Conservative Spin Magnitude Change in Orbital Evolution in General Relativity

TL;DR

This paper reveals that in general relativity, the scattering behavior of spinning compact objects depends on additional spin degrees of freedom beyond the spin vector, affecting observable quantities like impulses and waveforms.

Contribution

It introduces the concept that spin tensor degrees of freedom influence scattering observables and connects these effects to the eikonal phase in gravitational interactions.

Findings

Spin magnitude can change under conservative evolution.

Additional spin degrees of freedom relate to mass multipoles.

Effects are encoded in the eikonal phase.

Abstract

We show that physical scattering observables for compact spinning objects in general relativity can depend on additional degrees of freedom in the spin tensor beyond those described by the spin vector alone. The impulse, spin kick, and leading-order waveforms exhibit such a nontrivial dependence. A signal of this additional structure is the change in the magnitude of the spin vector under conservative Hamiltonian evolution, similar to our previous studies in electrodynamics. These additional degrees of freedom describe dynamical mass multipoles of compact objects and decouple for black holes. We also show that the conservative impulse, spin kick and change of the additional degrees of freedom are encoded in the eikonal phase.