1. The problem statement, all variables and given/known data What is the exact mechanism through which mass converts into energy and vice versa through Einstein's equation.In other words,what are the conditions under which this transformation happens and why? 2. Relevant equations E=m.c^2 3. The attempt at a solution Some argue that mass is the condensed form of energy so my question is invalid. I think it's valid even then.The question says "What are the conditions (and why these conditions only) under which this condensed form of energy converts into its free (usual) form and vice-versa?" I don't know the answer that's why I am curious to it.I have asked many people, even my teachers, but couldn't get a satisfactory answer.