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APS Open Science

Objective state reduction from microscopic irreversibility

Julio R. Gea-Banacloche

Published May 26, 2026• 1 citation
Publisher

Abstract

A realistic model of a projective quantum measurement is presented that provides a physical justification for the collapse postulate without invoking the macroscopic nature of a detector. The analysis adopts a minimally realist interpretation of the formalism, in which objective reality is ascribed only to quantum states associated with definite, empirically accessible physical properties. It is shown that objective state reduction arises from an irreversible microscopic process, enforced by appropriate boundary conditions, while the subsequent amplification stage merely reveals, at the macroscopic scale, the randomly selected measurement outcome.

DOI
10.1103/dq8f-rpgl

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A realistic model of a projective quantum measurement is presented that provides a physical justification for the collapse postulate without invoking the macroscopic nature of a detector. The analysis adopts a minimally realist interpretation of the formalism, in which objective reality is ascribed only to quantum states associated with definite, empirically accessible physical properties. It is shown that objective state reduction arises from an irreversible microscopic process, enforced by appropriate boundary conditions, while the subsequent amplification stage merely reveals, at the macroscopic scale, the randomly selected measurement outcome.
The role played by irreversible processes, at the microscopic scale, in several examples of quantum measurements is studied within a "minimally realist" interpretation of the quantum formalism introduced in a recent publication. The analysis extends and confirms the basic results of the earlier work: separation of the collapse and amplification phases of the measurement, and decoherence and collapse happening already as a consequence of irreversible microscopic processes at the level of the "probe" itself (typically an atom in the examples considered). Additionally, the proposed interpretation of the formalism is shown to be consistent with previous interpretations, and a case for true irreversibility in physics is argued in some detail.
The measurement problem is commonly taken to indicate a fundamental conflict betweenthe linear, unitary dynamics of quantum mechanics and the definite outcomes observed inmeasurement processes. Standard responses typically resolve this tension by modifying thedynamics, enriching the ontology, or revising the interpretation of the quantum state.In this paper, we analyze the measurement problem from a projection-based perspectivein which quantum evolution and measurement outcomes are associated with ontologicallydistinct effective descriptions defined by incompatible invariance requirements. Within thisframework, quantum mechanics is characterized by unitary invariance and coherent superpo-sition, whereas measurement records are defined by definiteness, stability, and reproducibil-ity. The measurement problem arises when these distinct projection structures are implicitlyidentified or required to coexist within a single effective description.By analyzing measurement as a transition between such projections, we show that theappearance of wavefunction collapse does not require a physical dynamical process. Instead,collapse emerges as a necessary structural consequence of projecting a quantum descriptiononto a measurement domain that enforces different invariance constraints. This accountpreserves the formal structure and empirical success of quantum mechanics while motivatinga restrained ontological reading of its elements.More broadly, the analysis situates the measurement problem within a general patternin which foundational paradoxes arise at the boundaries between effective descriptions. Inthis sense, the resolution proposed here does not introduce new physics, but clarifies theontological scope and limits of quantum-mechanical description.
Abstract The measurement problem in quantum mechanics is commonly framed as a conflict between unitary dynamics and the appearance of definite outcomes. This paper argues that the conflict arises from a misplaced assumption about what outcomes must be. In experimental practice, outcomes are not arbitrarily precise microscopic states or instantaneous events, but stable physical records produced in measurement interactions. Treating outcomes as such records allows them to be formalized as coarse-grained equivalence classes of physical states that persist in time and support consistent interaction across systems. On this basis, decoherence explains the stability of outcomes without selecting a unique global result or modifying quantum dynamics. Objectivity is recovered through the robustness of records rather than through absolute, observer-independent outcome facts. The measurement problem is thereby dissolved without invoking collapse mechanisms, branching universes, or observer-dependent postulates, while preserving all standard empirical predictions of quantum mechanics.
The quantum measurement problem—why and how the deterministic, unitary evolution of quantum states gives rise to definite, stochastic outcomes—has remained unresolved since the inception of quantum mechanics. This paper proposes that wave function reduction is not a separate physical process requiring its own postulate, but an instance of Level-0 projection within the Quantum Blueprint Formalism (QBF). The QBF Level-0 framework describes the constrained transition from a pre-coherent space M_s of distinction configurations to a coherence manifold M_Θ of macroscopic order parameters, governed by three admissibility conditions: local compatibility (A1), preserved Heisenberg structure (A2), and bounded compression (A3). We demonstrate that quantum state reduction exhibits the same formal structure: it is a constrained projection from Hilbert space (possibility space) to eigenstate space (actuality space), subject to conditions structurally isomorphic to (A1)–(A3).
The traditional, standard approach to quantum theory is to assume that the theory ``really'' contains only unitary physical dynamics--i.e., that the only physically quantifiable evolution is that given by the time-dependent Schrodinger equation. This leads to two distinct classes of interpretations for the standard theory in its orthodox form: (i) an Everettian-type approach assuming that all mutually exclusive outcomes occur in different ``branches'' of the universe; or (ii) single-outcome approaches that assume a ``projection postulate'' (PP) with no accompanying physical account within quantum theory. A contrasting, unorthodox approach is to suggest forms of quantum theory that involve physical non-unitarity; these are called ``objective collapse models.'' Among these are Penrose's theory of gravitation-induced collapse and the Transactional Interpretation. The primary focus of this paper is an example demonstrating that standard quantum theory (with or without the projection postulate) can in-principle yield empirically consequential inconsistencies. Thus, it appears that for quantum theory to be viable in a realist sense (as opposed to being an instrumentalist protocol in which inconsistencies are evaded by changing the protocol), it must possess genuine, physical non-unitarity yielding well-defined single outcomes. This leads to the conclusion that objective collapse models should be more seriously considered.
An operational approach to quantum state reduction, the state change of the measured system caused by a measurement of an observable conditional upon the outcome of measurement, is founded without assuming the projection postulate in any stages of the measuring process. Whereas the conventional formula assumes that the probe measurement satisfies the projection postulate, a new formula for determining the state reduction shows that the state reduction does not depend on how the probe observable is measured, or in particular does not depend on whether the probe measurement satisfies the projection postulate or not, contrary to the longstanding attempts in showing how the macroscopic nature of probe detection provokes state reduction. 1.