Maxwellian mirages in general relativity

TL;DR

This paper examines the Maxwellian analogies in linear general relativity, highlighting how coordinate choices influence the form of the equations and the potential for misleading interpretations of gravitational phenomena.

Contribution

It clarifies the conditions under which Maxwellian forms in linear gravity are valid and warns against coordinate-dependent mirages that can mislead gravitational analysis.

Findings

Maxwellian formalism depends on coordinate choice and can be a mirage.

Coordinate choices can produce Maxwell-like equations that do not correspond to physical Lorentzian forces.

Inconsistent approximations can lead to misleading conclusions about gravitational waves.

Abstract

Maxwellian approximations to linear general relativity are revisited in light of relatively recent results on the degrees of freedom in the linear gravitational field. The well-known Maxwellian formalism obtained in harmonic coordinates is compared with a Maxwellian formalism obtained under a coordinate choice where each of the metric components corresponds to each of the coordinate-invariant degrees of freedom of the linear gravitational field. The coordinate freedom of general relativity can be exploited to cast the field equations into Maxwellian form, but such forms can be mere mirages of the coordinate choice -- mirages such as vector gravitational waves. A coordinate choice that yields perfectly-Maxwellian field equations, will yield a force equation that is not Lorentzian. If field definitions are chosen to obtain Lorentz-like terms in the force equation, then Maxwellian forms are compromised in the field equations. Many treatments of gravito-electromagnetism will make inconsistent ordering choices between the field equations and force equations, or else truncate terms of relevant order from the force equation. Often such mistakes reflect an attempt to force exact Maxwellian analogs simultaneously in both the field equations and the force equation, with the result that terms dropped are as large as those kept.