This paper develops an information-theoretic framework distinguishing causal interaction from observability. By defining observability in terms of mutual information between physical systems and observers, the work examines conditions under which systems may remain unobservable in principle despite participating in causal physical processes. Examples from cosmological horizons and quantum no-signaling are used to illustrate limits on epistemic accessibility embedded within established physical theories.
This paper develops an information-theoretic framework distinguishing causal interaction from observability. By defining observability in terms of mutual information between physical systems and observers, the work examines conditions under which systems may remain unobservable in principle despite participating in causal physical processes. Examples from cosmological horizons and quantum no-signaling are used to illustrate limits on epistemic accessibility embedded within established physical theories.
The no-signaling principle preventing superluminal communication is a limiting paradigm for physical theories. Within the information-theoretic framework it is commonly understood in terms of admissible correlations in composite systems. Here we unveil its complementary incarnation---the ''dynamical no-signaling principle''---which forbids superluminal signaling via measurements on simple physical objects (e.g., particles) evolving in time. We show that it imposes strong constraints on admissible models of dynamics. The posited principle is universal; it can be applied to any theory (classical, quantum, or postquantum) with well-defined rules for calculating detection statistics in spacetime. As an immediate application we show how one could exploit the Schr\"odinger equation to establish a fully operational superluminal protocol in the Minkowski spacetime. This example illustrates how the principle can be used to identify the limits of applicability of a given model of quantum or postquantum dynamics.
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.
This sixth volume of Causal Physics 101 completes the operational layer of the CT–United framework by introducing a rigorous theory of measurement, observables, and epistemic access. While Parts I–V established the ontology of natural and reflective causes, will dynamics, attractors, and macro-causal systems, this paper formalizes how causal quantities are accessed, measured, inferred, and compared—without collapsing or confusing ontological structure with epistemic procedure. Causal observables are defined as functionals on the causal field and its derived quantities, and measurement is modeled as a reflective coupling between an observing system and the causal field. Measurement is shown to be a reflective causal act that perturbs the system it probes, yet does not alter the ontological classification of the cause being measured: natural causes remain natural even when accessed reflectively. This clean separation between ontological causation and epistemic access resolves long-standing ambiguities surrounding entities that are fundamental yet indirectly observed, such as neutrinos, black holes, vacuum tension, and the speed of light. The paper further develops measurement noise and resolution limits in terms of coherence alignment, introduces the statistical structure of Causal Physics as an epistemic construct rather than an ontological one, and assigns causal weight to observation itself based on alignment with coherence gradients. By closing the measurement loop, Part VI renders Causal Physics a complete, operationally closed physical theory and prepares the ground for causal thermodynamics and cosmology.
This work proposes a minimal informational perspective on physical observability. Instead of treating spacetime and physical states as fundamentally given, the model suggests that what we observe emerges through a structural constraint that filters possible configurations into physically accessible ones. In this view, physical reality is not defined by dynamics alone, but by limits on what information can consistently appear as an observable state. The model introduces the idea that unobserved or excluded configurations are not lost arbitrarily, but contribute indirectly by shaping the structure within which observable states emerge. The framework does not introduce new forces or speculative entities. Rather, it provides a conceptual basis for interpreting physical constraints as consequences of informational selection. The approach is deliberately minimal and compatible with existing physical theories, serving as a foundational step toward more detailed theoretical and empirical developments.
Signal causality, the prohibition of superluminal information transmission, is the fundamental property shared by quantum measurement theory and relativity, and it is the key to understanding the connection between nonlocal measurement effects and elementary interactions. To prevent those effects from transmitting information between the generating and observing process, they must be induced by the kinds of entangling interactions that constitute measurements, as implied in the Projection Postulate. They must also be nondeterministic as reflected in the Born Probability Rule. The nondeterminism of entanglement-generating processes explains why the relevant types of information cannot be instantiated in elementary systems, and why the sequencing of nonlocal effects is, in principle, unobservable. This perspective suggests a simple hypothesis about nonlocal transfers of amplitude during entangling interactions, which yields straightforward experimental consequences.
Abstract This paper proposes a novel framework that reinterprets the observer in quantum mechanics as a physical system governed by causal integration principles. By modeling the observer as an active participant in the measurement process—rather than a passive external entity—we show that quantum collapse can emerge as a natural consequence of the observer’s internal causal structure and information processing constraints. This framework avoids the ad-hoc postulates of standard quantum theory and provides a consistent, ontologically grounded account of measurement. Crucial experimental tests are proposed to validate the predictions of this model, including modified double-slit experiments and precision measurements of quantum correlations. These experiments aim to distinguish between the predictions of our causal integration model and those of conventional quantum mechanics, thereby opening new avenues for experimental investigation into the foundations of quantum theory.