Why would mass/energy increase between observers and charge remains same. I guess some will leap simply saying: "who said mass changes?" Feynman, Lectures on Physics Vol I, ch15: Secondly, under what situtations can we consider m changes with velocity?? Or is this a general statement? Third: is it possible to define a radius or charge density (delta function?) for electron? I remember some phrase like "SR doesn't allow electron to have a radius", but don't know what it's based on.