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arXiv:quant-ph/9812011

A realistic interpretation of the density matrix I: Basic concepts

A. Raiteri

Published Dec 4, 1998• 1 citation
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Abstract

A realistic interpretation of Schroedinger and Dirac equations for density matrices is proposed, in which the difference between the position arguments of the density matrix is considered as an objective extra space dimension. "Particle" solutions are found, which are perfectly localized both in position space and in momentum space (the position and momentum operators commute in the density matrix representation); definitions for all observable quantities are given and the values associated to the "particle" solutions are the correct ones, both for the non-relativistic and the relativistic case. Finally, a non-linear interaction (the electromagnetic one) is introduced in an attempt to single out the "particle" solutions of the Dirac equation from all other solutions; the dynamical evolution of the electromagnetic field is described by the classical (unquantized) Maxwell equations.

DOI
10.48550/arxiv.quant-ph/9812011

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A realistic interpretation of Schroedinger and Dirac equations for density matrices is proposed, in which the difference between the position arguments of the density matrix is considered as an objective extra space dimension. "Particle" solutions are found, which are perfectly localized both in position space and in momentum space (the position and momentum operators commute in the density matrix representation); definitions for all observable quantities are given and the values associated to the "particle" solutions are the correct ones, both for the non-relativistic and the relativistic case. Finally, a non-linear interaction (the electromagnetic one) is introduced in an attempt to single out the "particle" solutions of the Dirac equation from all other solutions; the dynamical evolution of the electromagnetic field is described by the classical (unquantized) Maxwell equations.
A realistic interpretation of Schroedinger and Dirac equations for density matrices is proposed, in which the difference between the position arguments of the density matrix is considered as an "objective" extra space dimension. "Particle" solutions are found, which are perfectly localized both in position space and in momentum space (the position and momentum operators commute in the density matrix representation); definitions for all observable quantities are given and the values associated to the "particle" solutions are the correct ones, both for the non-relativistic and the relativistic case. Finally, a non-linear interaction (the electromagnetic one) is introduced in an attempt to single out the "particle" solutions of the Dirac equation from all other solutions; the dynamical evolution of the electromagnetic field is described by the classical (unquantized) Maxwell equations.
The interpretation proposed in quant-ph/9812011 is extended to the general case of a non-relativistic particle moving in an arbitrary external potential. It is shown that, even in this general case, "particle" solutions exist which do not spread out with time, and remain well localized around their center of mass; it is postulated that these are the only solutions which represent individual physical particles. As a consequence two basic principles of standard QM, namely the superposition principle and the wave-function collapse, are shown to have no ontological meaning. Three simple applications of our approach are then examined: the free particle, the linear harmonic oscillator and the delta barrier potential; the corresponding "particle" solutions are explicitly shown. Finally, it is argued that the persisting confusion about the meaning of the wave-function (does it represent an individual particle or a statistical ensemble?) calls for a non-linear extension of the Schroedinger equation.
The objective of this series of three papers is to axiomatically derive quantum mechanics from classical mechanics and two other basic axioms. In this first paper, Schreodinger's equation for the density matrix is fist obtained and from it Schroedinger's equation for the wave functions is derived. The momentum and position operators acting upon the density matrix are defined and it is then demonstrated that they commute. Pauli's equation for the density matrix is also obtained. A statistical potential formally identical to the quantum potential of Bohm's hidden variable theory is introduced, and this quantum potential is reinterpreted through the formalism here proposed. It is shown that, for dispersion free {\it ensembles% }, Schroedinger's equation for the density matrix is equivalent to Newton's equations. A general non-ambiguous procedure for the construction of operators which act upon the density matrix is presented. It is also shown how these operators can be reduced to those which act upon the wave functions.
The search for a theory of the elementary particles which is founded on the well-established principles of quantum mechanics and conforms at the same time with the requirements of the principle of relativity has, in recent years, taken several divergent directions. On the one hand, the second quantization of wave fields derived from a Lagrangian by a variational procedure(1) has succeeded in accounting for the existence and most of the properties of the electron, the photon, and the meson. On the other hand, many generalizations of the Dirac wave equation of the electron(2) have been attempted, with applications to the meson(3) and the proton(4). Heisenberg(5) has considered the much more difficult problem of the interaction between different particles, and has found that the key to the situation is the so-called ‘scattering matrix’, which is nothing other than a limiting form of the relativistic density matrix, as defined in § 2 of this paper. It seems probable that the relativistic density matrix ρ; or statistical operator, as it may be called without reference to representation, will play an important part in relativistic quantum mechanics in the future. It satisfies the same equation as the wave function, but differs from it in being a real linear operator, or a dynamical variable, in the terminology of Dirac.
The objective of this series of three papers is to axiomatically derive quantum mechanics from classical mechanics and two other basic axioms. In this first paper, Schroedinger's equation for the density matrix is fist obtained and from it Schroedinger's equation for the wave functions is derived. The momentum and position operators acting upon the density matrix are defined and it is then demonstrated that they commute. Pauli's equation for the density matrix is also obtained. A statistical potential formally identical to the quantum potential of Bohm's hidden variable theory is introduced, and this quantum potential is reinterpreted through the formalism here proposed. It is shown that, for dispersion free {\\it ensembles% }, Schroedinger's equation for the density matrix is equivalent to Newton's equations. A general non-ambiguous procedure for the construction of operators which act upon the density matrix is presented. It is also shown how these operators can be reduced to those which act upon the wave functions.
Probabilistic description of results of measurements and its consequences for understanding quantum mechanics are discussed. It is shown that the basic mathematical structure of quantum mechanics like the probability amplitudes, Born rule, probability density current, commutation and uncertainty relations, momentum operator, rules for including scalar and vector potentials and antiparticles can be derived from the definition of the mean values of powers of space coordinates and time. Equations of motion of quantum mechanics, the Klein-Gordon equation, Schrödinger equation and Dirac equation are obtained from the requirement of the relativistic invariance of the theory. The limit case of localized probability densities leads to the Hamilton-Jacobi equation of classical mechanics. Many-particle systems are also discussed.