The non-zero energy of 2+1 Minkowski space

Abstract

We compute the energy of 2+1 Minkowski space from a covariant action principle. Using Ashtekar and Varadarajan's characterization of 2+1 asymptotic flatness, we first show that the 2+1 Einstein-Hilbert action with Gibbons-Hawking boundary term is both finite on-shell (apart from past and future boundary terms) and stationary about solutions under arbitrary smooth asymptotically flat variations of the metric. Thus, this action provides a valid variational principle and no further boundary terms are required. We then obtain the gravitational Hamiltonian by direct computation from this action. The result agrees with the Hamiltonian of Ashtekar and Varadarajan up to an overall addititve constant. This constant is such that 2+1 Minkowski space is assigned the energy E = -1/4G, while the upper bound on the energy is set to zero. Any variational principle with a boundary term built only from the extrinsic and intrinsic curvatures of the boundary is shown to lead to the same result. Interestingly, our result is not the flat-space limit of the corresponding energy -1/8G of 2+1 anti-de Sitter space.

arXiv comment16 pages, minor changes

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• Symmetries and conservation laws in Lagrangian gauge theories with applications to the mechanics of black holes and to gravity in three dimensions
• Covariant Counterterms and Conserved Charges in Asymptotically Flat Spacetimes
• Variational principle and 1-point functions in 3-dimensional flat space Einstein gravity
• Note on counterterms in asymptotically flat spacetimes
• Combing gravitational hair in 2+1 dimensions
• Energy in first order 2+1 gravity
• The energy and stability of D-term strings
• The point-particle solution and the asymptotic flatness in 2+1 Horava gravity
• Vacuum energy in asymptotically flat 2+1 gravity
• Conserved charges in (Lovelock) gravity in first order formalism