Justification of the Maximum Entropy Criterion in Quantum Mechanics

Araceli N. Proto

Petr Hájíček

B. Roy Frieden

A wide class of physical problems requires the estimation of probability laws. Diffraction patterns and quantum mechanical probability laws on position are examples. Minimum Fisher information is one approach to estimating such laws; maximum entropy is another. In this paper, we show that the minimum Fisher information approach may be derived from a prior principle of maximum Cramer-Rao bound (MCRB). The MCRB-Fisher approach is then applied to some fundamental physical problems, including diffraction theory and quantum mechanics. It is found to give the correct physical solutions, that is the Helmholtz and Schrödinger wave equations respectively. By comparison, the maximum entropy approach gives incorrect solutions to these problems, except in the special (thermodynamic) case of a harmonic oscillator potential.

Roger Balian, Service De Physique Théorique

We review with a tutorial scope the information theory foundations of quantum statistical physics. Only a small proportion of the variables that characterize a system at the microscopic scale can be controlled, for both practical and theoretical reasons, and a probabilistic description involving the observers is required. The criterion of maximum von Neumann entropy is then used for making reasonable inferences. It means that no spurious information is introduced besides the known data. Its outcomes can be given a direct justification based on the principle of indifference of Laplace. We introduce the concept of relevant entropy associated with some set of relevant variables; it characterizes the information that is missing at the microscopic level when only these variables are known. For equilibrium problems, the relevant variables are the conserved ones, and the Second Law is recovered as a second step of the inference process. For non-equilibrium problems, the increase of the relevant entropy expresses an irretrievable loss of information from the relevant variables towards the irrelevant ones. Two examples illustrate the flexibility of the choice of relevant variables and the multiplicity of the associated entropies: the thermodynamic entropy (satisfying the Clausius–Duhem inequality) and the Boltzmann entropy (satisfying the H-theorem). The identification of entropy with missing information is also supported by the paradox of Maxwell’s demon. Spin-echo experiments show that irreversibility itself is not an absolute concept: use of hidden information may overcome the arrow of time.

Ugo Farinelli, A. Gamba

Willy De Baere

The crucial role of counterfactual reasoning in generating inconsistencies between local subquantum theories and quantum mechanics is stressed. It is argued that the physical justification of such reasoning requires the fulfilment of a criterion of actualizability of hypothetical situations. It is demonstrated that in a subquantum framework this criterion is equivalent to the assumption of actual reproducibility of individual subquantum states. Therefore, inconsistencies between local subquantum theories and quantum mechanics disappear if a nonreproducibility property of nature, in full agreement with the empirical fact that past, present and future are basically distinct, is invoked. The proposal also leaves the universal validity of the locality principle untouched, in agreement with all existing empirical evidence.