Gravity and handedness of photons

Ivan Agullo 
Department of Physics & Astronomy,  Louisiana State University  

Maxwell's equations in absence of charges and currents are invariant under electric-magnetic duality transformations. This symmetry leads to a constant of motion, given by the net polarization of the electromagnetic field. I will describe how this symmetry remains at the quantum level in the absence of gravity, but not necessarily otherwise. This implies that the polarization of photons can change in time; or in other words,  a gravitational field can act as an optically active medium. This is a quantum effect, and it can be understood as the generalization of the well-known chiral anomaly of fermions to photons.