Classical worldlines from scattering amplitudes

TL;DR

This paper systematically investigates the classical limit of scattering amplitudes in quantum field theory, demonstrating the equivalence with worldline formalisms and showing how classical causality emerges, with applications to electrodynamics and scalar models.

Contribution

It introduces a diagrammatic approach that explicitly shows the equivalence between scattering amplitude and worldline formalisms in the classical limit, including divergence cancellation and causality emergence.

Findings

Manifest divergence cancellation at the integrand level beyond one loop.

Exact match between amplitude-based and worldline formalisms in examples.

Emergent classical causality flow from retarded propagator prescription.

Abstract

We present a systematic diagrammatic investigation of the classical limit of observables computed from scattering amplitudes in quantum field theory through the Kosower-Maybee-O'Connell (KMOC) formalism, motivated by the study of gravitational waves from black hole binaries. We achieve the manifest cancellation of divergences in the $\hbar \to 0$ limit at the integrand level beyond one loop by employing the Schwinger parametrisation to rewrite both cut and uncut propagators in a worldline-like representation before they are combined. The resulting finite classical integrand takes the same form as the counterpart in the worldline formalisms such as post-Minkowskian effective field theory (PMEFT) and worldline quantum field theory (WQFT), and in fact exactly coincides with the latter in various examples, showing explicitly the equivalence between scattering amplitude and worldline formalisms. The classical causality flow, as expressed by the retarded propagator prescription, appears as an emergent feature. Examples are presented for impulse observables in electrodynamics and a scalar model at two loops, as well as certain subclasses of diagrams to higher orders and all orders.