Considerations about the incompleteness of the Ehrenfest’s theorem in quantum mechanics

Abstract

We describe a study motivated by our interest to examine the incompleteness of the Ehrenfest’s theorem in quantum mechanics and to resolve a doubt regarding whether or not the hermiticity of the Hamiltonian operator is sufficient to justify a simplification of the expression of the macroscopic-observable time derivative that promotes the one usually found in quantum-mechanics textbooks. The study develops by considering the simple quantum system ‘particle in one-dimensional box’. We propose theoretical arguments to support the incompleteness of the Ehrenfest’s theorem in the formulation he gave, in agreement with similar findings already published by a few authors, and corroborate them with the numerical example of an electric charge in an electrostatic field. The contents of this study should be useful to Bachelor and Master students; the style of the discussions is tailored to stimulate, we hope, the student’s ability for independent thinking.