We present a unitary and operational account of quantum measurement in which outcome irreversibility arises from computational complexity rather than fundamental nonunitarity. Measurement is modeled as a two-stage physical process. First, local interactions dynamically select a pointer basis in which stable classical records are formed via decoherence. Second, subsequent local scrambling dynamics distribute these records globally across a finite quantum memory register. Using explicit circuit constructions based on local random circuits that form approximate unitary k-designs, we show that measurement records are encoded in high-order nonlocal correlations. We prove that reversing such records requires quantum circuits of exponential depth, even allowing ε-approximate reconstruction. This establishes a quantitative complexity barrier rendering outcome reversal physically unrealizable despite global unitarity. We apply the framework to multi-register (Wigner-type) scenarios and show that no physically realizable operation can expose inconsistency between distinct register-relative descriptions. Finally, we prove a conditional no-go theorem excluding any global collapse mechanism compatible with unitary dynamics, locality, and computational feasibility. Measurement irreversibility is therefore shown to be an emergent, complexity-protected phenomenon within standard quantum mechanics.
We present a unitary and operational account of quantum measurement in which outcome irreversibility arises from computational complexity rather than fundamental nonunitarity. Measurement is modeled as a two-stage physical process. First, local interactions dynamically select a pointer basis in which stable classical records are formed via decoherence. Second, subsequent local scrambling dynamics distribute these records globally across a finite quantum memory register. Using explicit circuit constructions based on local random circuits that form approximate unitary k-designs, we show that measurement records are encoded in high-order nonlocal correlations. We prove that reversing such records requires quantum circuits of exponential depth, even allowing ε-approximate reconstruction. This establishes a quantitative complexity barrier rendering outcome reversal physically unrealizable despite global unitarity. We apply the framework to multi-register (Wigner-type) scenarios and show that no physically realizable operation can expose inconsistency between distinct register-relative descriptions. Finally, we prove a conditional no-go theorem excluding any global collapse mechanism compatible with unitary dynamics, locality, and computational feasibility. Measurement irreversibility is therefore shown to be an emergent, complexity-protected phenomenon within standard quantum mechanics.
We present a unitary and operational account of quantum measurement in which outcome irreversibility arises from computational complexity rather than fundamental nonunitarity. Measurement is modeled as a two-stage physical process. First, local interactions dynamically select a pointer basis in which stable classical records are formed via decoherence. Second, subsequent local scrambling dynamics distribute these records globally across a finite quantum memory register. Using explicit circuit constructions based on local random circuits that form approximate unitary k-designs, we show that measurement records are encoded in high-order nonlocal correlations. We prove that reversing such records requires quantum circuits of exponential depth, even allowing ε-approximate reconstruction. This establishes a quantitative complexity barrier rendering outcome reversal physically unrealizable despite global unitarity. We apply the framework to multi-register (Wigner-type) scenarios and show that no physically realizable operation can expose inconsistency between distinct register-relative descriptions. Finally, we prove a conditional no-go theorem excluding any global collapse mechanism compatible with unitary dynamics, locality, and computational feasibility. Measurement irreversibility is therefore shown to be an emergent, complexity-protected phenomenon within standard quantum mechanics
Quantum mechanics lacks a physical definition of measurement, observer, probability, and experienced time. These notions are instead treated as primitive or interpretation-dependent, leading to persistent foundational problems including the measurement problem, the origin of the Born rule, observer disagreement in Wigner’s Friend scenarios, the arrow of time, and apparent information loss in black hole evaporation. In this work, we show that these problems share a single missing ingredient: the absence of an operational definition of observers as bounded physical systems. We define observers as physically realizable systems capable of forming irreversible internal records under unitary dynamics. Measurement is identified with record formation, probability with record redundancy, experienced time with record accumulation, and irreversibility with computational inaccessibility rather than non-unitarity. Using decoherence and quantum Darwinism, we show that the number of accessible, decoherence-stable records associated with an outcome scales with the squared amplitude, yielding the Born rule from record counting without invoking collapse, subjective probability, or additional axioms. Observer-relative paradoxes dissolve once outcome definiteness is indexed to record formation rather than to the global quantum state. Reconstruction of global quantum information is shown to require exponential resources, rendering measurement and black hole evaporation operationally irreversible despite global unitarity. The resulting framework closes quantum mechanics conceptually by eliminating undefined primitives, without modifying its mathematical structure or introducing new dynamical postulates.
We present an operational account of quantum measurement in which outcome definiteness and irreversibility arise from causal structure, decoherence, and computational constraints rather than from fundamental collapse. We show that in any globally unitary quantum system with environmental coupling and nontrivial causal partitioning, no physically realizable observer can access all information pathways simultaneously. As a result, observer-dependent descriptions—definite outcomes for embedded observers and coherent superpositions for external observers—are mutually consistent and cannot lead to operational contradiction. This framework resolves Wigner-type measurement paradoxes and clarifies the origin of irreversibility without modifying quantum mechanics.
Abstract This article presents a revised and conceptually refined formulation of a previously proposed Temporal Interpretation of quantum measurement, explicitly addressing the physical nature of wavefunction collapse. While the original proposal emphasised relativistic aspects of momentum superpositions, the present work clarifies the interpretation within the standard formalism of quantum mechanics, without introducing new dynamics or experimental predictions. Unitary evolution is read as a process of differentiated phase accumulation between branches of a quantum state. In relativistic contexts, phase differentials can be structurally related to effective proper-time differentials, allowing the introduction of a notion of temporal compatibility that provides a physically grounded account of decoherence. When such differentials exceed a characteristic scale, interference between branches becomes operationally inaccessible, while global unitarity is preserved. The observer is treated as a physical system endowed with an irreversible proper time associated with macroscopic, entropic dynamics. Within this framework, collapse is explained as an emergent physical phenomenon arising from the irreversibility of the observer’s proper time, which prevents the reconstruction of phase coherence after an interaction. Collapse is therefore neither a fundamental dynamical mechanism nor an objective event in space-time, but a necessary consequence of temporal irreversibility in physical observers. This formulation sharpens the distinction between decoherence and collapse, preserves full consistency with the Born rule, and positions the Temporal Interpretation as a competing interpretative framework that provides a concrete physical explanation for the quantum–classical transition, improving and clarifying the original proposal. This version clarifies the operational status of the interaction time, refines Sections 5.3 and 5.4, and standardises references and editorial structure. No changes to the formalism or conclusions.
We present a unified operational framework for the arrow of time grounded in irreversible information loss. While microscopic physical laws are time-reversal invariant, macroscopic irreversibility arises when systems overwrite metastable records. We show that recent reversible quantum-control experiments do not reverse time, but instead operate in regimes where no logically irreversible record is created. To formalize this, we introduce the **forgetting rate** (Ξcr), a coarse-grained rate defined on physically selected metastable record states (e.g., pointer states, memory registers, or macroscopic configurations). Unlike standard entropy production, which is expressed in terms of microscopic transition asymmetry, (Ξcr) captures the **rate of irreversible record overwriting**. In this formulation, entropy production obeys the lower boundS˙irr≥kBΞcr,consistent with Landauer's principle but expressed at the level of record dynamics. Within this framework, thermodynamic, quantum-measurement, psychological, and cosmological arrows emerge as substrate-dependent manifestations of Ξcr>0. Temporal asymmetry is therefore not a single global property, but a distributed feature of physical information-processing systems. The approach naturally explains the decoupling of arrows in composite systems and identifies experimentally accessible regimes where irreversibility can be locally suppressed. This work does not introduce a new microscopic law but provides a unifying, operational reformulation that connects information thermodynamics, decoherence theory, and memory dynamics through a single coarse-grained rate.
The quantum measurement problem, i.e., the apparent conflict between unitary quantum evolution and non-unitary, stochastic wave-function collapse, remains unresolved a century after the formulation of quantum mechanics. We first review the standard picture, from the Copenhagen prescription with its Heisenberg cut, through von Neumann's movable cut, to environment-induced decoherence, which explains the emergence of stable classical records but still presupposes the Born rule and therefore cannot by itself replace the measurement postulates. We then connect this problem to the Maximum Entropy Principle in two complementary ways. First, decoherence drives the measurement apparatus toward the least biased state compatible with the dynamically protected pointer distribution, so that the emergent collapse may be viewed as a thermodynamic relaxation toward constrained maximum entropy. Second, we propose a Timeless Histories formulation of quantum mechanics in which the primitive objects are ordered sequences of events rather than evolving wave functions. Their conditional probabilities are assigned, relative to specified refinements, by a single Born-Boltzmann rule that combines the quadratic structure of Born's rule with Boltzmann's counting of equally probable microstates. The textbook form of Born's rule, the projection postulate, density operators, and wave functions are then recovered as derived informational constructs, while collapse becomes Bayesian conditioning on recorded events. Time and space enter only subsequently through unitary translation symmetry. Since the probability rule is postulated without reference to any physical collapse or classical observer, the usual circularity of the decoherence program is avoided. Decoherence can then be invoked to explain the stability and approximate additivity of macroscopic records, i.e., the emergence of classicality.