles are cancelled inmediately, anyways.
This instantaneous emission of light (unexplained) creates a new state, called ONE.
The trick is to calculate the energies perceived by both observers: E0, E1 and H0, H1 and CALCULATE the total difference in
energies BEFORE and AFTER the light is turned ON.
Edited by rhertz, 18 June 2019 - 07:51 PM.