Quantum Field Theory, Worldline Theory, and Spin Magnitude Change in Orbital Evolution

Understand

A previous paper~\cite{Bern:2022kto} identified a puzzle stemming from the amplitudes-based approach to spinning bodies in general relativity: additional Wilson coefficients appear compared to current worldline approaches to conservative dynamics of generic astrophysical objects, including neutron stars.

• In this paper we clarify the nature of analogous Wilson coefficients in the simpler theory of electrodynamics.
• We analyze the original field-theory construction, identifying definite-spin states some of which have negative norms, and relating the additional Wilson coefficients in the classical theory to transitions between different quantum spin states.
• We produce a new version of the theory which also has additional Wilson coefficients, but no negative-norm states.