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.
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.
The consistent histories formalism can be used to describe histories comprised of events across many systems, times, and places, plausibly rich enough to describe our experiences of the classical world; however, many consistent history sets are nonclassical and thus not obviously relevant to our experiences. Meanwhile, the program of environmentally induced decoherence identifies dynamically privileged classical degrees of freedom, but provides no general account of when or how many such degrees of freedom consistently combine to form histories. This work shows that the strengths of these two approaches can be combined by adopting a dynamics-first perspective on decoherence. Inspired by quantum causal models and quantum Darwinism, we define the process of decoherence in terms of the causal influences through unitary dynamics required for the proliferation of information about observables. We characterise decoherence as a property of the unitary dynamics, without presupposing the existence of any quantum state. Instead, we show that the state emerges from dual decoherence, related to decoherence by time-reversal of the unitary dynamics. Indeed, for any set of systems in an arbitrary unitary circuit, decoherence and its dual single out a privileged consistent history set -- and we demonstrate through examples that states emerge from dual decoherence while outcomes emerge from decoherence. Hence the idea that quantum states emerge from the process of decoherence turns out to be the key missing ingredient for unifying environmentally induced decoherence and consistent histories. Taking this idea ontologically seriously leads to a recently proposed causal interpretation of quantum theory or a dynamics-first version of the Everett interpretation. The causal approach also sheds light on the suppression of off-diagonal terms, time asymmetry, and robustness of the pointer basis.
Theory and experiment have not fully resolved the apparent dichotomy , which has agonized physics for the past eighty years: on the one hand, the description of microsystems by quantum mechanics and, on the other, the description of macrosystems by classical dynamics or statistical mechanics. Derivations of the time-irreversible Liouville equation for an open quantum system, based on projecting out its environment, have narrowed the gap between the quantum and classical descriptions. Yet our `classical' intuition continues to be confronted by quantum-mechanical results like the Einstein--Podolsky--Rosen paradox that challenges the classical notion of locality, or the quantum Zeno effect which suggests that the isolation of a system is not the only way to preserve its quantum state. There are two key concepts in any discussion of such issues. The first, which is responsible for the most salient nonclassical properties, is entanglement , that is partial or complete correlation or, more generally, inseparability of the elements comprising a quantum ensemble . Even after their interaction has ceased, this inseparability, originating from their past interaction, can affect the state of one element when another element is subject to a nonunitary action, such as its measurement, tracing- out, or thermalization. The second key concept is decoherence of open quantum systems, which is the consequence of their entanglement with their environment, a `meter' or a thermal `reservoir', followed by the tracing-out of the latter. Despite new insights into entanglement and decoherence, there are still no complete, unequivocal answers to the fundamental questions of the transition from quantal to classical behaviour: how do irreversibility and classicality emerge from unitarity as systems and their environments become increasingly complex ? At what stage does system--meter entanglement give rise to a classical readout of the meter? Is there an upper limit on the size or complexity of systems displaying entanglement? Major developments have opened new vistas into controlled entanglement , which is the resource of quantum information processing. However, these developments have mainly focused on ensembles of simple two- and three-level systems that are thoroughly isolated from their environments. Treatments of coherence in quantum computing have mostly assumed that only a single or a few the elements of the quantum ensemble may simultaneously undergo an uncontrolled intervention---a quantum error. Decoherence-control protocols for more general types of errors are still incomplete. In order to resolve the outstanding issues of the quantum--classical transition, and study the control of entanglement and decoherence without the foregoing restrictions, we must venture into the domain of Quantum Complex Systems (QUACS), either consisting of a large number of inseparable elements or having many coupled degrees of freedom. Our conviction is that fundamental understanding and manipulation of entanglement within QUACS or their entanglement with the environment or a meter, call for the creation of a new conceptual framework or paradigm, that would encompass phenomena common to cold atoms in laser fields, large molecules, Josephson junctions, quantum gases and solids, with the view of employing these systems for quantum information processing and computing. Progress within this paradigm should allow us to answer the questions: does entanglement play an essential role in the evolution of large collections of complex systems? What are the size and complexity limits of systems and ensembles still controllable by an external intervention? What are the most appropriate decoherence protection schemes and control algorithms? This special issue addresses the challenge of understanding in depth and manipulating the basic quantum properties of optical, atomic, molecular and condensed-matter QUACS, and large ensembles thereof. In view of the interdisciplinary character of this issue, we find it expedient to look at the presented articles not only according to the physical objects they describe, but also from a unifying standpoint. Several unifying themes may be discerned: memory effects in dephasing and relaxation; protection of quantum information by its distribution throughout the ensemble; control of decoherence and entanglement by time-dependent interventions or measurements; manifestations of entanglement in scattering off statistical ensembles; measurment-induced dynamics. The concerted discussion of the topics outlined above should help us advance the new paradigm that addresses our abilities to diagnose and manipulate the entangled states of complex quantum objects and their robustness against decoherence. These abilities are required for quantum information (QI) applications in matter--wave interferometry in molecular, semiconducting or superconducting systems. On the fundamental level, this book may help establish the notion of dynamical information exchange between quantum systems and chart in detail the route from unitarity to classicality. Further developments within the outlined paradigm should yield novel, advantageous QI processing schemes and high-sensitivity interferometers, owing to decoherence suppression and effective control of many degrees of freedom. In the long run, these strategies may prove to be the first step towards the (hitherto unattempted) use of entanglement in nanotechnology, metrology and chemistry, with potentially remarkable novel applications.
Traditional physics and classical intuition rely heavily on the concept of unilateral, linear causality, in which causes strictly precede their effects in time. However, deep conceptual challenges arising from quantum mechanics, general relativity, and modern theories of quantum gravity indicate severe limitations of this one-way temporal causal chain at the fundamental level. This paper proposes an alternative physical paradigm: causality is not a localized process evolving unidirectionally along a fixed time axis, but rather a synchronic network jointly determined by the holistic physical structure, global boundary conditions, and topological-geometric features of the system. By analyzing delayed-choice experiments, quantum entanglement, action principles in classical and quantum field theories, and closed timelike curves in general relativity, we formulate a mathematical framework for "holistic structural causality" that operates independently of a unidirectional arrow of time. This framework demonstrates that the forward flow of time is merely an emergent thermodynamic property at the macroscopic scale. At the fundamental level, global phase-space constraints dictate the co-existence and mutual relations of events. This paradigm offers new perspectives for resolving the quantum measurement problem and the time-symmetry paradox, while unlocking valuable theoretical potential for modern cosmology and quantum gravity.
We consider the claim that decoherence explains the emergence of classicality in quantum systems, and conclude that it cannot. We show that, given a universe composed of a variety of subsystems, some of which are macroscopic and subject to decoherence-inducing interactions, and some of which are microscopic, the macroscopic subsystems do not display any distinctively classical behavior. Therefore, a universe in which macroscopic and microscopic do display distinct behavior must be in a very special, highly nongeneric quantum state.
This paper develops one of the foundational mechanisms of Radial Coherential Dynamics (RCD): coherence is global, but its rupture is radial. This asymmetry resolves the century-old tension between quantum non-locality and relativistic causal structure. In RCD, coherence can extend across spatially separated systems, explaining entanglement, while collapse propagates radially at speed ccc from a nucleation center. Causal structure is therefore not an axiom, but an emergent order defined by the arrival times of collapse fronts. This provides: A derivation of causality from first principles A resolution of the EPR paradox without non-local signaling A mechanism for single outcomes (why we observe one classical reality) A geometric origin for the arrow of time Experimental predictions related to decoherence delays and spatial asymmetries Radial Causality reframes collapse as a wave-like propagation in the coherence field C(x,t)C(x,t)C(x,t), whose emergent metric determines the propagation speed. The universe does not update globally — it updates in expanding radial cascades, each rewriting the boundary between coherence and actuality.
A thoroughgoing and consistent application of quantum theory shows that the connection between classical and quantum physics is very different from what can be found in standard textbooks. In the last two decades it has been shown that the elements of classical physics emerge through an irreversible quantum process called ‘decoherence’. According to decoherence theories, quantum objects acquire classical properties only through interactions with their natural environment as a consequence of the holistic features of quantum theory. In view of the interpretation problem of quantum theory, only two directions out of the host of proposals suggested in the course of time appear to remain consistent solutions. Whatever solution is preferred, the message of decoherence remains the same: classical notions are not needed as the starting point for physics. Instead, these emerge through the dynamical process of decoherence from the quantum substrate.