Polarization-dependent observational signatures of Weyl-coupled photons around a black hole
Year: 2026
Type: article
Abstract: A photon coupled non-minimally to the Weyl tensor propagates differently in its two linear polarization states: the vacuum around a black hole becomes a birefringent medium. On a Schwarzschild background the coupling α splits the photon sphere and the critical impact parameter of the two polarizations, producing a double shadow and a polarization-dependent photon ring; the two shadow edges separate by 62% of the shadow radius at α / M 2 = 0.75. We compute a unified set of wave-optical and geometric-optical observables for both polarizations across a range of α : greybody factors, absorption cross sections, differential cross sections and their glories, a backward birefringence signal, shadows, photon rings, and null trajectories. The two-polarization scattering matrices are extracted with a Riccati–Hankel matching scheme that keeps the phase-sensitive observables reliable, and the geometric-optics chain is cross-validated with three independent ray tracers. The cleanest diagnostic is the backward signal: it is protected by parity and vanishes identically at α = 0. Laboratory bounds on α render these effects unobservable for supermassive black holes; order-unity couplings would require a black hole of primordial mass.
Authors: Mao-Yuan Wan, Chen Wu
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funder: Zhejiang Normal University
Award: G2785111632
OpenAlex ID: W7168258697