A generally relativistic variational principle for irreversible processes is used to derive the Einstein field equations, the Maxwell equations, and generalized Ginzburg–Landau equations. Superconductivity of the medium is described by means of phenomenological thermodynamics with an order parameter in the sense of the Ginzburg–Landau theory. The variational principle yields the reversible parts by direct variation and the
irreversible parts (heat conduction, internal friction, and the time-dependent part of the superconductivity) by means of the generalized Onsager relations.