  We propose a generalization of the Chern-Simon (CS) Lagrangian which is
invariant under the SIM(2) transformations but not under the full Lorentz
group. We study the effect of such a term on radiation propagating over
cosmological distances. We find that the dominant effect of this term is to
produce circular polarization as radiation propagates through space. We use the
circular polarization data from distant radio sources in order to impose a
limit on this term.
