Joint Theory Seminar: Andrea Cristofoli

Title: Amplitudes in Strong Gravitational Fields and Classical Physics

Abstract: Scattering amplitudes on a flat background are a valuable tool for describing the post-Minkowskian approximation in classical two-body gravitational physics. However, in classical general relativity, perturbation theory can be established around any solution to Einstein's field equations, not just Minkowski space. This raises the question of which QFT tools can be used to describe these more general perturbative schemes.

In this talk, we will introduce the concept of "strong field amplitudes" and demonstrate how they can be formulated on backgrounds beyond Minkowski. Our focus will be on strong field amplitudes on Kerr-Schild backgrounds as a case study. We will highlight the similarities and differences with flat spacetime amplitudes and show how their use in the KMOC formalism leads to a perturbative scheme referred to as post-background (PB), which contains infinite contributions in the post-Minkowskian approximation.

We will also explain how this approach reduces to the well-known self-force (SF) paradigm in the case of a Schwarzschild background. Finally, we will provide a concrete example of an observable in this perturbative scheme by analyzing the waveform at 1PB generated by the scattering of a probe particle with a plane wave.