@jmax@mastodon.social You are likely going to regret that. (-: When discussing the energy–momentum relation, mass *is* by convention rest mass, as the usual formulation E^2=m^2×c^4+(p×c)^2 is in terms of rest mass m. The relation says that energy does not imply mass when m=0. Energy implies the momentum portion of the sum, which photons have, defined as p=h/λ. With m=0 the full form reduces to E^2=(p×c)^2 which after substitution for photon momentum becomes E=c×h/λ=h×f . But this does not become a statement about mass. It's fallacious to then substitute E=m×c^2 and solve for m to get m=h×f/c^2 . E=m×c^2 is a different reduced case for massive stationary objects (m>0, p=0), neither of which is the case for photons. Furthermore, the maths yields divergent γ=∞ Taylor series sums when u=c so thinking of K.E. terms for photons is aphysical. Energy-mass equivalence is a special form for the case of m>0, γ≠∞. Energy does not imply mass in the general case. @cstross@wandering.shop #physics #relativity