What guarantees the "peaceful coexistence" of quantum nonlocality and special relativity? The tension arises because entanglement leads to locally inexplicable correlations between distant events that have no absolute temporal order in relativistic spacetime. This paper identifies a relativistic consistency condition that is weaker than Bell locality but stronger than the no-signaling condition meant to exclude superluminal communication. While justifications for the no-signaling condition often rely on anthropocentric arguments, relativistic consistency is simply the requirement that joint outcome distributions for spacelike separated measurements (or measurement-like processes) must be independent of their temporal order. This is necessary to obtain consistent statistical predictions across different Lorentz frames. We first consider ideal quantum measurements, derive the relevant consistency condition on the level of probability distributions, and show that it implies no-signaling (but not vice versa). We then extend the results to general quantum operations and derive corresponding operator conditions. This will allow us to clarify the relationships between relativistic consistency, no-signaling, and local commutativity. We argue that relativistic consistency is the basic physical principle that ensures the compatibility of quantum statistics and relativistic spacetime structure, while no-signaling and local commutativity can be justified on this basis.
What guarantees the "peaceful coexistence" of quantum nonlocality and special relativity? The tension arises because entanglement leads to locally inexplicable correlations between distant events that have no absolute temporal order in relativistic spacetime. This paper identifies a relativistic consistency condition that is weaker than Bell locality but stronger than the no-signaling condition meant to exclude superluminal communication. While justifications for the no-signaling condition often rely on anthropocentric arguments, relativistic consistency is simply the requirement that joint outcome distributions for spacelike separated measurements (or measurement-like processes) must be independent of their temporal order. This is necessary to obtain consistent statistical predictions across different Lorentz frames. We first consider ideal quantum measurements, derive the relevant consistency condition on the level of probability distributions, and show that it implies no-signaling (but not vice versa). We then extend the results to general quantum operations and derive corresponding operator conditions. This will allow us to clarify the relationships between relativistic consistency, no-signaling, and local commutativity. We argue that relativistic consistency is the basic physical principle that ensures the compatibility of quantum statistics and relativistic spacetime structure, while no-signaling and local commutativity can be justified on this basis.
We investigate whether commutativity is necessary to represent relativistic locality for localization observables of relativistic quantum systems in Minkowski spacetime. A well known no-go theorem by Halvorson and Clifton shows that commutativity of localization effects for causally separated regions is incompatible with other seemingly natural assumptions about spatial localization. Since commutativity is taken to represent locality in the Araki-Haag-Kastler framework of QFT, this prompts the question whether it follows from more elementary locality principles of quantum theory. Using Busch's operational analysis in terms of no-signaling and relativistic consistency, we argue that for particle-like systems commutativity is not implied by these principles. Assuming a natural local detectability principle, elementary localization observables are not localized in arbitrarily small spacetime neighborhoods of the relevant spatial regions, but rather in regions containing the entire rest space (a Cauchy surface) on which the measurement is performed. This reflects the particle picture itself, where localization occurs at a unique place on a rest space filled with ideal detectors, and therefore does not directly conflict with the Araki-Haag-Kastler notion of locality. We also show that commutativity and localization can coexist for less idealized localization procedures. To this end, we introduce conditional localization POVMs associated with bounded spatial regions interpreted as laboratories. By the gentle measurement lemma, these observables describe conditional localization probabilities and can, in principle, satisfy commutativity for causally separated laboratories. They may therefore be represented by local observables in the Araki-Haag-Kastler sense. Explicit examples are presented in a second work within local QFT.
There are two issues concerning hidden variables in quantum mechanics: 1.Whether the outcomes of quantum measurements are hidden variables has long been debated. Experiments violating Bell's inequalities have essentially refuted local hidden variable theories. 2.Quantum entanglement provides nonlocal effects that alter wave functions. However, these changes cannot transmit signals, making the exact time of such nonlocal effects a hidden variable. Theories about hidden variables are difficult to verify experimentally due to their unobservability. This paper connects these two hidden variable theories in quantum mechanics, proposing the spacetime relationship that nonlocal effects from entanglement must satisfy to prevent quantum measurement outcomes from being local hidden variables. It further outlines the spacetime relationship that any causal effects should adhere to. Based on this, the paper demonstrates the universal temporal order between relativistic spacelike events. On this foundation, it integrates the locally deterministic relativistic spacetime structure with the nonlocal phenomena of probabilistic quantum mechanics, thereby establishing the universal ordered spacetime structure.
This is Paper 142 in the Anaxiomatic Mechanics / pq-Spacetime stack. It develops a finite mechanical account of quantum entanglement within pq-spacetime. Entanglement is identified with the persistence of a non-factorable family of admissible future histories. Two measurement regions can therefore appear as separate local systems in emergent spacetime while still remaining projections of one joint finite closure problem. Bell inequality violation follows because the joint outcomes are not generated by two independent local completion families. No-signalling is nevertheless preserved because complete changes to a distant measurement boundary do not alter the local marginal outcome weights. The paper establishes the Entanglement Closure Theorem and distinguishes microscopic non-factorability from observable entanglement after coarse-graining into stable measurement records. It also gives an explicit minimal singlet construction using two opposed history fibers carrying the relevant four-phase weights. The resulting finite ledger is antisymmetric, nonseparable, and exactly balanced at each local wing. Complete changes of basis on the distant side preserve those local marginals. Using the previously derived Hilbert-space and beta-Dirac bridges of pq-spacetime, the paper recovers the familiar spinor overlap law, the quantum singlet correlation, the maximal Bell-CHSH value, monogamy of maximal entanglement, and decoherence through environmental boundary enlargement. The analysis also explains why no preferred measurement ordering is required and why entanglement remains compatible with relativistic causality. The resulting picture is direct. Entangled systems do not exchange superluminal messages, carry classical answer sheets, or require a force acting across space. Their local outcomes arise from one shared finite admissibility structure whose joint correlations are nonlocal while its controllable local statistics remain causal. Entanglement is therefore physically real, nonclassical, and nonlocal in correlation, but not a signalling mechanism.
Entanglement and Nonlocal Coherence: Why Nonlocality Is Natural in a Phase Ontology (Phase Ontology — Part IV) reframes quantum entanglement by changing the usual starting assumption. The paper argues that entanglement only looks paradoxical if spacetime separation is assumed to imply ontological separation. If phase coherence is treated as a primary layer of reality (as developed in Parts I–III), then nonlocal correlations become expected rather than mysterious. In this view, entangled systems share a single coherent phase structure prior to measurement. Measurement is described as local phase locking: an interaction with a detector stabilizes a definite outcome locally, while the joint correlations arise because both outcomes are constrained by the same underlying phase structure. No superluminal signal is required; correlations are revealed rather than transmitted. The paper also addresses compatibility with relativity: since no information is communicated faster than light, relativistic causality remains intact. Locality is treated as an emergent property of spacetime, while coherence relations can be nonlocal because they are not fundamentally “in spacetime” to begin with. This manuscript is Part IV of a seven-part series (Parts I–VII). It connects the measurement framework of Part III to entanglement and points ahead to Part V (time, irreversibility) and Part VII (experimental implications and falsifiability).
The study of quantum correlations within relativistic spacetimes, and the consequences of relativistic causality on information processing using such correlations, has gained much attention in recent years. In this paper, we establish a unified framework in the form of operational no-signalling constraints to study both nonlocal and temporal correlations within general relativistic spacetimes. We explore several intriguing consequences arising from our framework. Firstly, we show that the violation of the operational no-signalling constraints in Minkowski spacetime implies either a logical paradox or an operational infringement of Poincaré symmetry. We thereby examine and subvert recent claims in [Phys. Rev. Lett. 129, 110401 (2022)] on the possibility of witnessing operationally detectable causal loops in Minkowski spacetime. Secondly, we explore the possibility of jamming of nonlocal correlations, controverting a recent claim in [Nat. Comm. 16, 269 (2025)] that a physical mechanism for jamming would necessarily lead to superluminal signalling. Finally, we show that in black hole spacetimes certain nonlocal correlations under and across the event horizon can be jammed by any agent without spoiling the operational no-signalling constraints.
There are two issues concerning hidden variables in quantum mechanics: 1.Whether the outcomes of quantum measurements are hidden variables has long been debated. Experiments violating Bell's inequalities have essentially refuted local hidden variable theories. 2.Quantum entanglement provides nonlocal effects that alter wave functions. However, these changes cannot transmit signals, making the exact time of such nonlocal effects a hidden variable. Theories about hidden variables are difficult to verify experimentally due to their unobservability. This paper connects these two hidden variable theories in quantum mechanics, proposing the spacetime relationship that nonlocal effects from entanglement must satisfy to prevent quantum measurement outcomes from being local hidden variables. It further outlines the spacetime relationship that any causal effects should adhere to. Based on this, the paper demonstrates the universal temporal order between relativistic spacelike events. On this foundation, it integrates the locally deterministic relativistic spacetime structure with the nonlocal phenomena of probabilistic quantum mechanics, thereby establishing the universal ordered spacetime structure. key words: ordered spacetime structure ; local hidden variable ; nonlocal effects; uncertainty principle;