Polarization-dependent observational signatures of Weyl-coupled photons around a black hole
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/M2=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.
Versions
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BibTeX
@misc{wan2026polarizationdependent,
title = {{Polarization-dependent observational signatures of Weyl-coupled photons around a black hole}},
author = {Mao-Yuan Wan and Chen Wu},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2607.10658},
eprint = {2607.10658},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.10658},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Polarization-dependent observational signatures of Weyl-coupled photons around a black hole
AU - Mao-Yuan Wan
AU - Chen Wu
PY - 2026
DA - 2026/08/13
PB - arXiv
DO - 10.48550/arxiv.2607.10658
UR - https://arxiv.org/abs/2607.10658
AB - 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.
ER -