Point particles in 2+1 dimensions: general relativity and loop gravity descriptions

TL;DR

This paper develops a Hamiltonian framework for point particles in (2+1)-dimensional gravity, linking general relativity and loop gravity, and introduces a dynamical model that captures continuous evolution and discrete topological changes.

Contribution

It presents a unified Hamiltonian description of (2+1)-dimensional gravity with point particles using connection, frame-field, and loop variables, incorporating dynamical triangulation and graph-changing moves.

Findings

The model reproduces general relativity dynamics for point particles.

It demonstrates a concrete realization of graph-changing moves in loop quantum gravity.

The framework is suitable for quantization using existing loop quantum gravity methods.

Abstract

We develop a Hamiltonian description of point particles in (2+1)-dimensions using connection and frame-field variables for general relativity. The topology of each spatial hypersurface is that of a punctured two-sphere with particles residing at the punctures. We describe this topology with a CW complex (a collection of two-cells glued together along edges), and use this to fix a gauge and reduce the Hamiltonian. The equations of motion for the fields describe a dynamical triangulation where each vertex moves according to the equation of motion for a free relativistic particle. The evolution is continuous except for when triangles collapse (i.e. the edges become parallel) causing discrete, topological changes in the underlying CW complex. We then introduce the loop gravity phase space parameterized by holonomy-flux variables on a graph (a network of one-dimensional links). By embedding a graph within the CW complex, we find a description of this system in terms of loop variables. The resulting equations of motion describe the same dynamical triangulation as the connection and frame-field variables. In this framework, the collapse of a triangle causes a discrete change in the underlying graph, giving a concrete realization of the graph-changing moves that many expect to feature in full loop quantum gravity. The main result is a dynamical model of loop gravity which agrees with general relativity and is well-suited for quantization using existing methods.