Post-1-Newtonian equations of motion for systems of arbitrarily structured bodies

TL;DR

This paper derives post-1-Newtonian equations of motion for arbitrarily structured bodies using surface integrals, extending previous weakly self-gravitating models to include strong internal gravity effects.

Contribution

It provides a surface integral derivation of post-1-Newtonian equations of motion for bodies with arbitrary structure, including strong internal gravity, generalizing prior weak-field results.

Findings

Derived explicit translational equations of motion.

Extended previous models to include strong internal gravity.

Unified definitions of multipole and tidal moments.

Abstract

We give a surface integral derivation of post-1-Newtonian translational equations of motion for a system of arbitrarily structured bodies, including the coupling to all the bodies' mass and current multipole moments. The derivation requires only that the post-1-Newtonian vacuum field equations are satisfied in weak-field regions between the bodies; the bodies' internal gravity can be arbitrarily strong. The derivation extends previous results of Damour, Soffel and Xu (DSX) for weakly self-gravitating bodies for which the post-1-Newtonian field equations are satisfied everywhere. The derivation consists of a number of steps: (i) The definition of each body's current and mass multipole moments and center-of-mass worldline in terms of the behavior of the metric in a weak-field region surrounding the body. (ii) The definition for each body of a set of gravitoelectric and gravitomagnetic tidal moments, again in terms of the behavior of the metric in a weak-field region surrounding the body. For the special case of weakly self-gravitating bodies, our definitions of these multipole and tidal moments agree with definitions given previously by DSX. (iii) The derivation of a formula, for any given body, of the second time derivative of its mass dipole moment in terms of its other multipole and tidal moments and their time derivatives. This formula was obtained previously by DSX for weakly self-gravitating bodies. (iv) A derivation of the relation between the tidal moments acting on each body and the multipole moments and center-of-mass worldlines of all the other bodies. A formalism to compute this relation was developed by DSX; we simplify their formalism and compute the relation explicitly. (v) The deduction from the previous steps of the explicit translational equations of motion, whose form has not been previously derived.