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arXiv:1011.2852

Non-differentiable Bohmian trajectories

Gebhard Grübl, Markus Penz

Published Nov 12, 2010• 1 citation
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Abstract

A solution $ψ$ to Schrödinger's equation needs some degree of regularity in order to allow the construction of a Bohmian mechanics from the integral curves of the velocity field $\hbar \Im \left( \bigtriangledown ψ/mψ\right) .$ In the case of one specific non-differentiable weak solution $Ψ$ we show how Bohmian trajectories can be obtained for $Ψ$ from the trajectories of a sequence $Ψ_{n}\rightarrow Ψ.$ (For any real $t$ the sequence $Ψ_{n}\left( t,\cdot \right) $ converges strongly.) The limiting trajectories no longer need to be differentiable. This suggests a way how Bohmian mechanics might work for arbitrary initial vectors $Ψ$ in the Hilbert space on which the Schrödinger evolution $% Ψ\mapsto e^{-iht}Ψ$ acts.

DOI
10.48550/arxiv.1011.2852

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A solution ψψψ to Schrödinger's equation needs some degree of regularity in order to allow the construction of a Bohmian mechanics from the integral curves of the velocity field ℏℑ(▽ψ/mψ).\hbar \Im \left( \bigtriangledown ψ/mψ\right) .ℏℑ(▽ψ/mψ). In the case of one specific non-differentiable weak solution ΨΨΨ we show how Bohmian trajectories can be obtained for ΨΨΨ from the trajectories of a sequence Ψn→Ψ.Ψ_{n}\rightarrow Ψ.Ψn​→Ψ. (For any real ttt the sequence Ψn(t,⋅)Ψ_{n}\left( t,\cdot \right) Ψn​(t,⋅) converges strongly.) The limiting trajectories no longer need to be differentiable. This suggests a way how Bohmian mechanics might work for arbitrary initial vectors ΨΨΨ in the Hilbert space on which the Schrödinger evolution % Ψ\mapsto e^{-iht}Ψ acts.
Bohmian mechanics is a non-relativistic quantum theory based on a particle approach. In this paper, we study the Schrödinger equation with a rapidly oscillating potential and the associated Bohmian trajectory. We prove that the corresponding Bohmian trajectory converges locally in a measure, and the limit coincides with the Bohmian trajectory for the effective Schrödinger equation on a finite time interval. This is beneficial for efficient simulation of the Bohmian trajectories in oscillating potential fields.
de Broglie-Bohm theory (dBBT), treating quantum particles as point objects moving along well defined (Bohmian) trajectories, offers an appealing solution of the measurement problem in quantum mechanics; it has, however, problems relating to spin, relativity and lack of proper integration with the Hilbert space based framework. In this work, we present a consistent formalism which has the traditional state-observable framework integrated with the desirable features of dBBT. We adopt ensemble interpretation for the Schrodinger wave function ψψψ. Given a Schrodinger wave function ψψψ, we use its value ψ0ψ_0ψ0​ at some fixed time (say, t=0t = 0t=0) to define the probability measure ∣ψ0∣2dx|ψ_0|^2 {\rm d}x∣ψ0​∣2dx on the system configuration space MMM (=Rn=\mathbb{R}^n=Rn). On the resulting probability space M0\mathcal{M}_0M0​, we introduce a stochastic process ξ(t)ξ(t)ξ(t) corresponding to the Heisenberg position operator XH(t)X_H(t)XH​(t) such that, in the Heisenberg state ∣ψh⟩|ψ_h\rangle∣ψh​⟩ corresponding to ψ0ψ_0ψ0​, the expectation value of XH(t)X_H(t)XH​(t) equals that of ξ(t)ξ(t)ξ(t) in M0\mathcal{M}_0M0​. This condition leads to the de Broglie-Bohm guidance equation for the sample paths of the process ξ(t)ξ(t)ξ(t) which are, therefore, Bohmian trajectories supposedly representing time-evolutions of individual members of the ψ0ψ_0ψ0​-ensemble. Stochastic processes and Bohmian trajectories corresponding to observables with discrete eigenvalues (in particular spin) are treated by extending the configuration space to the spectral space of the commutative algebra obtained by adding appropriate discrete observables to the position observables. Pauli's equation is treated as an example. A straightforward derivation of von Neumann's projection rule employing the Schrodinger-Bohm evolution of individual systems along their Bohmian trajectories is given. Some comments on the potential application of the formalism developed here to quantum mechanics of the universe are included.
Abstract Chaos in Bohmian quantum mechanics is an open field of research. In general, most of the 3D Bohmian trajectories are free to wander around the 3D space. However, there are cases where the evolution of the trajectories is dictated by exact or approximate integrals of motion. A first case corresponds to partial integrability, where the trajectories (ordered and chaotic) evolve on certain integral surfaces. A second case corresponds to ordered trajectories. In this paper we extend our previous work in 3D Bohmian chaos by using both forms of integrability and discuss their physical implications.
We present here the exact, trajectory based wave-mechanical description of particle dynamics provided by the time-independent Schroedinger equation, of which the time-dependent equation allows a statistical treatment. Let us remind, for comparison, that Bohmian mechanics is largely recognized to provide a basically hydrodynamic formulation of the quantum theory, describing the evolution of physical systems in terms of statistical streamlines which indicate the regions of space where the moving particles preferentially gather: the very concept of true particle path is avoided, in Bohmian theory, by a weak, non-perturbing measurement. In the present paper the wave-dynamical Hamiltonian equations providing the exact particle trajectories are directly obtained from the time-independent Schroedinger equation, and numerical solutions are presented for a number of different external potentials acting on the moving particles. A Wave-Potential function (holding for any - even classical - wave process described by a Helmholtz-like equation) is shown to be the one and only cause of wave-like features such as diffraction and interference, and to act, for matter waves, without any wave-particle energy exchange.
It is often argued that measurable predictions of Bohmian mechanics cannot be distinguished from those of a theory with arbitrarily modified particle velocities satisfying the same equivariance equation. By considering the wave function of a closed system in a state with definite total energy, we argue that a distinction in measurable predictions is possible. Even though such a wave function is time-independent, the conditional wave function for a subsystem depends on time through the time-dependent particle trajectories not belonging to the subsystem. If these trajectories can be approximated by classical trajectories, then the conditional wave function can be approximated by a wave function which satisfies Schrodinger equation in a classical time-dependent potential, which is in good agreement with observations. However, such an approximation cannot be justified for particle velocities significantly deviating from the Bohmian ones, implying that Bohmian velocities are observationally preferred.