A new study explores the effects of black hole spin in effective field theories (EFTs) of gravity, analyzing how these properties influence wave scattering. Researchers calculated key observables such as the polarization rotation angle, the wavenumber kick (or deflection angle), and the (Shapiro/Wigner-Smith) time delay. These phenomena are related to gravitational Faraday rotation, the gravitational spin Hall effect, and infrared causality, providing a new perspective on the interaction between gravity and spin.

The calculations were performed using the Magnusian formalism, extended to a matrix to incorporate helicity information of the waves. This approach allowed scientists to investigate how EFTs of gravity, which are low-energy approximations of a more complete quantum theory of gravity, differ from Einstein's General Relativity in the presence of rotating black holes. Black hole spin, a fundamental characteristic of these astrophysical objects, thus becomes a crucial probe for distinguishing between different theoretical models of gravity.

Among the most important findings, the study reveals that the gravitational spin Hall effect in EFTs of gravity shows significant qualitative differences compared to that predicted by General Relativity, due to what they call "noncommutative wavenumber kicks". Furthermore, it was found that black hole spin slightly enhances the causality constraints on the EFT coefficients. This implies that the inclusion of spin not only modifies scattering predictions but also imposes stricter limits on the parameters describing deviations from General Relativity in these effective theories.