  Open systems may be perturbed out of equilibrium states either by subjecting
them to nonconservative forces or by injecting external currents. For small
perturbations, the linear response is quantified by two different matrices. In
the framework of network thermodynamics, under very broad assumptions, we show
that the two matrices are connected by a supersymmetry, predicting that they
have the same spectrum - up to the degeneracy of the ground state. Our approach
brings into the mathematics of supersymmetry a new ingredient, namely oblique
projection operators.
