  We prove an instance of the Reciprocity Theorem that demonstrates that Kerr
rotation, also known as the magneto-optical Kerr effect, may only arise in
materials that break microscopic time reversal symmetry. This argument applies
in the linear response regime, and only fails for nonlinear effects. Recent
measurements with a modified Sagnac Interferometer have found finite Kerr
rotation in a variety of superconductors. The Sagnac Interferometer is a probe
for nonreciprocity, so it must be that time reversal symmetry is broken in
these materials.
