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arXiv:1309.6753 · Physical Review A

Semiclassical and quantum analysis of a free-particle Hermite wave function

P. Strange

Published Apr 1, 2014• 8 citations
arXivHTMLPDF

Abstract

In this Brief Report we discuss a solution of the free-particle Schr\"odinger equation in which the time and space dependence are not separable. The wave function is written as a product of exponential terms, Hermite polynomials, and a phase. The peaks in the wave function decelerate and then accelerate around $t=0$. We analyze this behavior within both a quantum and a semiclassical regime. We show that the acceleration does not represent true acceleration of the particle but can be related to the envelope function of the allowed classical paths. Comparison with other ``accelerating'' wave functions is also made. The analysis provides considerable insight into the meaning of the quantum wave function.

DOI
10.1103/physreva.89.044101

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Comment on "Semiclassical and Quantum Analysis of a Focussing Free Particle Hermite Wavefunction", by Paul Strange (arXiv:1309.6753 [quant-ph])

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In the recent and very enjoyable paper (Paul Strange, "Semiclassical and Quantum Analysis of a Focussing Free Particle Hermite Wavefunction", arXiv:1309.6753[quant-ph]), Professor Strange has studied a particular solution of the free particle Schroedinger equation in which the time and space dependence are not separable. After recognizing the fact that "The Schroedinger equation has an identical mathematical form to the paraxial wave equation [...]", he claims to "describe and try to gain insight into an exotic, apparently accelerating solution of the free particle Schroedinger equation that is square integrable and which also displays some unusual characteristics." It is the main aim of this short note to show that the wavefunction described by Prof. Strange is simply one particular Hermite-Gauss solution of the paraxial wave equation and even more "exotic" examples can be found by considering, e.g., Laguerre-Gauss solutions.

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In this Brief Report we discuss a solution of the free-particle Schr"odinger equation in which the time and space dependence are not separable. The wave function is written as a product of exponential terms, Hermite polynomials, and a phase. The peaks in the wave function decelerate and then accelerate around t=0t=0t=0. We analyze this behavior within both a quantum and a semiclassical regime. We show that the acceleration does not represent true acceleration of the particle but can be related to the envelope function of the allowed classical paths. Comparison with other ``accelerating'' wave functions is also made. The analysis provides considerable insight into the meaning of the quantum wave function.
In this paper we discuss a solution of the free particle Schroodinger equation in which the time and space dependence are not separable. The wavefunction is written as a product of exponential terms, Hermite polynomials and a phase. The peaks in the wavefunction decelerate and then accelerate around t = 0. We analyse this behaviour within both a quantum and a semi-classical regime. We show that the acceleration does not represent true acceleration of the particle but can be related to the envelope function of the allowed classical paths. Comparison with other accelerating wavefunctions is also made. The analysis provides considerable insight into the meaning of the quantum wavefunction.
In the recent and very enjoyable paper (Paul Strange, "Semiclassical and Quantum Analysis of a Focussing Free Particle Hermite Wavefunction", arXiv:1309.6753[quant-ph]), Professor Strange has studied a particular solution of the free particle Schroedinger equation in which the time and space dependence are not separable. After recognizing the fact that "The Schroedinger equation has an identical mathematical form to the paraxial wave equation [...]", he claims to "describe and try to gain insight into an exotic, apparently accelerating solution of the free particle Schroedinger equation that is square integrable and which also displays some unusual characteristics." It is the main aim of this short note to show that the wavefunction described by Prof. Strange is simply one particular Hermite-Gauss solution of the paraxial wave equation and even more "exotic" examples can be found by considering, e.g., Laguerre-Gauss solutions.
Despite the didactic importance of a free particle in quantum mechanics, its coherent state analysis has long been untouched. It is only recently that it has been noticed and studied in the semiclassical domain. While the previously known solutions, reported by Bagrov et al. for a free particle, are described using the linear non-Hermitian invariant operator, we show in this work that the general solution of the Schrödinger equation can also be naturally derived using a simpler method based on an Hermitian linear invariant operator. According to this, an exact Gaussian wave function that corresponds to a coherent state solution is obtained. An interpretation for such general quantum solution designated within the Lewis-Riesenfeld framework is provided and, further, quantum-classical correspondence principle for the system is reexamined.
We analyze the non-Markovian dynamics of a quantum system subject to spontaneous collapse in space. After having proved, under suitable conditions, the separation of the center-of-mass and relative motions, we focus our analysis on the time evolution of the center of mass of an isolated system (free-particle case). We compute the explicit expression of the Green's function, for a generic Gaussian noise, and analyze in detail the case of an exponential correlation function. We study the time evolution of average quantities, such as the mean position, momentum, and energy. We eventually specialize to the case of Gaussian wave functions and prove that all basic facts about collapse models, which are known to be true in the white-noise case, hold also in the more general case of non-Markovian dynamics.
A new method is proposed for constructing approximate solutions to the Schrödinger equation. In place of the wave function, its Gaussian-windowed Fourier transform is used as the fundamental entity. This allows an intuitively attractive connection to be made with a family of classical trajectories and, at all times, the wave function is inferred from the present state of these trajectories. The fact that the connection between the wave function and the classical trajectories is consistently constructed in phase space allows this method to be free of the limitations of other methods.
In spite of outstanding achievement of the quantum theory, the theory is under never-ending judgment due to probabilistic character and certain unresolved paradoxes arising from the very fundamentals. Additionally, it is debated that a more sophisticated theory beyond the quantum theory emerges as a need for a better understanding of the atomic and sub atomic worlds. This paper, for that reason, focuses on the very fundamental structure of the quantum theory such as the physical and philosophical meanings of the wave functions, the probability and the Schrödinger Wave Equation (SWE). Specifically deals with the time evolution of the free quantum particles. The work just arises from the difference of the classical and quantum waves, specifically the comparison of the time and space dependence of classical waves and Schrödinger waves, for the free space, indicates that time dependence shows a clear dissimilarity. In order to tackle the problem, a time dependent momentum operator, for the first time, is defined and consequently an alternative TDSWE is straightforwardly derived and its philosophical implications have been discussed. Additionally, time and position dependence of the probability functions, relating free particles, have been physically and philosophically discussed and some alternative approaches and comments are forwarded. Finally, the approaches, comments and mathematical equations are in good agreement with the standard quantum theory and interestingly go further possibly tracing some signs of new pathways to resolve the mysterious quantum world. Keywords: Schrödinger wave equation, fundamentals of quantum mechanics, momentum operator, quantum probabilities, time evolution of quantum systems.