Quantum measurement is a fundamental concept in the field of quantum mechanics. The action of quantum measurement, leading the superposition state of the measured quantum system into a definite output state, not only reconciles contradictions between quantum and classical mechanics but also facilitates quantum state manipulations, including reading and resetting. Despite its significance, four fundamental issues -- randomness, instantaneousness, irreversibility, and preferred-basis -- continue to pose significant challenges to the broader application of quantum measurement and our overall understanding of quantum mechanics. In this work, we employ a new space-time approach to relativistic quantum mechanics to address these issues systematically. Our approach provides a comprehensive elucidation of the intricate connections between quantum measurement and quantum unitary evolution, as well as an in-depth analysis for the interdependence of non-local correlations and relativistic theories. We thereby reveal a more fundamental dynamical theory, beyond the traditional time-evolution equation in quantum mechanics, where the axioms of quantum measurement naturally emerge as a corollary. These findings contribute to the advancement of related fields, and our work holds potential implications for future research and applications in the realm of quantum mechanics.
Quantum measurement is a fundamental concept in the field of quantum mechanics. The action of quantum measurement, leading the superposition state of the measured quantum system into a definite output state, not only reconciles contradictions between quantum and classical mechanics but also facilitates quantum state manipulations, including reading and resetting. Despite its significance, four fundamental issues -- randomness, instantaneousness, irreversibility, and preferred-basis -- continue to pose significant challenges to the broader application of quantum measurement and our overall understanding of quantum mechanics. In this work, we employ a new space-time approach to relativistic quantum mechanics to address these issues systematically. Our approach provides a comprehensive elucidation of the intricate connections between quantum measurement and quantum unitary evolution, as well as an in-depth analysis for the interdependence of non-local correlations and relativistic theories. We thereby reveal a more fundamental dynamical theory, beyond the traditional time-evolution equation in quantum mechanics, where the axioms of quantum measurement naturally emerge as a corollary. These findings contribute to the advancement of related fields, and our work holds potential implications for future research and applications in the realm of quantum mechanics.
Space-time is one of the most essential, yet most mysterious concepts in physics. In quantum mechanics it is common to understand time as a marker of instances of evolution and define states around all the space but at one time; while in general relativity space-time is taken as a combinator, curved around mass. Here we present a unified approach on both space and time in quantum theory, and build quantum states across spacetime instead of only on spatial slices. We no longer distinguish measurements on the same system at different times with measurements on different systems at one time and construct spacetime states upon these measurement statistics. As a first step towards non-relativistic quantum field theory, we consider how to approach this in the continuous-variable multi-mode regime. We propose six possible definitions for spacetime states in continuous variables, based on four different measurement processes: quadratures, displaced parity operators, position measurements and weak measurements. They are motivated by the pseudo-density matrix formulation among indefinite causal structures and the path integral formalism. We show that these definitions lead to desirable properties, and raise the differences and similarities between spatial and temporal correlations. An experimental proposal for tomography is presented, construing the operational meaning of the spacetime states.
Abstract Quantum mechanics and relativity are two of the most successful theoretical frameworks in modern physics, yet their conceptual foundations remain difficult to reconcile at the deepest level. Quantum theory describes physical systems through states, observables, probabilities, measurement contexts, superposition, entanglement, and nonclassical correlations. Relativity describes physical processes through space-time structure, causal order, invariant physical law, observer-dependent decomposition of time and space, and constraints on signal propagation. This article examines the conceptual relationship between quantum measurement, relativistic causality, and space-time description. The purpose is not to present a mathematical unification but to analyze how the two frameworks organize physical explanation differently and where their interpretive tensions arise. Special attention is given to the measurement problem, the status of quantum states, the role of observers and reference frames, the meaning of locality, the distinction between correlation and causal influence, and the challenge of describing quantum processes without privileging a single global time. The article argues that many apparent conflicts between quantum theory and relativity result from importing classical intuitions into domains where they no longer apply. Quantum entanglement does not permit controllable faster-than-light communication, yet it challenges classical separability. Relativity forbids superluminal signaling, yet it does not require that all physically meaningful correlations be reducible to local classical properties. Measurement introduces another difficulty because standard quantum theory often describes state update in a way that appears instantaneous, while relativistic space-time does not provide a universal present. The discussion develops a conceptual framework in which quantum measurement is treated as a physical interaction, relativistic causality as a constraint on operational influence, and space-time description as an organizing structure for events rather than a simple background stage. This analysis is relevant for quantum field theory, quantum information, relativistic quantum foundations, and future approaches to quantum gravity.
The quantum measurement problem remains one of the most profound unresolved issues in fundamental physics. While quantum mechanics delivers extraordinarily precise statistical predictions, it does not specify a physical mechanism by which definite classical outcomes arise from quantum superpositions. Decoherence theory accounts for the suppression of interference but does not explain outcome selection or the stability of classical reality. This paper investigates quantum measurement within the framework of Relativistic Coherent Vacuum Gravity Theory (rCVGT). In rCVGT, the vacuum is modeled as a structured relativistic medium characterized by a coherence order parameter ψ(x), a coherence magnitude Q(x) = |ψ(x)|², a vacuum-flow four-field u^μ(x), and a physical time-rate field τ(x). We propose that quantum measurement corresponds to a dynamical instability of vacuum coherence induced by macroscopic coupling to matter and radiation. Classical outcomes emerge when local vacuum coherence is driven below a critical stability threshold, resulting in an irreversible reconfiguration of the vacuum state. The collapse timescale is determined by the vacuum-coherence self-energy defined within rCVGT. This mechanism simultaneously provides a physical foundation for classicality, irreversibility, and the arrow of time. The framework preserves the formal structure and empirical predictions of quantum mechanics while offering a unified physical explanation of measurement grounded in relativistic vacuum dynamics.
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.
Measurements play a crucial role in quantum mechanics because they serve two fundamental purposes. First, they enable the extraction of information about a quantum system; second, they give rise to new physical effects that do not appear in classical physics, one important example being the measurement-induced state update, where the act of measurement alters the system’s state. For these reasons, the mathematical description of measurements plays a primary role in the foundational postulates of the theory. However, the essence of what happens during a measurement remains poorly understood, even within the relatively easy realm of non-relativistic quantum theory. The situation becomes even less transparent in the relativistic setting, where the requirement to preserve causality and prevent faster-than-light signalling introduces both formal and interpretational challenges, complicating the very mathematical formulation of a measurement theory. As for quantum gravity, the idea of applying quantum measurements within or onto a quantum spacetime remains virtually unexplored, presenting a vast and uncharted frontier for theoretical inquiry. Building on recent advances across several areas at the intersection of quantum foundations, quantum information theory, relativistic quantum theory, and quantum gravity, the last few years saw a surge of interest in formulating a consistent framework for describing local measurements in a relativistic context. The main results of this endeavour are two distinct proposals for a relativistic measurement postulate for quantum field theory in generic spacetimes. While these two proposals are still being understood, and a growing research community is working to clarify their implications and potential interconnections, the field of relativistic quantum measurements has now reached a level of maturity that allows us to ask both formal and interpretive questions on a solid footing. All this only applies to the regime in which quantum field theory in curved spacetime remains valid: nothing can yet be said about measurements at the Planck energy, where quantum gravitational effects are expected to become important. This thesis presents an introduction to the theory of measurements in non-relativistic and relativistic quantum mechanics, along with four original research articles that explore aspects of measurements in both fields, as well as the potential extension of measurement theory to the domain of quantum gravity. The first article examines the limitations of realistic measurement apparatuses in non-relativistic quantum theory, establishing reasonable bounds for the precision that can be achieved with a given apparatus composed of many quantum components. The second article explores how to extract information from a newly introduced class of quantum systems, defined as those that cannot be replicated and thus fall outside the usual scope of quantum mechanics. Then, it applies the methodology to quantum field theory, demonstrating that classical results, such as the Unruh effect, can still be observed in this setting. The third article discusses detector-based measurements in quantum field theory. This is a recently proposed method that extends the non-relativistic measurement postulate to the relativistic regime by means of Unruh-de Witt detectors. The article highlights some conceptual and mathematical issues with the framework and proposes a solution using algebraic quantum field theory. The outcome is a self-consistent scheme for introducing local measurements in quantum field theory via measuring detectors. Finally, the fourth article uses the measurement scheme above to study how what we learned in the past years about local measurements can be extended to quantum gravity. This is achieved through the holographic principle realised in terms of the AdS-CFT correspondence. In conclusion, this thesis contributes to the development of both non-relativistic and relativistic measurement theories and introduces ideas and questions related to the role of measurement in quantum gravity. Though preliminary, this work opens a pathway toward studying measuring observers in quantum gravity.
We provide an overview of standard "projective" quantum measurements with the goal of elucidating connections between theory and experiment. We make use of a unitary "Stinespring" representation of measurements on a dilated Hilbert space that includes both the physical degrees of freedom and those of the measurement apparatus. We explain how this unitary representation (i) is guaranteed by the axioms of quantum mechanics, (ii) relates to both the Kraus and von Neumann representations, and (iii) corresponds to the physical time evolution of the system and apparatus during the measurement process. The Stinespring representation also offers significant conceptual insight into measurements, helps connects theory and experiment, is particularly useful in describing protocols involving midcircuit measurements and outcome-dependent operations, and establishes that all quantum operations are compatible with relativistic locality, among other insights.