Understanding the concept of causality is fundamental in physics, as it conditions the very possibility of any exchange of information between physical systems. This article examines and compares three main interpretations of the correlative operational concept of "causal separation" that have been respectively proposed in special relativity, quantum theory, and according to the statistical "non-signalling" condition. These interpretations, which can be classified regarding the range of CHSH correlation degrees they allow, lead to radically different results regarding the exchange of information between physical systems. Namely, it can be shown that, in opposition with the non-communication theorem of quantum theory, non-local communication can be established between the two entangled parts of a physical system when the relativistic criterium of causal separation is satisfied but not the quantum one.
Understanding the concept of causality is fundamental in physics, as it conditions the very possibility of any exchange of information between physical systems. This article examines and compares three main interpretations of the correlative operational concept of "causal separation" that have been respectively proposed in special relativity, quantum theory, and according to the statistical "non-signalling" condition. These interpretations, which can be classified regarding the range of CHSH correlation degrees they allow, lead to radically different results regarding the exchange of information between physical systems. Namely, it can be shown that, in opposition with the non-communication theorem of quantum theory, non-local communication can be established between the two entangled parts of a physical system when the relativistic criterium of causal separation is satisfied but not the quantum one.
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 theory is in principle compatible with scenarios where physical processes occur in an indefinite order, potentially yielding advantages in a broad range of information processing tasks. However, advantages in communication, the most basic form of information processing, have so far remained controversial and hard to prove. Here we provide a framework for assessing the role of causal order in communication, by comparing different causal structures under the constraint that the allowed operations must not generate signaling from signaling-incapable devices. Using this framework, we establish a clear-cut advantage of indefinite causal order, and, at the same time, we identify a series of fundamental limits to the communication power of causal structures in quantum mechanics. Notably, we find that a special form of indefinite causal order, obtained by coherently controlling the order of two processes, enhances the transmission of classical messages in a one-shot scenario, but no quantum operation with indefinite order can offer advantages over shared entanglement when asymptotically many uses of the same communication device are employed. Overall, our results unveil non-trivial relations between communication, causal order, entanglement, and no-signaling quantum processes.
Quantum mechanics has achieved unparalleled success as an operational theory, describing a wide range of experiments to remarkable accuracy. However, the physical foundations on which it rests remain as puzzling as they were a century ago, and a concise statement of the physical principles that underlie quantum mechanics is still outstanding. One promising approach holds that these principles should be formulated in terms of information, since many of the counter-intuitive effects in quantum theory concern questions such as what one can know about a quantum system and how the information encoded therein can be processed and distributed between parties. Another challenging feature of quantum theory is its incompatibility with the other cornerstone of modern physics, general relativity. In order to reconcile the two, one must identify and retain only the essential concepts and principles of each theory, and in the case of general relativity, causality has been identified as such a concept. This raises the question of how our classical understanding of causality must change when quantum theory is taken into account. In order to address these questions about information, knowledge and causality, I turn to the framework of causal models. In classical statistics, causal models explain the relations among a set of variables in terms of causal influences, which makes them a powerful tool for structuring our knowledge about complex systems and developing strategies for interacting with them. More importantly, the framework provides the conceptual underpinnings and mathematical methods for addressing questions about causation, information and knowledge in a rigorous manner. In this thesis, I develop a version of the classical causal models framework that is compatible with quantum theory and explore its physical implications. \n \n \nA first step is to define quantum versions of fundamental elements such as variables, conditionals and belief propagation rules. This allows one to consider the question of what one can come to know about a quantum variable from the point of view of causal modelling. The conditionals relating quantum variables are found to have a richer structure than their classical counterparts, which can be exploited for the task of discerning causal relations given limited data -- a central problem in classical causal modelling. The mathematical properties of quantum conditionals also establish a correspondence between various classes of two-party correlations, such as bound and distillable entanglement, and the types of causal structures that can give rise to them, which may become a useful tool for entanglement theory and quantum information processing. In the context of open quantum systems dynamics, quantum causal models provide a clear physical explanation of not completely positive maps and, more broadly, of non-Markovian quantum dynamics. Finally, I consider the possibility of non-classical effects in the way that different causal mechanisms are combined -- that is, non-classical causal structures. For the simple case of two causally ordered variables, I propose indicators that witness different classes of combinations of causal mechanisms, including a non-classical mixture, and describe an experiment realizing examples of the different classes. \n \n \nAs these results illustrate, the framework of quantum causal models provides both greater conceptual clarity and a comprehensive mathematical formalism for studying a diverse set of problems, ranging from foundational questions to applications in open systems dynamics and the exploration of non-classical causal structures, which are likely to feature in a future theory of quantum gravity. The material presented in this thesis is intended as a foundation and inspiration for further applications of quantum causal models.
The paradigmatic view where information is seen as a more fundamental concept than the laws of physics leads to a different understanding of spacetime, where the causal order of events emerges from correlations between random variables representing physical quantities. In particular, such an information-theoretic approach does not enforce a global spacetime structure. By following this path, we conclude that perfect signaling correlations among three parties are possible which do not obey the restrictions imposed by global spacetime. We show this using a recent framework based on the sole assumptions that locally, quantum theory is valid and random variables can be described by probability distributions. Our result is of zero-error type and can be seen as an analog to a three-party appearance of quantum non-locality which manifests itself by satisfying a condition with certainty, whereas the same is impossible for any local theory.
Recent developments in quantum information allow for a new understanding of quantum correlations. The aim of this paper is to physically explain why quantum mechanics obeys a stronger bond than the non-sig- naling requirement or alternatively why it obeys a principle of information causality. It is shown that a physical theory violating the quantum bond allows for correlations between settings while quantum mechanics only allows for correlations between possible outcomes. In fact, correlations between settings would violate the protocols used in quantum cryptography. The conclusion is that information codification is a local operation and quantum mechanics sets the general conditions for information exchanging in our universe since it satisfies and saturates the bond that is imposed by the principle of information causality, and in so doing it also sets specific constraints on both the possible interdependencies and the possible interactions (also causal interconnections) in our universe.
Emergent Causality Through Decoherence An imaginary hypothesis, not a theory. What if causality were not fundamental? What if it emerged, like temperature, from the degree of decoherence in a system? This hypothesis proposes that there is no single causality, but three regimes of the same process. The core idea:There is no single causality. There are causalities that emerge according to the degree of decoherence in a system. In other words: causality is not a property of the universe. It is a property of the level of description. The three regimes: Quantum causality (zero or low decoherence). The system is isolated. What exists are pure correlations: entanglement, superposition. There is no causality in the classical sense. There is correlation without direction. There is no defined "before" or "after." Relativistic causality (medium decoherence). The system interacts with the environment, but not completely. There is a causal structure defined by spacetime. There is causality, but it is relative: it depends on the observer. Classical causality (high decoherence). The system is fully coupled to the environment. It is classical. There is absolute, local causality: cause → effect, without ambiguity. The mechanism: decoherence.The more degrees of freedom interact with the system, the more decoherence. More decoherence → more classical → more causal. There is no "line" that is crossed. There is a continuum. Causality emerges gradually, just as temperature emerges from the number of particles. Why this makes the three regimes fit together: They are not separate levels. They are degrees of the same process. Time emerges together with causality. Without decoherence, there is no defined time. With decoherence, irreversibility emerges. And with it, the arrow of time. Relativity is the intermediate regime. What it would resolve: The problem of time: it emerges with decoherence. The arrow of time: it is the irreversibility of decoherence. The relativity–quantum clash: it is not a clash. They are different regimes of the same process. Causality: it is not fundamental. It is emergent. What it does not resolve: What decoherence exactly is. How to quantize gravity. Why there is decoherence. The information paradox. The label (important):This is an imaginary hypothesis, not a theory. It is coherent with what we know, but it is not demonstrated. It lacks mathematics, predictions, and confirmation. It does not say that time does not exist. It says that time could emerge. It does not say that causality does not exist. It says that causality could emerge. All of this, if one takes decoherence seriously, without adding anything else. But no one has demonstrated it. That is why it is a hypothesis, not a theory. A note on the anchor:This hypothesis does not invent its premise. It takes it from established physics: quantum mechanics already treats information as correlation, not as substance. Entanglement is pure correlation. Decoherence is entanglement with the environment. If information is always relational, then causality could be relational too. Related work: "Measure Reinforcement: Emergent Causality From Quantum Interactions" (Zenodo, 2025). Proposes emergent causality from quantum interactions without predefined time. Difference: includes a conscious observer. This hypothesis does not. Here decoherence is the mechanism, without consciousness. "Spacetime as an Emergent Causal Structure: A Quantum Mechanical Perspective" (Preprints.org, 2026). Derives proper time from the degree of decoherence. Difference: focuses on time, not causality as a regime. This hypothesis unifies the three regimes (quantum, relativistic, classical) into a single continuum. "Causal Space-Time: Emergent Physics from Causality" (2010). Proposes that physics emerges from a causal network. Difference: opposite direction. There causality is fundamental; here it emerges from decoherence. "Radial Coherential Dynamics: Emergent Causality" (2025). Proposes collapse as a decoherence front that generates time and causality. Difference: it is radial and has specific predictions. This hypothesis is more general: it describes causality as a regime of description, not as a radial process. The gap: no one has unified the three regimes (quantum, relativistic, classical) into a continuum parametrized by the degree of decoherence, nor has proposed that causality is a property of the level of description, not of the universe. Keywords: decoherence, emergent causality, quantum mechanics, foundations of physics, emergent time, quantum-classical transition.