Despite its enormous empirical success, the formalism of quantum theory still raises fundamental questions: why is nature described in terms of complex Hilbert spaces, and what modifications of it could we reasonably expect to find in some regimes of physics? Here we address these questions by studying how compatibility with thermodynamics constrains the structure of quantum theory. We employ two postulates that any probabilistic theory with reasonable thermodynamic behaviour should arguably satisfy. In the framework of generalised probabilistic theories, we show that these postulates already imply important aspects of quantum theory, like self-duality and analogues of projective measurements, subspaces and eigenvalues. However, they may still admit a class of theories beyond quantum mechanics. Using a thought experiment by von Neumann, we show that these theories admit a consistent thermodynamic notion of entropy, and prove that the second law holds for projective measurements and mixing procedures. Furthermore, we study additional entropy-like quantities based on measurement probabilities and convex decomposition probabilities, and uncover a relation between one of these quantities and Sorkin’s notion of higher-order interference.
This thesis investigates the connection between quantum theory, thermodynamics and information theory. Theories with structure similar to that of quantum theory are considered, mathematically described by the framework of "Generalized Probabilistic Theories". For these theories, a thought experiment by von Neumann is adapted to obtain a natural thermodynamic entropy definition, following a proposal by J. Barrett. Mathematical properties of this entropy are compared to physical consequences of the thought experiment. The validity of the second law of thermodynamics is investigated. In that context, observables and projective measurements are generalized to prove an entropy increase for projective measurements of ensembles. Information-theoretically motivated definitions of the entropy are compared to the entropy from the thermodynamic thought experiment. The conditions for the thermodynamic entropy to be well-defined are considered in greater detail. Several further properties of the theories under consideration (e.g. whether there is higher order interference, Pfister's state discrimination principle) and their relation to entropy are investigated.
We investigate the extent to which thermodynamic reasonableness, formalized as two operational postulates on entropy and reversible dynamics within the framework of Generalized Probabilistic Theories (GPTs), uniquely determines the complex Hilbert space formalism of standard quantum mechanics. Building on Krumm, Müller, and Masanes (2017), we provide an extended classification of state spaces satisfying thermodynamic consistency for systems of (N \ge 3) distinguishable states, proving that self-duality of the state cone, spectral entropy compatibility, and continuous reversible dynamics jointly eliminate real and quaternionic quantum theories as viable candidates. We show that augmenting the thermodynamic postulates with local tomographic closure and a purification principle is sufficient to recover full complex Hilbert space structure, deriving analogues of the Born rule via entropy maximization arguments. A central novel result is a no-go theorem establishing that non-complex state spaces cannot support a continuous, reversible, and entropy-preserving dynamics compatible with thermodynamic equilibration in composite systems. We further propose experimentally falsifiable signatures of post-quantum deviations using higher-order interference, generalized Landauer erasure, and multi-slit protocols. Also available on Academia.edu: https://www.academia.edu/attachments/132550729/download_file?s=portfolio
Despite its unparalleled empirical success, the standard formalism of quantum mechanics, strictly bound to complex Hilbert spaces, raises profound foundational questions regarding its axiomatic origins. Among the most enduring mysteries is why quantum theory permits pairwise, second-order interference, yet strictly forbids genuine higher-order multi-path interference. While Rafael Sorkin's quantum measure theory formalized this absence of higher-order interference mathematically, a comprehensive physical justification rooted in thermodynamics has remained elusive. This report provides an exhaustive analysis bridging Sorkin's hierarchy of interference with operational thermodynamics within the framework of Generalized Probabilistic Theories (GPTs). Building upon Krumm et al.'s seminal Theorem 12—which establishes a strict equivalence between the absence of third-order interference and the coincidence of measurement and decomposition max-entropies—this paper generalizes the Mamun, SM (2026) ……………………………….1 Higher-Order Interference and Thermodynamic Entropy in Generalized Hilbert Spaces Bridging Sorkin's Framework with Operational Thermodynamics (Vol-III) equivalence to arbitrary-order Rényi entropies across non-Hilbert state spaces. Also available on Academia.edu: https://www.academia.edu/attachments/132551016/download_file?s=portfolio
This article examines the scope and limitations of quantum mechanics in its Schrödinger and Dirac formulations. Quantum theory provides an extraordinarily precise description of how physical states evolve, interfere, and produce measurable probabilities. However, like other foundational theories, it presupposes a specific mathematical structure—complex Hilbert spaces, linear unitary dynamics, an external time parameter, and the Born rule—that it does not itself explain. The work argues that quantum mechanics answers the question of how quantum states behave, but not the deeper question of why physical reality admits a state-based, probabilistic description at all. To address this missing level of explanation, the article places quantum mechanics within the broader framework of the Thermodynamic Imperative of Order (TIO) and the Granular Entropic Program (GEP). Within this framework, quantum states are interpreted as emergent, statistically stable macroscopic representations of typical correlations in high-dimensional configuration spaces. Linearity, unitarity, and the Born rule arise as necessary consequences of measure concentration and stability, rather than as fundamental postulates. The article does not propose modifications to quantum mechanics. Instead, it clarifies the domain of validity of the theory and explains why a framework with its structure and remarkable universality must exist if stable quantum phenomena are to be observed at all.
It is a scientific truism nowadays that quantum mechanical predictions are of statistical or rather of probabilistic nature: a quantum theory assigns a probability distribution to each measurable quantity (observable) of a physical system prepared according to a given preparation prescription (state). Such sets of probability distributions represent the most general quantitative predictions of physical theories. The aim of this study is to determine all theoretical frameworks which yield such a kind of output taking the probabilistic nature of quantitative physical predictions as initial hypothesis. Astonishingly, this project succeeds with a minimal number of additional assumptions, which arise quite naturally in the course of the analysis, and which essentially amount to regularity conditions. The result is that there exist only two classes of physical theories providing quantitative probabilistic predictions: classical theories where observables are represented by real functions over a phase space and states are represented by probability measures upon the phase space, and quantum theories where observables are represented by selfadjoint operators acting on a complex Hilbert space and states are represented by state operators. On empirical grounds, classical theories cannot provide the all-encompassing, fundamental frame for the mathematical description of physical systems. Quantum theories remain, and there is no more complete formal frame for quantitative physical theories. As an additional result, it is shown that there exist physical systems which possess a quantum mechanical but no classical description; a prominent example is the spin system of two spin-1/2 particles. The classical hidden variables model, which J. Bell relates to this system in order to prove his theorem, cannot be a true description of it because no such classical model exists, and Bell's theorem loses its foundation.