Local symmetries as constraints on the motion of freely-falling extended bodies

TL;DR

This paper establishes universal constraints on the motion of extended bodies in gravity, linking local symmetries to internal structure limitations and spacetime properties in both Newtonian and relativistic contexts.

Contribution

It introduces the concept of local symmetries as constraints on extended-body motion, connecting internal structure effects to the algebraic properties of the tidal tensor and spacetime symmetries.

Findings

Certain quadrupole components do not influence motion.

Forces and torques are constrained by local symmetries.

Local symmetries are prevalent across various spacetime types.

Abstract

Different extended objects can fall in different ways, depending on their internal structures. Some motions are nevertheless impossible, regardless of internal structure. This paper derives universal constraints on extended-body motion, both in Newtonian gravity and in general relativity. In both theories, we identify a weak notion of "local symmetry" which precludes certain force and torque combinations. Local symmetries imply that certain components of a body's quadrupole moment cannot affect its motion. They also imply that some forces arise only in combination with appropriate torques. Many of these symmetries are shown to be determined by the algebraic structure of the tidal tensor. In general relativity, we thus relate qualitative features of extended-body motion to the Petrov type of the spacetime. Doing so shows that local symmetries are in fact ubiquitous. In general relativity, there are at least two in all algebraically-special spacetimes. Some of these are generated by Killing vectors and some by conformal Killing-Yano tensors. However, many local symmetries do not fall into either of these classes.