I'm trying to wrap my head around what happens as mass is converted to energy. In a nuclear reaction, it is my understanding that mass is converted to energy. It is also my understanding that as matter approaches the speed of light it's mass approaches infinity. If that is so, why is the mass of light itself zero? (In my mind, matter that has been converted to energy is going the speed of light). I apologize if this is a really elementary question, but could you set me straight on where the error in my thinking is? I suppose what I am trying to figure out is what physical process takes place at the instant mass is converted to energy?