Gravitational Radiation Reaction

TL;DR

This paper develops a method to derive the equations of motion for a spinning particle in curved spacetime, accounting for gravitational radiation reaction via tail effects, using matched asymptotic expansions to avoid divergences.

Contribution

It introduces a novel approach combining matched asymptotic expansions and tail effects to derive radiation reaction forces for spinning particles in curved backgrounds.

Findings

Reaction force is due to gravitational wave tails and depends on the particle's entire history.

Equations of motion reduce to geodesic motion on a regularized metric including tail effects.

Spin-induced forces are locally expressed in terms of curvature and spin tensors.

Abstract

We consider the radiation reaction to the motion of a point-like particle of mass $m$ and specific spin $S$ traveling on a curved background. Assuming $S=O(Gm)$ and $Gm\ll L$ where $L$ is the length scale of the background curvature, we divide the spacetime into two regions; the external region where the metric is approximated by the background metric plus perturbations due to a point-like particle and the internal region where the metric is approximated by that of a black hole plus perturbations due to the tidal effect of the background curvature, and use the technique of the matched asymptotic expansion to construct an approximate metric which is valid over the entire region. In this way, we avoid the divergent self-gravity at the position of the particle and derive the equations of motion from the consistency condition of the matching. The matching is done to the order necessary to include the effect of radiation reaction of $O(Gm)$ with respect to the background metric as well as the effect of spin-induced force. The reaction term of $O(Gm)$ is found to be completely due to tails of radiation, that is, due to curvature scattering of gravitational waves. In other words, the reaction force is found to depend on the entire history of the particle trajectory. Defining a regularized metric which consists of the back- ground metric plus the tail part of the perturbed metric, we find the equations of motion reduce to the geodesic equation on this regularized metric, except for the spin-induced force which is locally expressed in terms of the curvature and spin tensors. Some implications of the result and future issues are briefly discussed.