The concept of time presents a profound challenge at the intersection of quantum mechanics and thermodynamics. While fundamental microscopic laws are largely time-symmetric, macroscopic phenomena universally exhibit a distinct thermodynamic arrow of time, characterized by increasing entropy. This paper explores the hypothesis that quantum entanglement plays a crucial role in the emergence of this thermodynamic arrow. We review how the entanglement between a quantum system and its environment leads to decoherence, effectively driving the system towards a state of higher entropy and classical-like behavior. By analyzing concepts such as von Neumann entropy, entanglement entropy, and the Page-Wootters mechanism, we propose a theoretical framework where the degradation and spreading of entanglement act as the underlying mechanism for the perceived unidirectional flow of time. This approach offers a potential resolution to the "problem of time" in quantum gravity and provides a unified perspective on the genesis of irreversibility from the inherently reversible laws of quantum mechanics. We discuss the implications of this entanglement-driven temporal emergence for our understanding of the universe's initial low-entropy state and the fundamental nature of reality.
The concept of time presents a profound challenge at the intersection of quantum mechanics and thermodynamics. While fundamental microscopic laws are largely time-symmetric, macroscopic phenomena universally exhibit a distinct thermodynamic arrow of time, characterized by increasing entropy. This paper explores the hypothesis that quantum entanglement plays a crucial role in the emergence of this thermodynamic arrow. We review how the entanglement between a quantum system and its environment leads to decoherence, effectively driving the system towards a state of higher entropy and classical-like behavior. By analyzing concepts such as von Neumann entropy, entanglement entropy, and the Page-Wootters mechanism, we propose a theoretical framework where the degradation and spreading of entanglement act as the underlying mechanism for the perceived unidirectional flow of time. This approach offers a potential resolution to the "problem of time" in quantum gravity and provides a unified perspective on the genesis of irreversibility from the inherently reversible laws of quantum mechanics. We discuss the implications of this entanglement-driven temporal emergence for our understanding of the universe's initial low-entropy state and the fundamental nature of reality.
This work investigates the fundamental connection between quantum mechanics and thermodynamics through the concept of the arrow of time. While the second law of thermodynamics identifies the arrow of time with the monotonic increase of entropy in isolated systems, closed quantum systems evolve unitarily and conserve von Neumann entropy, making the origin of irreversibility less apparent. Here we establish a no-go theorem proving that correlations in unknown states of closed quantum systems cannot be universally removed by any physical operation. This result demonstrates that correlation generation is inherently irreversible, providing a microscopic and universal basis for the arrow of time. In thermodynamic settings, this irreversibility ensures that heat flows spontaneously from hotter to colder subsystems, reproducing the Clausius formulation of the second law. More generally, the irreversible growth of correlations explains entropy increase, the approach to equilibrium, and decoherence as observable manifestations of the same principle. Our findings ground a quantum-mechanical foundation for the second law of thermodynamics and demonstrate the universality of the arrow of time.
The problem of time’s arrow is (roughly) to understand why macroscopic physical processes typically evolve in a specific direction (the one we call “the future”) even though the underlying laws of physics describing the individual microscopic systems are time-reversal invariant. The standard answer to this problem is given by Boltzmann’s explanation of the II law of thermodynamics: a classical system starting in a non-equilibrium state spontaneously (i.e. with a high level of probability) tends towards equilibrium, thus implementing an irreversible process. However, this explanation has two drawbacks: 1. it needs a further assumption on the entropy of the initial state of the universe (past hypothesis); 2. microscopic systems (e.g. molecules in a box) should be represented as quantum states and not classical states. In this talk, I want to propose a different hypothesis to explain the origin of time’s arrow, based on quantum mechanics, specifically on decoherence theory. Differently from isolated systems, open quantum systems described by decoherence theory showirreversible behavior, as they evolve towards decoherence and von Neumann entropy increase. Is this behavior a sign of genuine irreversibility? And, if so, can this be linked to thermodynamic irreversibility?
We propose a speculative but quantitatively constrained phenomenological framework in which the thermodynamic arrow of time emerges from entropy-dependent decoherence in open quantum systems. Higher environmental entropy strengthens decoherence, preferentially de- stroying quantum coherence in one temporal direction, thus providing a microscopic mechanism for irreversibility. The formalism is constructed within an effective Lindblad master-equation approach, recovering unitary quantum mechanics and standard thermodynamics in appropriate limits. A toy two-level system illustrates the emergence of coherence decay and an effective microscopic arrow of time.
This work explores the hypothesis that time is an emergent phenomenon arising from underlying quantum informational processes. By integrating principles from quantum mechanics, thermodynamics, and general relativity, we develop a comprehensive framework that elucidates the relationship between quantum complexity, entanglement, and the nature of time. We investigate the role of quantum complexity in the evolution of quantum states and demonstrate how the increase in entanglement entropy provides a microscopic basis for the arrow of time. Using the AdS/CFT correspondence and the holographic principle, we establish connections between spacetime geometry and quantum informational measures like entanglement entropy. We address the black hole information paradox within this context, arguing that information is preserved in a highly scrambled form. Additionally, we propose an experimental design using black hole analogues to empirically validate our theoretical predictions, focusing on fast scrambling and the growth of quantum complexity. Our findings suggest that quantum informational dynamics govern the emergence of time and spacetime geometry. This work bridges quantum mechanics, thermodynamics, information theory, and general relativity, offering potential applications in quantum technologies and new insights into the fundamental nature of reality.
This preprint develops a unified coherence-based formulation of physical time grounded in the χ–η–I invariant structure of the Aletheia framework. Across quantum, thermodynamic, gravitational, and cosmological regimes, the paper introduces a scalar coherence potential Ψ constructed from purity, entropy, and structural persistence, and proposes that the physical time increment satisfies the differential law: dt = –κ dΨ In this formulation, time flow is the measurable rate at which coherence is depleted through decoherence, coarse-graining, or environmental coupling. When Ψ is constant—as in globally stationary, Page–Wootters–style quantum states—no internal time elapses. When coherence degrades, time emerges as the bookkeeping of informational loss. The paper demonstrates that this temporal-coupling law naturally reproduces: Thermodynamic time in the macroscopic limit (dt ∝ dS), Quantum decoherence arrows in open-system dynamics, Proper-time–like behavior in weakly decohering or isolated systems, Cosmological time flow via mode decoherence in an expanding FRW universe. Explicit models including qubit dephasing, Lindblad oscillators, and cosmological field modes illustrate how temporal coupling unifies arrows of time across domains traditionally treated with separate frameworks. This formulation provides a coherence-theoretic bridge between general relativity, quantum decoherence, and thermodynamic irreversibility, offering an empirically grounded alternative to conventional parameter-time, thermal time, and entanglement-clock approaches.
Abstract If an asymmetry in time does not arise from the fundamental dynamical laws of physics, it may be found in special boundary conditions. The argument normally goes that since thermodynamic entropy in the past is lower than in the future according to the Second Law of Thermodynamics, then tracing this back to the time around the Big Bang means the universe must have started off in a state of very low thermodynamic entropy: the Thermodynamic Past Hypothesis. In this paper, we consider another boundary condition that plays a similar role, but for the decoherent arrow of time, i.e. the subsystems of the universe are more mixed in the future than in the past. According to what we call the Entanglement Past Hypothesis, the initial quantum state of the universe had very low entanglement entropy. We clarify the content of the Entanglement Past Hypothesis, compare it with the Thermodynamic Past Hypothesis, and identify some challenges and open questions for future research.