By analogy with the chiral anomaly - in which coupling to a background electromagnetic field breaks the left-right handed chiral symmetry of massless fermions - it is argued that the conservation of left/right handed polarizations for photons is broken by background spacetime curvature. While the rate of spontaneous creation of pairs of handedness violating photons estimated from simulations is much too low to ever be detected experimentally, it is suggested that the stimulated process may have an observable effect on CMB photons.

The classical symmetry associated with the conservation of the Stoke's V parameter (measuring the difference between right and left-handed polarizations) is the electro-magnetic duality which rotates the field strength into its dual. By exploiting a (0,1) + (1,0) representation of the gauge field which resembles a Dirac bispinor, a strong case is made for this duality to be anomalous in much the same way that the chiral symmetry of fermions is anomalous, and the expected change in the Stoke's parameter is related to a product of electric (tidal) and magnetic (frame-dragging) parts of the Weyl tensor. The complete calculation is given in 1607.08879.