Plane wave holonomies in loop quantum gravity I: symmetries and gauges

TL;DR

This paper investigates the classical behavior of SU(2) holonomies in loop quantum gravity under plane gravitational waves, focusing on symmetries, gauges, and non-local features in a weak-field, semiclassical regime.

Contribution

It analyzes the classical limits of LQG holonomies in plane wave backgrounds, highlighting non-local features that persist in the semiclassical approximation.

Findings

Weak fields lead to nonlinear field equations.

Classical limits retain non-local features from quantum theory.

Holonomies behave similarly to classical analogs in the semiclassical regime.

Abstract

This is the first of two papers which study the behavior of the SU(2) holonomies of loop quantum gravity (LQG), when they are acted upon by a unidirectional, plane gravity wave. Initially, the LQG flux-holonomy variables are treated as classical, commuting functions rather than quantized operators, in a limit where variation from vertex to vertex are small and fields are weak. Despite the weakness of the fields, the field equations are not linear. Also, the theory can be quantized, and the expectation values of the quantum operators behave like their classical analogs. Exact LQG theories may be either local or non-local. The present paper argues that a wide class of non-local theories share non-local features which survive to the semiclassical limit, and these non-local features are included in the classical limit studied here. An appendix computes the surface term required when the propagation direction is the real line rather than $\mathrm{S}_1$. Paper II introduces coherent states, constructs a damped sine wave solution to the constraints, and solves for the behavior of the holonomies in the presence of the wave.