Does anyone know behind stories? As it seems that $p=U/c$ was known before the advent of special theory of relativity, (Halliday, Resnick, Walker says that Maxwell derived this.) when $E=pc$ was first derived by plugging in $m=0$ for $E^2=m^2c^4+p^2c^2$, were physicists happy that the two formulas agreed? Who first noticed this agreement? What was the reaction? And, who derived $E=pc$ from the special theory of relativity point of view? I know that $p=γmv$ was first derived by Planck in 1906, but I do not know much anything else such as who derived $E^2=m^2c^4+p^2c^2$ and who first plugged in $m=0$.