Symplectic mechanics of relativistic spinning compact bodies I.: Covariant foundations and integrability around black holes

TL;DR

This paper develops a covariant Hamiltonian framework for the motion of spinning bodies in general relativity, demonstrating integrability in Kerr spacetime and implications for gravitational waveform modeling.

Contribution

It introduces a covariant, 10-dimensional Hamiltonian formalism for spinning particles, handling gauge degeneracies and proving integrability in Kerr backgrounds at linear spin order.

Findings

Hamiltonian system admits five integrals of motion.

Linear-in-spin corrections preserve integrability.

Trajectory analysis suggests non-chaotic, predictable motion.

Abstract

In general relativity, the motion of an extended body moving in a given spacetime can be described by a particle on a (generally non-geodesic) worldline. In first approximation, this worldline is a geodesic of the underlying spacetime, and the resulting dynamics admit a covariant and 4-dimensional Hamiltonian formulation. In the case of a Kerr background spacetime, the Hamiltonian was shown to be integrable by B.~Carter and the now eponymous constant. At the next level of approximation, the particle possesses proper rotation (hereafter \textit{spin}), which couples the curvature of spacetime and drives the representative worldline away from geodesics. In this article, we lay the theoretical foundations of a series of works aiming at exploiting the Hamiltonian nature of the equations governing the motion of a spinning particle, at linear order in spin. Our formalism is covariant and 10-dimensional. It handles the degeneracies inherent to the local Lorentz invariance of general relativity with tools from Poisson geometry, and accounts for the center-of-mass/spin-supplementary-condition using constrained Hamiltonian system theory. As a first application, we consider the linear-in-spin motion in a Kerr background. We show that the resulting Hamiltonian system admits exactly five functionally independent integrals of motion related to Killing symmetries, thereby proving that the system is integrable. We conclude that linear-in-spin corrections to the geodesic motion do not break integrability, and that the resulting trajectories are not chaotic. We explain how this integrability feature can be used to reduce the computational cost of waveform generation schemes for asymmetric binary systems of compact objects.