Polarization effects on light propagation in gravitational fields

The effects of a gravitational field on the propagation of electromagnetic waves in a vacuum are typically analyzed using geometric optics. In this approach, high-frequency electromagnetic waves are treated as rays that follow null geodesics. However, this model is only precisely accurate when light frequencies approach infinity. For finite frequencies, there exists a polarization-dependent deviation known as the gravitational spin-Hall effect of light. This effect becomes particularly significant when considering propagation over substantial astronomical distances and emissions from regions with strong gravitational fields, especially for radio and microwaves.

We use a recently developed covariant spin-optics method to investigate this effect in Schwarzschild spacetimes. These provide both the simplest mathematical setting that gives a chance to obtain analytical results that are important for qualitative understanding of the phenomena, and adequate description for the Solar-system observations. We conduct perturbative calculations, starting from exact solutions for null geodesics. As a result, we derive explicit expressions for the resulting trajectories and assess the magnitude of the effect concerning the gravitational deflection of light.

Key findings are as follows: (1) Polarization-induced corrections are absent for radially propagating light. (2) The corrections occur orthogonally to the plane of the unperturbed geodesic trajectory. (3) These corrections are particularly significant for low-energy electromagnetic waves, such as radio waves, and may even be observable with the current technology.