Was Einstein inconsistent between his theories of Special and General Relativity? In the theory of Special Relativity we learn that energy and mass are interchangeable E = mc^2. In the theory of General Relativity we learn that because of Einstein's equivalence principle (EEP) the mass of a particle is invariant. When a uranium atom undergoes fission, the energy released is only the energy of the system, bound up in the atom, that is being re-allocated; the masses of all the constituent particles making up the atom remain invariant. Are these two theories therefore mutually contradictory?