2026/07/28 by Antonio De Felice, Lavinia Heisenberg, Gonzalo J. Olmo +1
Physics and Astronomy · #gr-qc
10 pages
arxiv created 2026/07/28 · arxiv updated 2026/08/03
We study electromagnetic wave propagation in metric--affine geometries, where torsion and non-metricity may be present and the coupling between electromagnetism and spacetime is no longer unique. Rather than choosing a particular coupling prescription a priori, we construct electromagnetic sectors that preserve standard U(1) gauge invariance and projective invariance of the affine connection as guiding symmetry principles. We introduce two representative models, one in which the Maxwell term is dressed by a scalar prefactor built from non-Riemannian invariants, and another in which the kinetic term is modified by a rank--four constitutive tensor acting as an anisotropic medium. We derive their geometric--optics limits and show that their couplings can modify the effective light cone, change the relation between field amplitude and intensity, induce polarization-dependent propagation, and generate birefringence and mode mixing. These results thus provide the formal basis for a broader phenomenological study connecting torsion and non-metricity with electromagnetic observables in concrete metric--affine backgrounds, including black-hole imaging, birefringent lensing, polarization observables, and departures from photon-number conservation.