1. The problem statement, all variables and given/known data Show that if mass = 0, then E=pc and u=c. 2. Relevant equations E^2 = (pc)^2 + (mc^2)^2 B = u/c = pc/E 3. The attempt at a solution I understand that if m=0, then E^2=(pc)^2 => E=pc. But isn't p = Ymu? then: E^2 = (Ymuc)^2 + (mc^2)^2 Plugging in m=0 sets E=0... It seems strange to me that we can remove (mc^2)^2 from the equation due to m=0, but we can leave (Ymuc)^2 in. I'm thinking that this definition of p is not the same as Ymu? EDIT: wait... Y would equal infinity. Which multiplied with 0 makes ??? I'm just plain confused now.