Geometry of Thermodynamic States

Abstract

A novel geometric formalism for statistical estimation is applied here to the canonical distribution of classical statistical mechanics. In this scheme thermodynamic states, or equivalently, statistical mechanical states, can be characterised concisely in terms of the geometry of a submanifold ${\cal M}$ of the unit sphere ${\cal S}$ in a real Hilbert space ${\cal H}$. The measurement of a thermodynamic variable then corresponds to the reduction of a state vector in ${\cal H}$ to an eigenstate, where the transition probability is the Boltzmann weight. We derive a set of uncertainty relations for conjugate thermodynamic variables in the equilibrium thermodynamic states. These follow as a consequence of a striking thermodynamic analogue of the Anandan-Aharonov relations in quantum mechanics. As a result we are able to provide a resolution to the controversy surrounding the status of `temperature fluctuations' in the canonical ensemble. By consideration of the curvature of the thermodynamic trajectory in its state space we are then able to derive a series of higher order variance bounds, which we calculate explicitly to second order.