Geometric Algebra, Gravity and Gravitational Waves

TL;DR

This paper explores gravitational waves using Geometric Algebra and Gauge Theory Gravity, deriving solutions for black holes and plane waves, and highlighting a preferred gauge that clarifies the memory effect and particle motion in non-linear waves.

Contribution

It introduces a novel approach to gravitational waves using Geometric Algebra, deriving exact solutions and identifying a preferred gauge that simplifies analysis and reveals physical effects.

Findings

A preferred gauge for gravitational plane waves is identified.

The memory effect manifests clearly in this gauge.

Exact solutions for particle motion in impulsive waves are provided.

Abstract

We discuss an approach to gravitational waves based on Geometric Algebra and Gauge Theory Gravity. After a brief introduction to Geometric Algebra (GA), we consider Gauge Theory Gravity, which uses symmetries expressed within the GA of flat spacetime to derive gravitational forces as the gauge forces corresponding to making these symmetries local. We then consider solutions for black holes and plane gravitational waves in this approach, noting the simplicity that GA affords in both writing the solutions, and checking some of their properties. We then go on to show that a preferred gauge emerges for gravitational plane waves, in which a `memory effect' corresponding to non-zero velocities left after the passage of the waves becomes clear, and the physical nature of this effect is demonstrated. In a final section we present the mathematical details of the gravitational wave treatment in GA, and link it with other approaches to exact waves in the literature. Even for those not reaching it via Geometric Algebra, we recommend that the general relativity metric-based version of the preferred gauge, the Brinkmann metric, be considered for use more widely by astrophysicists and others for the study of gravitational plane waves. These advantages are shown to extend to a treatment of joint gravitational and electromagnetic plane waves, and in a final subsection, we use the exact solutions found for particle motion in exact impulsive gravitational waves to discuss whether backward in time motion can be induced by strongly non-linear waves.