Andrea Cristofoli

Amplitudes in Strong Gravitational Fields and Classical Physics

Scattering amplitudes are a useful tool for describing the post-Minkowskian approximation in the classical two-body problem in gravitational physics. However, classical general relativity allows for perturbation theory to be set up around any background solution to Einstein's field equations, not just Minkowski space. This raises the question: can scattering amplitudes also be used to describe these more general perturbative schemes? In this talk, we will answer this question by introducing the concept of "strong field amplitudes". We will demonstrate how these amplitudes can be formulated on any background and will focus on 2 and 3-point 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 naturally leads to a perturbative scheme referred to as post-background (PB). We will also explain how this approach reduces to the well-known self-force paradigm in the case of a Schwarzschild background. Lastly, we will provide a concrete example of an observable in this perturbative scheme by analyzing the waveform generated by the scattering of a probe particle with a plane wave using the classical limit of a 3-point amplitude.