The zeroth principle of thermodynamics in the form "temperature is uniform at equilibrium" is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.
I. NON-UNIFORM EQUILIBRIUM TEMPERATUREAccording to non-relativistic thermodynamics, a thermometer (say, a line of mercury in a glass tube), moved up and down a column of gas at equilibrium in a constant gravitational field, measures a uniform temperature. But this prediction is wrong. Relativistic effects make the gas warmer at the bottom and cooler at the top, by a correction proportional to c −2 , where c is the speed of light. This is the well known Tolman-Ehrenfest effect, discovered in the thirties [1, 2] and later derived in a variety of different manners [3][4][5][6][7][8][9][10][11]. The temperatures T 1 and T 2 measured by the same thermometer at two altitudes h 1 and h 2 in a Newtonian potential Φ(h) are related by the Tolman lawThe general-covariant version of this law readswhere |ξ| is the norm of the timelike Killing field with respect to which equilibrium is established. A violation of the uniformity of temperature seems counterintuitive at first, especially if one has in mind a definition of "temperature" as a label of the equivalence classes of all systems in equilibrium with one another. In a relativistic context a physical thermometer does not measure this label and we must therefore distinguish two notions: (i) a quantity T o defined as this label (proportional to the constant in (2)), and (ii) the temperature T measured by a standard thermometer.In the micro-canonical framework the entropy S(E) is the logarithm of the number of microstates N (E) that have energy E and T can be identified with the inverse of the derivative of S(E),where k is the Boltzmann constant. The fact that two systems in equilibrium have the same T can be derived by maximizing the total number of states N = N 1 N 2 under an energy transfer dE between the two. This gives easily T 1 = T 2 . In the presence of relativistic gravity, this derivation fails because conservation of energy becomes tricky: intuitively speaking, the energy dE reaching the upper system is smaller than the one leaving the lower system because "energy weighs". Is there a more general statistical argument that governs equilibrium in a relativistic context? Can the Tolman law be derived from a principle generalizing the maximization of the number of micro...