But e=mc2 is not true. That is why you are struggling to derive it properly. The factor c, which looks like light speed, is just a meaningless constant. This stems from incorrectly equating the photon based representation of reality received, with the reality it represents, hence deeming light speed as the fastest rate of change possible, and then using it as a calibration reference, but not as light, just a conceptual constant.
The derivation of c in section 1, part1, 1905 is nonsense:
By substituting c for v, ie a specific velocity of a particular entity, c is asserted to be: 2AB/(t'(a) - t(a)). Which is both wrong because that time involves duration incurred from subsequent timings, apart from being deemed an elapsed time in both cases anyway, which it is not. Assuming the quantity is doubled, it should be either twice A to B or B to A, or the sum of A to B and B to A incurred at the same time.
So it should be, when specifically considering light speed:
c = 2AB/2(t(a) - t(b)). Or simply, as considering either direction is superfluous, c = AB/(t(a) - t(b)). Which, although correct, is a statement of the obvious. That is, the velocity of light is a ratio of total distance travelled to the time taken to do so, ie the definition of velocity. Light is just another entity, which can be presumed to travel at a constant speed.
The point being that it is not light, just a constant. Einstein has caused readers to think it is light, but is no observational light by which an observer could observe, just a dissassociated "ray of light", an entity referred to as an "observer", and the concept of "frames of reference".
[Eckard, your point is spot on. All these 'effects' are actually differentials. When they refer to gravitational force causing changing momentum (and possibly dimension alteration), it is a differential in forces incurred. In just the same way that being 'at rest' is a state of moving but not moving differently from the reference used to calibrate the movement, etc, etc]
