Coherent states and classical radiative observables in the S-matrix formalism

Abstract

In this thesis, we study classical radiative observables perturbatively in terms of on-shell scattering amplitudes. In particular, we focus primarily on the two-body problem in gauge and gravitational theories by using an effective field theory ap- proach. The Kosower-Maybee-O Connell (KMOC) approach, which follows from the classical on-shell reduction of the in-in formalism by using appropriate massive particle wavefunctions, is extended to include classical waves which are naturally described by coherent states. Global observables like the impulse and localized observables like the waveform and gravitational event shapes are then studied in the amplitude approach, making contact also with asymptotic symmetries and light-ray operators defined near null infinity. The classical factorization of radiative observables from the uncertainty principle is proved to be equivalent to a Poisson distribution in the Fock space, and this provides new evidence in favor of a representation of the classical S-matrix in terms of an eikonal phase and a coherent state of gravitons.