What can be measured asymptotically?

TL;DR

This paper explores a broad class of asymptotic observables in quantum field theories with a well-defined S-matrix, including inclusive and out-of-time-order quantities, and discusses their calculation and physical implications.

Contribution

It introduces methods to compute inclusive and out-of-time-order observables in quantum field theories, extending the traditional focus on scattering amplitudes.

Findings

Relation of inclusive observables to amplitudes and products of amplitudes

Generalization of LSZ reduction formula for new observables

Analysis of gravitational radiation integrals and infrared physics

Abstract

We consider asymptotic observables in quantum field theories in which the S-matrix makes sense. We argue that in addition to scattering amplitudes, a whole compendium of inclusive observables exists where the time-ordering is relaxed. These include expectation values of electromagnetic or gravitational radiation fields as well as out-of-time-order amplitudes. We explain how to calculate them in two ways: by relating them to amplitudes and products of amplitudes, and by using a generalization of the LSZ reduction formula. As an application, we discuss one-loop master integrals contributing to gravitational radiation in the post-Minkowski expansion, emphasizing the role of classical cut contributions and highlighting the different infrared physics of in-in observables.