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Physical Review A

Nature of the stochastic processes defined by Bohm’s momentum and quantum force

Boon Leong Lan

Published Feb 26, 2002• 3 citations
Publisher

Abstract

The nature of the stochastic process defined by Bohm's momentum is elucidated for stationary energy eigenstates and nonstationary states in classically nonchaotic and chaotic Hamiltonian systems. In addition, the nature of the stochastic process defined by Bohm's quantum force is elucidated for stationary energy eigenstates in nonchaotic and chaotic systems, and for nonstationary states in nonchaotic systems. From these results, the following can be concluded. For stationary energy eigenstates, the process defined by the momentum is generically a stationary, Dirac-delta process in both nonchaotic and chaotic systems; in contrast, the process defined by the quantum force is nongeneric. For nonstationary states, the processes defined by the momentum and quantum force are both nongeneric in nonchaotic systems. Furthermore, for nonstationary states, the process defined by the momentum is, with a high level of confidence, a stationary, stable, brown ${(f}^{\ensuremath{-}2}$ power spectrum) process in the chaotic kicked pendulum. It is conjectured that this is also true for other chaotic systems. The preceding conclusion and conjecture complement those in [Phys. Rev. A 63, 042105 (2001)] for the process defined by the quantum force for nonstationary states in chaotic systems.

DOI
10.1103/physreva.65.032117

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The nature of the stochastic process defined by Bohm's momentum is elucidated for stationary energy eigenstates and nonstationary states in classically nonchaotic and chaotic Hamiltonian systems. In addition, the nature of the stochastic process defined by Bohm's quantum force is elucidated for stationary energy eigenstates in nonchaotic and chaotic systems, and for nonstationary states in nonchaotic systems. From these results, the following can be concluded. For stationary energy eigenstates, the process defined by the momentum is generically a stationary, Dirac-delta process in both nonchaotic and chaotic systems; in contrast, the process defined by the quantum force is nongeneric. For nonstationary states, the processes defined by the momentum and quantum force are both nongeneric in nonchaotic systems. Furthermore, for nonstationary states, the process defined by the momentum is, with a high level of confidence, a stationary, stable, brown (f\ensuremath−2{(f}^{\ensuremath{-}2}(f\ensuremath−2 power spectrum) process in the chaotic kicked pendulum. It is conjectured that this is also true for other chaotic systems. The preceding conclusion and conjecture complement those in [Phys. Rev. A 63, 042105 (2001)] for the process defined by the quantum force for nonstationary states in chaotic systems.
New insight into the nature of Bohm's quantum force is presented. Based on our numerical study, we conclude that, for the kicked pendulum, Bohm's quantum-force time-series for nonstationary states is typically or generically non-Gaussian stable distributed with a flat power spectrum. For fixed system parameters and initial wave function, the stable parameters and the constant value of the power spectrum are independent of the initial Bohmian angle. We conjecture that these properties of the quantum-force time-series are also typical or generic for other classically chaotic Hamiltonian dynamical systems since the kicked pendulum is a prototypical member of this class of systems. A new method of calculating the quantum probability density of a particle's position implied by these quantum-force properties is described.
Previously [2004, AIP Conf. Proc., 750, 361], we considered a stochastic mechanics in the form of a stochastic differential equation mdxdt = μ(t) where μ(t) is a stochastic process defined by the set of Bohmian momentum time histories from an ensemble of particles. In this paper, we consider another stochastic differential equation d2xdt2 = 1m[−dV(x)dx + ζ(t)] where the stochastic force ζ(t) is a stochastic process defined by the set of quantum force time histories from an ensemble of particles. We show that, if the stochastic process is a purely random process characterized by an n‐th order joint probability density in the form of products of delta functions, then the stochastic mechanics with the stochastic force ζ(t) is equivalent to quantum mechanics in the sense that the former yields the same position probability density as the latter. However, for a particular non‐purely random process, we show that the stochastic mechanics with the stochastic force ζ(t) is not equivalent to quantum mechanics. Therefore, this stochastic mechanics is also generally not equivalent to quantum mechanics.
We show that a bounded, isolated quantum system of many particles in a specific initial state will approach thermal equilibrium if the energy eigenfunctions which are superposed to form that state obey Berry's conjecture. Berry's conjecture is expected to hold only if the corresponding classical system is chaotic, and essentially states that the energy eigenfunctions behave as if they were Gaussian random variables. We review the existing evidence, and show that previously neglected effects substantially strengthen the case for Berry's conjecture. We study a rarefied hard-sphere gas as an explicit example of a many-body system which is known to be classically chaotic, and show that an energy eigenstate which obeys Berry's conjecture predicts a Maxwell-Boltzmann, Bose-Einstein, or Fermi-Dirac distribution for the momentum of each constituent particle, depending on whether the wave functions are taken to be nonsymmetric, completely symmetric, or completely antisymmetric functions of the positions of the particles. We call this phenomenon eigenstate thermalization. We show that a generic initial state will approach thermal equilibrium at least as fast as O(\ensuremath{\Elzxh}/\ensuremath{\Delta})${\mathit{t}}^{\mathrm{\ensuremath{-}}1}$, where \ensuremath{\Delta} is the uncertainty in the total energy of the gas. This result holds for an individual initial state; in contrast to the classical theory, no averaging over an ensemble of initial states is needed. We argue that these results constitute a sound foundation for quantum statistical mechanics.
Introduction The apparent generality of the title to this chapter is appropriately reduced by sufficiently precisely defining the terms used. We have in mind the natural extension to quantum mechanical settings of the ideas usually covered in a discussion of stochastic processes in classical mechanical settings. In classical mechanical settings, the fundamental process is Brownian motion, which describes the influence of external fluctuating forces on a small particle. The mathematical apparatus developed for the study of Brownian motion has been generalized and applied to irreversible thermodynamics (Onsager and Machlup, 1953), fluctuating hydrodynamics (Fox and Uhlenbeck, 1970), light scattering from fluids and mixtures (Berne and Pecora, 1976) and chemical reactions (McQuarrie and Keizer, 1981). In each case, the stochastic process involves fluctuations generated by thermal motions. We are not considering the stochastic trajectories (chaos) of classically deterministic motion in non-linear systems (Lichtenberg and Lieberman, 1983), which is an entirely separate context for stochastic processes. Our goal in this chapter is to study the effects of thermal fluctuations in quantum mechanical settings. Therefore, we exclude a discussion of interpreting quantum mechanics as a classical stochastic motion (Nelson, 1966), an idea which should have been put to rest by Grabert, Hänggi and Talkner (1979). We also exclude a discussion of quantum chaos, which is currently a very active area of research (Zaslavsky, 1981). Just as Brownian motion provides the paradigm for stochastic processes in classical mechanical settings, the Anderson–Kubo oscillator (Anderson, 1954; Kubo, 1954; Kubo, Toda and Nashitsume, 1985) is the paradigm for stochastic processes in quantum mechanical settings.
The behavior of the periodically kicked pendulum is determined classically and quantum mechanically. Our earlier analytic study [Phys. Rev. A 41, 2952 (1990)] is confirmed and extended by the numerical results presented here. We are able to construct minimum uncertainty, Gaussian wave packets that maintain their form, and shadow the corresponding classical motion for long times when the classical motion is nonchaotic or only weakly chaotic. However, when the classical motion is robustly chaotic, the quantum-mechanical probability distribution grows macroscopically broad exponentially fast and the quantum-classical correspondence breaks down.