Can anybody point out if there is a NON-QED derivation of E=pc ? I would need a reference if such a derivation existed. Thank you in advance. PS: I am not looking for the definition of 4 vector invariants that leads to [tex]E^2 = (pc)^2 + (mc^2)^2[/tex]. In the same vein, I am not looking for setting m=0 in the above equation because it brings up the question of deriving [tex]E^2 = (pc)^2 + (mc^2)^2[/tex] from first principles without resorting to QED.