We study the dynamics of particles coupled to gravity in (2 + 1) dimensions. Using the ADM formalism, we derive the general Hamiltonian for an N -body system and analyze the dynamics of a two-particle system. Nonlinear terms are found up to second order in κ in the general case, and to every order in the quasi-static limit.
IntroductionThe study of 2+1 dimensional gravity [1,2,3,4,5,6,7,8,9] has been around for several decades. Its attraction was rooted in its mathematical simplicity, which afforded some insight into the dynamics and behaviour of general relativistic gravity. Indeed, in this framework, the Einstein tensor can be expressed in terms of the curvature tensor, such that vacuum must be locally flat. This framework became even more popular after the discovery of the BTZ black hole [10,11] and is now a standard tool practitioners of quantum gravity employ in understanding their subject [12,13,14].We are concerned here with the problem of N -body dynamics, which is a long-standing one in relativity due to its notorious difficulty. In lower dimensional settings, this problem becomes much simpler. For example, the general form of the solution has been obtained for lineal gravity [15] for arbitrary N , after which a variety of exact solutions for N = 2 were obtained in various contexts that include both charge and cosmological expansion/contraction [16,17,18,19], by investigating the Hamiltonian of such a system through canonical reduction. Several interesting exact solutions to the N -body equilibrium problem [20,21,22,23] in (1+1) dimensions have also been obtained.In this paper, we follow a similar approach in (2+1) gravity to obtain canonical equations of motion to analyze a two-body system. The analysis of the N -body problem in (2+1) dimensions also has an interesting history, beginning with construction of a spinning point-particle solution [4] and then a consideration of the quantum scattering problem [24]. Further developments came upon realizing that the problem could be analyzed from a topological perspective [25], and an implicit solution for the metric and the motion of N interacting particles was obtained [26]. Based on a mapping from multivalued Minkowskian coordinates to single-valued ones, it becomes explicit for two particles with any speed and for any number of particles with small speed. It is possible to show that the collision of point particles in 2+1 AdS spacetime can result in the formation of a black hole [27].The connection between these approaches and more traditional canonical methods as employed in (1+1) [15] and (3+1) dimensions [28] has not been explicated. In this paper, we address this issue. We begin by finding the total action corresponding to the system, which consists of the Einstein-Hilbert action for the field as well as a term corresponding to coupling gravity to matter. A variational approach will then lead to coordinate conditions and constraints, which will in turn produce the total Hamiltonian as an expansion in powers of the gravitational coupling κ. The Hamilton...