Quote by sjw40364
First you must realize that E = mc2 is not what Einstein originally wrote.
The correct equation is E subscript 0 = mc2.
In the first equation m depends on energy (on velocity) and the photon has mass.
In the second equation m is constant and the photon has no mass.
Over the years his orginal equation was changed from E subscript 0 = mc2 to E = mc2.

Yea, you are correct , p. 47 "General Results of the Theory," Section XV
Relativity, Einstein AE.
He derives that using a Taylor (I believe) expansion.