E = mc2 is not really true in all cases?? I am new to the field of SR and GR so my knowledge is not much but in the last thread I red following . "That is an incorrect statement. It assumes that E = mc2 in all cases, which it does not. It therefore "relativistic mass" cannot be replaced by "mass-energy". This assumption can lead a person to make statements which are very serious errors. E.g. people will almost always believe that mass density is identical to energy density. It is not. Rindler explains this in his 1982 intro to SR book." So please explain, E = mc2 is not really true in all cases, in which cases this doesnt hold true.