PROSPECTIVE The concern of this paper will be different from that of most studies of quantum gravity. There will be no discussion of the problem of quantizing the gravitational field equations, hence nothing about the modifications which quantization requires of space-time structure at distances of the order of the Planck length (∼ 10 −33 cm). The emphasis will be rather upon the implications of quantum mechanics for certain general properties of events and processes which occur in the theatre of space-time, specifically with non-locality and nonlinearity . Quantum mechanics is undoubtedly a non-local theory when it treats correlated spatially separated systems. When one examines closely the character of this non-locality, however, one does not find reasons for modifying the causal structure of space-time as described by special or general relativity theory, but rather reasons for refining the concept of an event. The occurrence of definite outcomes of measurements implies that there are processes in nature governed by nonlinear laws. Nonlinearity is very peculiar from the standpoint of quantum mechanics, since it is contrary to the linearity of the time-dependent Schrödinger equation, which is commonly assumed to govern the dynamics of any isolated physical system. Hence it is necessary to inquire whether a rational treatment of the occurrence of outcomes is possible without some modifications of current quantum mechanics. Whatever decision is reached on this question, however, it does not appear to bear directly on space-time structure, but rather on the general character of processes.
PROSPECTIVE The concern of this paper will be different from that of most studies of quantum gravity. There will be no discussion of the problem of quantizing the gravitational field equations, hence nothing about the modifications which quantization requires of space-time structure at distances of the order of the Planck length (∼ 10 −33 cm). The emphasis will be rather upon the implications of quantum mechanics for certain general properties of events and processes which occur in the theatre of space-time, specifically with non-locality and nonlinearity . Quantum mechanics is undoubtedly a non-local theory when it treats correlated spatially separated systems. When one examines closely the character of this non-locality, however, one does not find reasons for modifying the causal structure of space-time as described by special or general relativity theory, but rather reasons for refining the concept of an event. The occurrence of definite outcomes of measurements implies that there are processes in nature governed by nonlinear laws. Nonlinearity is very peculiar from the standpoint of quantum mechanics, since it is contrary to the linearity of the time-dependent Schrödinger equation, which is commonly assumed to govern the dynamics of any isolated physical system. Hence it is necessary to inquire whether a rational treatment of the occurrence of outcomes is possible without some modifications of current quantum mechanics. Whatever decision is reached on this question, however, it does not appear to bear directly on space-time structure, but rather on the general character of processes.
Introduction Our concern in this chapter is locality in quantum mechanics (QM). Locality is a heuristic physics principle based on the following propositions. No Action-at-a-Distance All the evidence points to the principle that physical actions, taken within restricted (localized) regions of space and time by observers or other agencies such as systems under observation (SUOs), do not cause instantly observable effects on other SUOs at large distances. This does not apply to mathematical/ metaphysical concepts such as quantum wave functions or correlations, as these are conceptual objects (Scarani et al., 2000). Statements about instantaneous wave function collapse are vacuous (have no empirical significance) and are therefore not an issue of significance in physics. Such statements are an issue to theorists who objectivize wave functions, as in Hidden Variables (HV) theory. Action-at-a-distance is generally regarded as anathema by most physicists. For example, Newton's law of universal gravitation is well known for mathematically encoding action-at-a-distance. There is direct evidence, however, in the form of a letter written by Newton to Bentley, that Newton believed that gravity acting “at a distance through a vacuum without the mediation of anything else” was an absurdity (Newton, 2006). The no action-at-a-distance principle is encoded in quantized detector networks (QDN) by the requirement that labstate preparation and consequent signal detection never occur at the same stage. Causal Transmission All physically observable consequences of local actions taken by an observer or SUO are transmitted by identifiable physical processes, such as electromagnetic waves or neutrinos. There is no such thing as magic or action-at-a-distance . In QDN this proposition is taken into account implicitly in the labstate outcome amplitudes at each stage, as these model how information is propagated from stage to stage. When necessary, the information void can be modeled as if there were fields and/or particles propagating through it, giving scope for different mathematical models, such as Euclidean space, curved spacetime, noncommuting spacetimes, and so on. The structure of the information void is essentially a discussion of whatever modules have to be taken into account between labstate preparation and signal detection.
Recent developments in quantum theory have focused attention on fundamental questions, in particular on whether it might be necessary to modify quantum mechanics to reconcile quantum gravity and general relativity. This book is based on a conference held in Oxford in the spring of 1984 to discuss quantum gravity. It brings together contributors who examine different aspects of the problem, including the experimental support for quantum mechanics, its strange and apparently paradoxical features, its underlying philosophy, and possible modifications to the theory.
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.
Quantum mechanics is the study of uncertainty. But what does uncertainty mean within the structure of physical reality? From Newton onward, mathematical physics has largely approached motion by specifying a state, identifying the laws governing its evolution, and determining the subsequent behavior of the system. Fourier extended this program to heat, Maxwell to electromagnetic fields, Navier and Stokes to fluids, Hamilton and Lagrange to variational and canonical mechanics, Einstein to spacetime, and quantum mechanics to the evolution and measurement of the quantum state. Across these theories, increasingly powerful mathematics has described what a physical system may do and how its state may evolve. This work asks a logically prior question: Does everything that is mathematically accessible participate equally in physical realization? We propose that it does not. The central distinction developed in this work is between accessibility, admissibility, participation, and realization. A mathematical representation may contain a large space of possible states, trajectories, modes, or outcomes. Physical realization, however, need not occupy that space uniformly. We propose that relational structure establishes an admissible geometry within the accessible space; that geometry determines which possibilities participate strongly or weakly; and probability subsequently quantifies the residual uncertainty among physically supported realizations. The resulting ordering is: Potentiality → Relations → Structure → Geometry → Admissibility → Participation → Realization → Evolution. Under this interpretation, probability is not abolished. Superposition is not abolished. Quantum mechanics is not rejected. Rather, their physical ordering is reconsidered. Superposition describes what the mathematical representation can contain; relational geometry determines how strongly those possibilities can participate; probability describes the uncertainty remaining within that structured support; and observation records the realization that occurs. The NIST loophole-free two-photon Bell experiment provides the principal empirical test developed in this work. Beginning from the experimentally characterized entangled state—not from subsequently observed outcome counts—we construct a relational geometry and the corresponding outcome-dependent participation and information-cost landscapes. The NIST measurement configurations then become experimental sampling coordinates on a geometry determined independently from the later event records. The resulting analysis identifies several mutually reinforcing features: strong inequality among the participation of mathematically accessible Bell outcomes; a corresponding hierarchy of geometric information cost; concentration of experimental variation into reduced effective dimensions; agreement between predicted geometric support and held-out experimental records; preservation of Bell-inequality violation on the reduced realization geometry; compatibility with no-signalling; and robustness tests against multinomial, temporal, detector, and other alternative explanations. The interpretation is not a restoration of Bell-local hidden variables and does not require superluminal signalling. Bell's theorem continues to exclude the relevant locally factorized descriptions. Instead, the observed correlations are interpreted as local measurements of a common non-separable relational structure. Spatial separation does not imply relational or informational separability. This distinction also reframes the quantum measurement problem. Wave-function collapse need not be regarded as the event that creates the physical structure responsible for an outcome. The framework instead proposes that admissibility constrains participation prior to observation, with measurement revealing a realization from an already structured physical domain. A definitive replacement for the standard measurement postulate will require discriminating predictions beyond those already shared with conventional quantum mechanics. The broader implication concerns the theory of motion itself. Newtonian mechanics, Hamiltonian dynamics, Maxwell's equations, Fourier transport, Navier–Stokes flow, relativity, and quantum mechanics employ very different evolution equations, yet all operate upon structured physical domains. The admissibility framework proposes that their deeper commonality lies not in a single dynamical equation but in an ontological ordering: the laws of motion govern how reality evolves; admissibility governs the structure within which reality can become realized. This has consequences beyond quantum foundations. In engineering and computational physics, enormous mathematical state spaces are routinely explored through CFD, finite-element simulation, optimization, Monte Carlo methods, and digital twins. If physically significant behavior occupies a substantially smaller admissible geometry, computation can be redirected from exhaustive exploration toward the structures in which realizable behavior is concentrated. This provides a theoretical basis for reduced-order physical prediction, targeted simulation, transport localization, and deterministic Physics Intelligence. In AI infrastructure and high-performance computing, the same principle suggests that workloads, thermal transport, energy utilization, routing, scheduling, and hardware states need not be treated as equally relevant points within enormous combinatorial spaces. Identifying admissible low-dimensional structure before exhaustive computation offers a route toward reduced search complexity, improved orchestration, higher computational and energy efficiency, and physics-informed predictive control. In aerospace, energy, advanced manufacturing, climate systems, and data-center infrastructure, the framework similarly motivates identifying the geometry of physically participating transport before expending computational resources resolving the entire mathematically accessible domain. The work therefore proposes a common principle extending from fundamental physics to computational engineering: Mathematical possibility is larger than physical realization. Uncertainty may remain genuine at the level of an individual event without implying an absence of antecedent structure. The results presented here provide empirical evidence consistent with the proposition that physical realization is organized by relational geometry prior to probability, while identifying the further tests required to establish whether this ordering constitutes a fundamental law of quantum and classical motion. The next decisive tests include prospective Bell predictions, multipartite and contextuality experiments, Lorentz-covariant formulation, recovery of the classical limit, and extension to quantum field theory and curved spacetime. Nature need not explore everything mathematics permits. Motion is the realization of what physical structure admits. That description captures the actual architecture of the finished 130-page work: it preserves Bell's theorem rather than attempting to evade it, explicitly separates mathematical accessibility from physical participation, and treats the NIST comparison as evidence for the proposed framework while acknowledging that a distinctive prediction beyond standard quantum mechanics remains necessary for a physical replacement of collapse. Principal Implications What This Means for Bell’s Inequality Bell's theorem remains intact. The present framework does not attempt to restore Bell-local hidden variables, weaken Bell's result, or explain the observed correlations through faster-than-light signalling. Bell establishes that the correlations observed between Alice and Bob cannot be reproduced by the relevant locally factorized description. The question developed here begins after that result: if Bell-local separability fails, what physical structure carries the correlations? The proposed answer is relational geometry. Alice and Bob remain spatially separated and perform local measurements, but their outcomes arise from different interrogations of a common non-separable quantum structure. Spatial separation therefore does not imply informational or relational separability. The correlation need not be interpreted as something transmitted from Alice to Bob at the moment of measurement; it can instead be understood as a property of the joint structure to which both measurements belong. The NIST analysis gives this interpretation a quantitative form. The characterized entangled state determines the relational geometry before the subsequent detector records are used for evaluation. Alice's and Bob's measurement settings sample different regions of that geometry. The resulting Bell outcomes exhibit strongly unequal participation, while Bell violation and no-signalling are preserved. The proposed implication is therefore: Bell violation demonstrates the failure of Bell-local factorization; admissibility proposes a geometry for the non-separable physical structure that remains. This reframes the familiar intuition of "action at a distance." The deeper question may not be how information crosses the distance between Alice and Bob, but why spatial distance should require the relational information defining their joint state to become separable in the first place. What This Means for Wave-Function Collapse The framework does not discard wave-function collapse as an operational rule for updating the quantum state after an observation. It changes the proposed ontological role assigned to collapse. In the conventional collapse-centered picture, the quantum state contains multiple possible outcomes and measurement marks the transition to a definite result. This leaves a foundational question: what physically distinguishes the outcome that becomes real from those that do not? Admissibility inserts a structural stage before observation. The accessible quantum
The idea that events are equipped with a partial causal order is central to our understanding of physics in the tested regimes: given two pointlike events A and B , either A is in the causal past of B , B is in the causal past of A , or A and B are space-like separated. Operationally, the meaning of these order relations corresponds to constraints on the possible correlations between experiments performed in the vicinities of the respective events: if A is in the causal past of B , an experimenter at A could signal to an experimenter at B but not the other way around, while if A and B are space-like separated, no signaling is possible in either direction. In the context of a concrete physical theory, the correlations compatible with a given causal configuration may obey further constraints. For instance, space-like correlations in quantum mechanics arise from local measurements on joint quantum states, while time-like correlations are established via quantum channels. Similarly to other variables, however, the causal order of a set of events could be random, and little is understood about the constraints that causality implies in this case. A main difficulty concerns the fact that the order of events can now generally depend on the operations performed at the locations of these events, since, for instance, an operation at A could influence the order in which B and C occur in A ’s future. So far, no formal theory of causality compatible with such dynamical causal order has been developed. Apart from being of fundamental interest in the context of inferring causal relations, such a theory is imperative for understanding recent suggestions that the causal order of events in quantum mechanics can be indefinite. Here, we develop such a theory in the general multipartite case. Starting from a background-independent definition of causality, we derive an iteratively formulated canonical decomposition of multipartite causal correlations. For a fixed number of settings and outcomes for each party, these correlations form a polytope whose facets define causal inequalities. The case of quantum correlations in this paradigm is captured by the process matrix formalism. We investigate the link between causality and the closely related notion of causal separability of quantum processes, which we here define rigorously in analogy with the link between Bell locality and separability of quantum states. We show that causality and causal separability are not equivalent in general by giving an example of a physically admissible tripartite quantum process that is causal but not causally separable. We also show that there are causally separable quantum processes that become non-causal if extended by supplying the parties with entangled ancillas. This motivates the concepts of extensibly causal and extensibly causally separable (ECS) processes, for which the respective property remains invariant under extension. We characterize the class of ECS quantum processes in the tripartite case via simple conditions on the form of the process matrix. We show that the processes realizable by classically controlled quantum circuits are ECS and conjecture that the reverse also holds.