Takamasa Kanai

We investigate the impact of higher-curvature corrections on photon propagation within an effective field theory framework and explore their observational consequences in strong gravitational fields. In particular, we consider polarization-dependent modifications to photon trajectories induced by higher-order curvature terms and analyze their effects in static and spherically symmetric spacetimes, focusing on Schwarzschild and Reissner-Nordström backgrounds. Using the geometrical optics approximation, we derive the effective metric governing photon propagation and study the resulting shifts in the photon sphere. Based on this modified propagation, we compute the quasinormal modes in the eikonal limit and examine their dependence on the polarization modes. We further analyze gravitational lensing observables, focusing on the deflection angle, incorporating the polarization-dependent corrections. Our results clarify how contributions from beyond-general-relativity effects manifest in both quasinormal mode spectra and strong gravitational lensing observables. These findings further suggest the possibility of placing meaningful constraints on effective field theories.