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European Journal of Physics

The position–momentum correlation for quantum and classical probability distributions

Natascha Riahi

Published Feb 12, 2013• 6 citations
Publisher

Abstract

We compare the definition and the interpretation of the position–momentum correlation in classical statistical physics and quantum mechanics. The introduction of a regional momentum distribution makes a rigorous discussion in both theories possible. We investigate the mutual dependence of position and momentum for one- and two-centred probability distributions.

DOI
10.1088/0143-0807/34/2/461

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We compare the definition and the interpretation of the position–momentum correlation in classical statistical physics and quantum mechanics. The introduction of a regional momentum distribution makes a rigorous discussion in both theories possible. We investigate the mutual dependence of position and momentum for one- and two-centred probability distributions.
Present quantum theory, which is statistical in nature, does not predict joint probability distribution of position and momentum because they are noncommuting. We propose a deterministic quantum theory which predicts a joint probability distribution such that the separate probability distributions for position and momentum agree with usual quantum theory. Unlike the Wigner distribution the suggested distribution is positive definite. The theory predicts a correlation between position and momentum in individual events.
It is shown that quantum mechanics cannot be formulated as a stochastic theory involving a probability distribution function of position and momentum. This is done by showing that the most general distribution function which yields the proper quantum mechanical marginal distributions cannot consistently be used to predict the expectations of observables if phase space integration is used. Implications relating to the possibility of establishing a “hidden” variable theory of quantum mechanics are discussed.
The discovery that any classical random variable with all moments gives rise to a full quantum theory (that in the Gaussian and Poisson cases coincides with the usual one) implies that a quantum-type formalism will enter into practically all applications of classical probability and statistics. The new challenge consists in finding the classical interpretation, for different types of classical contexts, of typical quantum notions such as entanglement, normal order, equilibrium states, etc. As an example, every classical symmetric random variable has a canonically associated conjugate momentum. In usual quantum mechanics (associated with Gaussian or Poisson classical random variables), the interpretation of the momentum operator was already clear to Heisenberg. How should we interpret the conjugate momentum operator associated with classical random variables outside the Gauss-Poisson class? The Introduction is intended to place in historical perspective the recent developments that are the main object of the present exposition.
We show that the quantum wavefunction, interpreted as the probability density of finding a single non-localized quantum particle, which evolves according to classical laws of motion, is an intermediate description of a material quantum particle between the quantum and classical realms. Accordingly, classical and quantum mechanics should not be treated separately, a unified description in terms of the Wigner distribution function being possible. Although defined on classical phase space coordinates, the Wigner distribution function accommodates the nonlocalization property of quantum systems, and leads to both the Schrodinger equation for the quantum wavefunction and to the definition of position and momentum operators.