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Physics World

Probing the limits of quantum theory

Colin Bruce Jack

Published Jul 1, 1996• 0 citations
Publisher

Abstract

The "infamous boundary" is the apparent dichotomy between the quantum behaviour of microscopic systems and the classical behaviour of macroscopic systems. A lifetime after the invention of quantum theory, the problem of "quantum collapse" remains unsolved. Wavelike, probabilistic behaviour at the quantum level is resolved into specific outcomes at the observer's end of the measuring apparatus in an instantaneous way that appears to transcend the normal laws of physics.

DOI
10.1088/2058-7058/9/7/29

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The "infamous boundary" is the apparent dichotomy between the quantum behaviour of microscopic systems and the classical behaviour of macroscopic systems. A lifetime after the invention of quantum theory, the problem of "quantum collapse" remains unsolved. Wavelike, probabilistic behaviour at the quantum level is resolved into specific outcomes at the observer's end of the measuring apparatus in an instantaneous way that appears to transcend the normal laws of physics.
Ever since quantum theory was developed during the first quarter of the 20th century, we have lived with a strange division. Objects in our daily lives behave “normally” – they appear to obey classical physics – whereas microscopic objects can behave counter-intuitively and reveal intriguing features of quantum physics. But where exactly is the boundary between the quantum and classical worlds – if, indeed, there is one? If quantum physics is a universal theory, why is it respectable to talk about the quantum behaviour of electrons but not, say, of footballs?
One of the main goals of no-collapse interpretations of quantum mechanics is to show how results of measurements are objectified, or, in a more poignant terminology, how in a measurement situation facts arise. One needs to show how in their dynamical evolution the measured system and, perhaps more importantly, the measuring apparatus acquire, or at least appear to acquire, the definite properties that are otherwise explained by the collapse of the state. This can be seen as a special case of the problem whether quantum mechanics alone can account for the classical behaviour of macroscopic systems; in particular, the permanent definiteness of at least those properties of macroscopic systems that are directly observable, such as the position of the pointer of a measuring apparatus. In other words, whether quantum mechanics can account for macrofacts of the familiar kind.
The quantum measurement problem is commonly framed as a tension between unitary microscopic dynamics and the appearance of definite macroscopic outcomes. Decoherence theory explains the suppression of interference but does not explain why a single, persistent outcome is realized rather than a superposition of mutually incompatible macroscopic records. Interpretive responses—including collapse models, many-worlds branching, and observer-centric accounts—add ontological or epistemic structure beyond standard quantum mechanics. In this work, we show that such supplementation is unnecessary. We demonstrate that decoherence alone is generically insufficient to stabilize consistent macroscopic measurement histories under irreversible record formation. We introduce history-level coherence selection as a minimal structural requirement for persistent outcomes. This mechanism operates without modifying unitary dynamics, introducing collapse postulates, or invoking observers. We show that measurement outcomes correspond to stable, selection-supported histories rather than instantaneous state reductions. This reframing resolves the measurement problem by identifying the missing macroscopic mechanism—history selection under irreversible constraint—rather than by altering quantum theory. Keywords: quantum measurement problem, history selection, decoherence insufficiency, classical records, interpretation of quantum mechanics, consistent histories, irreversibility, information flow, foundational physics, measurement theory
Several theories have been advanced recently which appear to offer a resolution to that portion of the measurement problem which previously dealt with a possible reduction of the state vector in a subjective fashion by the brain, mind or consciousness. It now appears, based on both biological and mathematical analysis, that collapse of the wave function always takes place in an objective fashion in the retinal rod-rhodopsin molecule, and that only measured information is ever presented to the brain, mind or consciousness for possible subjective analysis. The remaining portion of the measurement problem has to do with the use and legitimacy of such terms as boundary (Heisenberg or von Neumann cut), information, irreversible, measurement, microscopic, macroscopic, observer (apparatus or measuring system), observed (measured system) and wave function collapse. This portion of the measurement problem may be resolved in two ways. First, by adopting Dirac's theory that it is nature that makes the choice of measuremental result. Second, by the insertion of a mesoscopic bridge between the microscopic and macroscopic worlds, in the existing form of the rhodopsin molecule with its retinal and opsin components.