Researchers have developed a new methodology to define the dynamical tidal response of neutron stars, a crucial factor for distinguishing these objects from black holes in gravitational-wave observations. The tidal response of a neutron star, which reflects its internal structure and equation of state, is fundamental for understanding the physics of matter at extreme densities. However, its definition within the framework of general relativity is complex due to coordinate ambiguities and the difficulty of linking the stellar response to binary dynamics and the resulting gravitational waveforms.

The new approach utilizes the worldline effective field theory (EFT) framework, connecting the problem to gravitational-wave scattering off an isolated neutron star. The scattering amplitudes were computed both within the EFT, employing standard quantum field theory techniques, and within stellar perturbation theory (the corresponding ultraviolet theory). In the latter, the coupled metric and matter perturbation equations are solved in the stellar interior within general relativity, and matched to the analytical Mano-Suzuki-Takasugi (MST) solutions in the exterior.

By matching the scattering amplitude between the effective theory and the ultraviolet theory, the scientists obtained the dynamical tidal response. The results are consistent with known expectations, such as the static limit and the behavior near the neutron star's resonant modes. Furthermore, the method recovers the imaginary part of the dominant oscillation mode induced by gravitational-wave dissipation, opening avenues for future improvements in both EFT and perturbation theory.