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Brazilian Journal of Physics

On the linearity of the Schrödinger equation

Iwo Białynicki‐Birula

Published Jun 1, 2005• 11 citations
Publisher

Abstract

The problem of the linearity of the Schrödinger equation is described from a historical perspective. It is argued that the Schrödinger picture on which this equation is based cannot be retained in relativistic quantum theory. A closer analysis of realistic experiments might offer a clue how to modify the evolution equation for the state vectors in quantum field theory.

DOI
10.1590/s0103-97332005000200003

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The problem of the linearity of the Schrödinger equation is described from a historical perspective. It is argued that the Schrödinger picture on which this equation is based cannot be retained in relativistic quantum theory. A closer analysis of realistic experiments might offer a clue how to modify the evolution equation for the state vectors in quantum field theory.
It is argued that the Schrödinger equation does not yield a correct description of the quantum-mechanical time evolution of states of isolated (open) systems featuring events. A precise general law for the time evolution of states replacing the Schrödinger equation is formulated within the so-called ETH-Approach to Quantum Mechanics. This law eliminates the infamous "measurement problem." Our general results are illustrated by an analysis of simple models describing a very heavy atom coupled to the quantized radiation field in a limit where the speed of light tends to infinity. The discussion of these models is the main subject of our paper.
The nature of electron spin has presented an enigma right from the beginning of quantum mechanics. We suggest that a simple realistic picture of a real coherently rotating vector field can account for both the Schrodinger equation and electron spin in a consistent manner. Such a rotating field carries distributed angular momentum and energy in the same way as a circularly polarized electromagnetic wave. We derive the Schrodinger equation from the relativistic Klein-Gordon Equation, where the complex wave function maps onto a fixed-axis real rotating vector. Such a realistic picture can also explain the Stern-Gerlach experiment which first identified electron spin. Remarkably, the predictions of a two-stage Stern-Gerlach experiment within this realistic picture differ from those of the orthodox quantum superposition approach. This two-stage experiment has not actually been done, and could provide insights into the limits of realistic models. This realistic picture also avoids quantum paradoxes and enables realistic explanations for a variety of quantum phenomena.
A number of authors have proposed stochastic versions of the Schrödinger equation, either as effective evolution equations for open quantum systems or as alternative theories with an intrinsic collapse mechanism. Here we discuss two directions for the generalization of these equations. First, we study a general class of norm preserving stochastic evolution equations, and show that even after making several specializations there is an infinity of possible stochastic Schrödinger equations for which state vector collapse is provable. Second, we explore the problem of formulating a relativistic stochastic Schrödinger equation, using a manifestly covariant equation for a quantum field system based on the interaction picture of Tomonaga and Schwinger. The stochastic noise term in this equation can couple to any local scalar density that commutes with the interaction energy density, and leads to collapse onto spatially localized eigenstates. However, as found in a similar model by Pearle, the equation predicts an infinite rate of energy nonconservation proportional to δ 3 ( ), arising from the local double commutator in the drift term.
The time-dependent Schrödinger equation is a cornerstone of quantum physics and governs all phenomena of the microscopic world. However, despite its importance, its origin is still not widely appreciated and properly understood. We obtain the Schrödinger equation from a mathematical identity by a slight generalization of the formulation of classical statistical mechanics based on the Hamilton-Jacobi equation. This approach brings out most clearly the fact that the linearity of quantum mechanics is intimately connected to the strong coupling between the amplitude and phase of a quantum wave.