Arc II — Observation Paper 1 Title Thermalization Without Equilibrium Zenodo DOI 10.5281/zenodo.18180820 Description This paper demonstrates that thermal behavior does not require equilibrium ensembles, microscopic ergodicity, or underlying statistical assumptions. Instead, thermalization is shown to arise from the progressive degradation of accessible correlations under finite observational capacity. The analysis reframes temperature, entropy production, and apparent equilibration as operational phenomena: when observers lose the ability to resolve fine-grained correlations, remaining observables converge toward thermal statistics regardless of the underlying dynamics. Thermal distributions therefore emerge as inference-fixed points under accessibility constraints, not as properties of microscopic phase space. By constructing thermalization without reference to microstates or equilibration timescales, the paper resolves long-standing conceptual tensions between thermodynamics and unitary dynamics. It clarifies why thermal behavior appears robust across systems with vastly different microscopic laws. This framework applies equally to classical, quantum, and gravitational settings, including horizon-induced thermality and detector responses in curved spacetime. Thermal behavior is shown to be a consequence of observational saturation rather than dynamical relaxation. Keywords thermalization; entropy production; irreversibility; operational thermodynamics; accessibility limits; non-equilibrium physics Notes Arc II — Observation (foundational) Thermal behavior derived without equilibrium assumptions Temperature as an inference artifact Applies to classical, quantum, and gravitational systems Conceptual bridge between entropy (Arc I) and measurement
Arc II — Observation Paper 1 Title Thermalization Without Equilibrium Zenodo DOI 10.5281/zenodo.18180820 Description This paper demonstrates that thermal behavior does not require equilibrium ensembles, microscopic ergodicity, or underlying statistical assumptions. Instead, thermalization is shown to arise from the progressive degradation of accessible correlations under finite observational capacity. The analysis reframes temperature, entropy production, and apparent equilibration as operational phenomena: when observers lose the ability to resolve fine-grained correlations, remaining observables converge toward thermal statistics regardless of the underlying dynamics. Thermal distributions therefore emerge as inference-fixed points under accessibility constraints, not as properties of microscopic phase space. By constructing thermalization without reference to microstates or equilibration timescales, the paper resolves long-standing conceptual tensions between thermodynamics and unitary dynamics. It clarifies why thermal behavior appears robust across systems with vastly different microscopic laws. This framework applies equally to classical, quantum, and gravitational settings, including horizon-induced thermality and detector responses in curved spacetime. Thermal behavior is shown to be a consequence of observational saturation rather than dynamical relaxation. Keywords thermalization; entropy production; irreversibility; operational thermodynamics; accessibility limits; non-equilibrium physics Notes Arc II — Observation (foundational) Thermal behavior derived without equilibrium assumptions Temperature as an inference artifact Applies to classical, quantum, and gravitational systems Conceptual bridge between entropy (Arc I) and measurement
One of the distinguishing achievements of physics has been the ability to describe the Universe based upon underlying microscopic principles. When there are many participating objects being governed by these microscopic principles, the resultant behavior often harbors emergent collective phenomena that may not be obviously related to the microscopic description. Phase transitions, quantum or classical, are a canonical example of such many-body phenomena that are understood in equilibrium through a framework of thermodynamic constraints, statistical ensembles, and a growth of system-wide correlations at the transition. However, a new class of quantum phase transitions has recently emerged for non-equilibrium systems with behavior that is not captured by this traditional framework. In the case of isolated quantum systems, these transitions occur among excited eigenstates and requires knowledge of the structure of these individual eigenstates in the quantum system. Understanding what drives such a phase transition requires a deeper microscopic knowledge of the role of correlations at the transition. \nThe use of ultracold atoms in optical lattices has paved the way for faithfully realizing quantum phase transitions. In particular, the advent of the quantum gas microscope realized an experimental platform that is highly controllable and provides microscopic access to the many-bodied wave functions. All experiments presented in this thesis use such a quantum gas microscope architecture where a quantum degenerate gas of Rb is used to probe the behavior of non-equilibrium quantum systems in one spatial dimension.\nThis thesis briefly discusses some studies of entanglement in equilibrium quantum phase transitions near the ground state of the system. In the first experiment, we directly measure the second-order R\\'enyi entanglement entropy that develops as a Bose-Hubbard system transitions from a Mott insulator to a superfluid. This is followed by a study of entanglement growth at the transition in the transverse Ising model. \nThese studies progress to the investigation of two particular non-equilibrium quantum phases and their transition into one another: quantum thermalization and many-body localization. We investigate the development of entanglement entropy that mimics thermodynamic entropy for an isolated quantum system which locally thermalizes under its own unitary dynamics. We then investigate the only known robust exception to this thermalizing behavior by studying the onset of many-body localization due to disorder. We observe the breakdown of thermalization in this phase and the logarithmically slow growth of entanglement throughout the system, which identifies the localized system to be an interacting one. Lastly, we investigate the role of correlations in the critical dynamics that emerge at high-orders where the system transitions from thermalizing behavior to many-body localization.
We build a framework for the thermodynamics of macroscopic quantum systems. In contrast with approaches requiring access to the full density matrix, our framework relies on a coarse-grained description, based on measurement statistics of a few observables. When these observables commute, the outcomes define classical macrostates whose entropy is quantified by observational entropy, accounting for uncertainty about both the macrostate and the microstate within it. We extend this notion to non-commuting observables forming a subalgebra of the operator space, and use Jaynes' principle to define an algebra-dependent entropy interpolating between von Neumann and observational entropies. Given initial and final measurement sets, connected by internal and/or environment-induced dynamics, we derive a second law for the coarse-grained dynamics. Unlike formulations based on von Neumann entropy, our inequality captures irreversibility from both non-unitary environment-induced dynamics and internal equilibration. It takes the usual form of a positive entropy production when the system is initially at internal equilibrium, while correction terms capture nonequilibrium resources ignored by the coarse-graining. We also derive fluctuation theorems for coarse-grained thermodynamic quantities. Along a quasi-static path of measurement schemes, we identify quantum macroscopic notions of work and heat fulfilling the first and second laws, including an additional work contribution from manipulating the algebra to which the system is confined, through external constraints or quantum measurement backaction. Finally, we apply our framework to examples illustrating the impact of varying the coarse-graining scheme. Our approach unifies macroscopic and stochastic thermodynamics in a genuinely quantum framework, laying the basis for a versatile, experimentally friendly toolbox to analyze complex quantum dynamics.
The emergence of classical behavior and the origin of thermal equilibrium are two central problems in the foundations of physics. In the standard accounts, both phenomena are typically explained through interactions with an external environment: decoherence suppresses quantum interference, while coupling to a thermal bath drives relaxation toward equilibrium. Over the last decades, however, it has become clear that equilibration and thermalization can arise even in fully isolated quantum systems, in the operational sense that the expectation values of relevant observables remain close to equilibrium values for most of the time. Here, we ask whether the same intrinsic equilibration mechanism can also account for the emergence of classical behavior. Using rigorous bounds on equilibration in closed systems, we derive sufficient conditions under which a time-evolved pure state becomes, for most times, operationally indistinguishable from a classical mixture associated with a chosen physical property. We identify two complementary routes to such operational classicality: either the chosen property almost commutes with the system Hamiltonian or the observables used to probe the system lose access to the remaining coherence after equilibration. Our results show that classical behavior need not be confined to the energy basis and may emerge even when substantial coherence remains present in the equilibrium state. This establishes a direct connection between two foundational questions: the origin of thermalization in isolated quantum systems and the quantum-to-classical transition
Quantum thermalization describes how closed quantum systems can effectively reach thermal equilibrium, resolving the apparent incongruity between the reversibility of Schrödinger's equation and the second law of thermodynamics. Despite its ubiquity and conceptual significance, the precise conditions that give rise to quantum thermalization are still not well understood. After nearly a century of efforts, we have yet to find a complete mathematical proof that an effective statistical description naturally emerges the underlying quantum dynamics in generic settings. Here, we prove that quantum thermalization must occur in any qubit system with local interactions under three conditions: (i) high effective temperature, (ii) translation invariance, and (iii) no perfect resonances in the energy spectrum. Specifically, we show that a typical, low-complexity pure state drawn from any ensemble with large entropy and well-defined effective temperature becomes locally indistinguishable from a Gibbs state upon unitary evolution. In this setting, our rigorous results prove the widely anticipated notion that statistical physics should be understood as an emergent phenomenon, explicitly derived from the first principles of quantum mechanics.
We identify a general mechanism for the finite-time thermalization of macroscopic observables, such as coarse-grained charge densities, in terms of elementary forms of the quantum dynamics of initial states: (1) aperiodicity, which provides a computable measure of (2) a dynamical partially ergodic exploration of the Hilbert space. Specifically, this mechanism predicts the equilibration of all (concentrated) macroscopic observables, in almost all states in an initial ensemble and almost all times within finite and longer intervals, given only the observable-independent information that the return probability of the ensemble of initial states is small over a finite time range. As a special case, it also accesses standard results on equilibration over infinitely long times in terms of (stronger versions of) the effective dimension of initial state delocalization in the energy eigenbasis. Our results incorporate macroscopic thermalization into the domain of operational quantum statistical mechanics, recently developed to provide finitely computable criteria for microscopic thermalization. We discuss an overall characterization of this approach as establishing connections between (1) the decay of a (theoretically or experimentally) computable probe indicating memorylessness, (2) a fundamental invariant mechanism in terms of the alignment of observables or states in the Hilbert space, and (3) predicting different natural forms of (classical and) quantum thermalization, most of which rigorously recover conventional eigenstate-based descriptions of infinite-time thermalization as a special case but provide stronger accessible predictions over finite observation times in the thermodynamic limit.
Classical thermodynamics treats temperature as a state variable characterizing systems in equilibrium with idealized infinite reservoirs. We argue that this framing, while computationally exact, obscures an essential physical reality: any system at finite characteristic energy Ec=kBT continuously emits thermal radiation and cools unless energy input compensates these losses. What thermodynamics calls ``thermal equilibrium'' is, at the microscopic level, a dynamically sustained steady state maintained by continuous photon exchange.
We derive that the average photon energy required to sustain a Planck distribution is ⟨hν⟩=π4Ec/[30ζ(3)]≈2.701Ec, quantifying the energetic throughput that any real system must sustain to maintain a given temperature. We resolve the apparent contradiction with the purely mechanical Maxwell velocity distribution: billiard-ball kinetics correctly describe the \emph{shape} of the distribution at a given Ec, but cannot account for how Ec is established or maintained against radiative losses in any real system of charged particles. We further show that every finite thermal reservoir is itself maintained by photon exchange at a larger scale, organizing physical systems into a natural hierarchy from individual samples through cryostats, laboratories, and planetary surfaces to stellar interiors, with the classical infinite reservoir emerging as the large-capacity limit within that hierarchy rather than a fundamental physical entity. We also comment on the relation between thermodynamic entropy S=kBlnW and the dimensionless entropy S=lnW, emphasizing that kB primarily fixes units (J/K) rather than introducing new statistical content. These results do not modify thermodynamics but provide its mechanistic interpretation in terms of quantum electrodynamics.