Interpreting spacetimes of any dimension using geodesic deviation

TL;DR

This paper introduces a universal method for interpreting the geometry and physics of spacetimes in any dimension by analyzing the relative motion of free particles, revealing detailed effects of gravitational fields and waves.

Contribution

It provides a systematic approach to decompose geodesic deviation into canonical components, applicable to higher-dimensional spacetimes, and demonstrates its utility with examples like pp-waves.

Findings

Gravitational effects can be decomposed into transverse, longitudinal, and Newton-Coulomb components.

Higher-dimensional gravitational waves exhibit multiple polarization states.

Deformations caused by higher-dimensional waves in 4D are not necessarily tracefree.

Abstract

We present a general method which can be used for geometrical and physical interpretation of an arbitrary spacetime in four or any higher number of dimensions. It is based on the systematic analysis of relative motion of free test particles. We demonstrate that local effect of the gravitational field on particles, as described by equation of geodesic deviation with respect to a natural orthonormal frame, can always be decomposed into a canonical set of transverse, longitudinal and Newton-Coulomb-type components, isotropic influence of a cosmological constant, and contributions arising from specific matter content of the universe. In particular, exact gravitational waves in Einstein's theory always exhibit themselves via purely transverse effects with D(D-3)/2 independent polarization states. To illustrate the utility of this approach we study the family of pp-wave spacetimes in higher dimensions and discuss specific measurable effects on a detector located in four spacetime dimensions. For example, the corresponding deformations caused by a generic higher-dimensional gravitational waves observed in such physical subspace, need not be tracefree.