Vilas Tamhane wrote:In fact Einstein derived momentum form energy-mass relationship. If mass is energy then energy is mass. E=mc^2 and so p=mc=(E/c^2)c=hf/c. However if we admit that photon has mass then we contradict mass/velocity relationship of SR. Therefore we admit what is convenient.
Your logic is wrong because you misuse equations.
E = mc2 is properly correct in a rest frame. [There is a pedagogically-disreputable idea called relativistic mass, but I won't talk about that here unless forced to.] The actual equation is:
E2 = m2c4 + p2c2
Further, p = mc isn't correct. p = mv for massive particles and E = pc for massless particles.
This is the source of your confusion.