Consider the two spin eigenstates of the electron, at rest. You are given that the Hamiltonian of this system commutes with the spin generators Sz, Sy, and S-. Since the electron has no kinetic energy, the energy eigenvalue corresponding to an eigenstate of the Hamiltonian is equal to mc^2, where m is the rest mass of the electron in that state.
(a) Show that the electron with spin-Up has the same rest mass as the electron with spin down.