Mass of composite systems Point 11 or write the energy momentum relation m E p and infer that in a closed system mass must be conserved because the energy momentum 4 vector is conserved Yes but we are still allowed to consider the mass of the closed system which is not the same as the sum of the masses that are in the system If you let an atomic bomb go off inside an infinitely strong container then the mass of the container is the same as it was before the explosion Even if there is no container one can still define the invariant mass of the system after the explosion which is not the same as the sum of the invariant masses of the particles after the explosion That sum will have become less of course