Classical Mechanics and Causality

Asrarul Haque

Ognyan Oreshkov, Fabio Costa, Časlav Brukner

The idea that events obey a definite causal order is deeply rooted in our understanding of the world and at the basis of the very notion of time. But where does causal order come from, and is it a necessary property of nature? Here, we address these questions from the standpoint of quantum mechanics in a new framework for multipartite correlations that does not assume a pre-defined global causal structure but only the validity of quantum mechanics locally. All known situations that respect causal order, including space-like and time-like separated experiments, are captured by this framework in a unified way. Surprisingly, we find correlations that cannot be understood in terms of definite causal order. These correlations violate a 'causal inequality' that is satisfied by all space-like and time-like correlations. We further show that in a classical limit causal order always arises, which suggests that space-time may emerge from a more fundamental structure in a quantum-to-classical transition. Causal order is a concept that is engrained in the standard understanding of time, both in classical and quantum mechanics. Oreshkovet al.generalize the standard formalism of quantum theory to a framework with no pre-existing causal order, and find a new class of correlations that have no analogue in the classical world.

Patrick Suppes, Mario Zanotti

Rafael Eduardo Lorca

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.

T. M. Helliwell, D. A. Konkowski

A potential causality paradox in quantum mechanics involving the measurement of a correlated system of two atoms that have moved far apart is constructed along the lines of a classical causality paradox involving the exchange of superluminal signals between distant observers. It is shown that the quantum measurement example does not lead to a paradox, but that the results have implications for the interpretation of quantum mechanics. The thought experiments presented have been used in undergraduate courses in special relativity and quantum mechanics.

Kalervo V. Laurikainen