Gravitational statistical mechanics: a model

Using the quantum Hamiltonian for a gravitational system with boundary, we find the partition function and derive the resulting thermodynamics. The Hamiltonian is the boundary term required by functional differentiability of the action for the Lorentzian general relativity. In this model, states of quantum geometry are represented by spin networks. We show that the statistical mechanics of the model reduces to that of a simple non-interacting gas of particles with spin. Using both canonical and grand canonical descriptions, we investigate two temperature regimes determined by the fundamental constant in the theory, in. In the high temperature limit (kT much greater than in), the model is thermodynamically stable. For low temperatures (kT much less than in) and for macroscopic areas of the bounding surface, the entropy is proportional to the area (with logarithmic correction), providing a simple derivation of the Bekenstein-Hawking result. By comparing our results to known semiclassical relations we are able to fix the fundamental scale in. Also in the low temperature, macroscopic limit, the quantum geometry on the boundary forms a 'condensate' in the lowest energy level (j = 1/2).