Quantum physics frequently involves a need to count the states, subspaces, measurement outcomes, and other elements of quantum dynamics. However, with quantum mechanics assigning probabilities to such objects, it is often desirable to work with the notion of a "total" that takes into account their varied relevance. For example, such an effective count of position states available to a lattice electron could characterize its localization properties. Similarly, the effective total of outcomes in the measurement step of a quantum computation relates to the efficiency of the quantum algorithm. Despite a broad need for effective counting, a well-founded prescription has not been formulated. Instead, the assignments that do not respect the measure-like nature of the concept, such as versions of the participation number or exponentiated entropies, are used in some areas. Here, we develop the additive theory of effective number functions (ENFs), namely functions assigning consistent totals to collections of objects endowed with probability weights. Our analysis reveals the existence of a minimal total, realized by the unique ENF, which leads to effective counting with absolute meaning. Touching upon the nature of the measure, our results may find applications not only in quantum physics, but also in other quantitative sciences.
Quantum physics frequently involves a need to count the states, subspaces, measurement outcomes, and other elements of quantum dynamics. However, with quantum mechanics assigning probabilities to such objects, it is often desirable to work with the notion of a "total" that takes into account their varied relevance. For example, such an effective count of position states available to a lattice electron could characterize its localization properties. Similarly, the effective total of outcomes in the measurement step of a quantum computation relates to the efficiency of the quantum algorithm. Despite a broad need for effective counting, a well-founded prescription has not been formulated. Instead, the assignments that do not respect the measure-like nature of the concept, such as versions of the participation number or exponentiated entropies, are used in some areas. Here, we develop the additive theory of effective number functions (ENFs), namely functions assigning consistent totals to collections of objects endowed with probability weights. Our analysis reveals the existence of a minimal total, realized by the unique ENF, which leads to effective counting with absolute meaning. Touching upon the nature of the measure, our results may find applications not only in quantum physics, but also in other quantitative sciences.
A marquee feature of quantum behavior is that, upon probing, the microscopic system emerges in one of multiple possible states. While quantum mechanics postulates the respective probabilities, the effective abundance of these simultaneous ``identities'', if a meaningful concept at all, has to be inferred. To address such problems, we construct and analyze the theory of functions assigning the quantity (effective number) of objects endowed with probability weights. In a surprising outcome, the consistency of such probability-dependent measure assignments entails the existence of a minimal amount, realized by a unique effective number function. This result provides a well-founded solution to identity-counting problems in quantum mechanics. Such problems range from counting the basis states contained in an output of a quantum computation, and relevant in the analysis of quantum algorithms, to a novel way to characterize complex states such as QCD vacuum or eigenstates of quantum spin systems. In accompanying works, we analyze notable consequences of these findings, namely expressing quantum uncertainty as a measure, the ensuing universal treatment of localization phenomena, and effective description of quantum states. At the basic level, our results point to useful extensions for concepts of measure and support, and to a new probability notion of effective choices.
Indeterminacy associated with the probing of a quantum state is commonly expressed through spectral distances (metric) featured in the outcomes of repeated experiments. Here, we express it as an effective amount (measure) of distinct outcomes instead. The resulting μ-uncertainties are described by the effective number theory whose central result, the existence of a minimal amount, leads to a well-defined notion of intrinsic irremovable uncertainty. We derive μ-uncertainty formulas for arbitrary set of commuting operators, including the cases with continuous spectra. The associated entropy-like characteristics, the μ-entropies, convey how many degrees of freedom are effectively involved in a given measurement process. In order to construct quantum μ-entropies, we are led to quantum effective numbers designed to count independent, mutually orthogonal states effectively comprising a density matrix. This concept is basis-independent and leads to a measure-based characterization of entanglement.
Quantum mechanics is basically a mathematical recipe on how to construct physical models. Historically its origin and main domain of application has been in the microscopic regime, although it strictly seen constitutes a general mathematical framework not limited to this regime. Since it is a statistical theory, the meaning and role of probabilities in it need to be defined and understood in order to gain an understanding of the predictions and validity of quantum mechanics. The interpretational problems of quantum mechanics are also connected with the interpretation of the concept of probability. In this thesis the use of probability theory as extended logic, in particular in the way it was presented by E. T. Jaynes, will be central. With this interpretation of probabilities they become a subjective notion, always dependent on one's state of knowledge or the context in which they are assigned, which has consequences on how things are to be viewed, understood and tackled in quantum mechanics. For instance, the statistical operator or density operator, is usually defined in terms of probabilities and therefore also needs to be updated when the probabilities are updated by acquisition of additional data. Furthermore, it is a context dependent notion, meaning, e.g., that two observers will in general assign different statistical operators to the same phenomenon, which is demonstrated in the papers of the thesis. It is also presented an alternative and conceptually clear approach to the problematic notion of "probabilities of probabilities", which is related to such things as probability distributions on statistical operators. In connection to this, we consider concrete numerical applications of Bayesian quantum state assignment methods to a three-level quantum system, where prior knowledge and various kinds of measurement data are encoded into a statistical operator, which can then be used for deriving probabilities of other measurements. The thesis also offers examples of an alternative quantum state assignment technique, using maximum entropy methods, which in some cases are compared with the Bayesian quantum state assignment methods. Finally, the interesting and important problem whether the statistical operator, or more generally quantum mechanics, gives a complete description of "objective physical reality" is considered. A related concern is here the possibility of finding a "local hidden-variable theory" underlying the quantum mechanical description. There have been attempts to prove that such a theory cannot be constructed, where the most well-known impossibility proof claiming to show this was given by J. S. Bell. In connection to this, the thesis presents an idea for an interpretation or alternative approach to quantum mechanics based on the concept of space-time.
Quantum mechanics is the study of uncertainty. But what does uncertainty mean within the structure of physical reality? From Newton onward, mathematical physics has largely approached motion by specifying a state, identifying the laws governing its evolution, and determining the subsequent behavior of the system. Fourier extended this program to heat, Maxwell to electromagnetic fields, Navier and Stokes to fluids, Hamilton and Lagrange to variational and canonical mechanics, Einstein to spacetime, and quantum mechanics to the evolution and measurement of the quantum state. Across these theories, increasingly powerful mathematics has described what a physical system may do and how its state may evolve. This work asks a logically prior question: Does everything that is mathematically accessible participate equally in physical realization? We propose that it does not. The central distinction developed in this work is between accessibility, admissibility, participation, and realization. A mathematical representation may contain a large space of possible states, trajectories, modes, or outcomes. Physical realization, however, need not occupy that space uniformly. We propose that relational structure establishes an admissible geometry within the accessible space; that geometry determines which possibilities participate strongly or weakly; and probability subsequently quantifies the residual uncertainty among physically supported realizations. The resulting ordering is: Potentiality → Relations → Structure → Geometry → Admissibility → Participation → Realization → Evolution. Under this interpretation, probability is not abolished. Superposition is not abolished. Quantum mechanics is not rejected. Rather, their physical ordering is reconsidered. Superposition describes what the mathematical representation can contain; relational geometry determines how strongly those possibilities can participate; probability describes the uncertainty remaining within that structured support; and observation records the realization that occurs. The NIST loophole-free two-photon Bell experiment provides the principal empirical test developed in this work. Beginning from the experimentally characterized entangled state—not from subsequently observed outcome counts—we construct a relational geometry and the corresponding outcome-dependent participation and information-cost landscapes. The NIST measurement configurations then become experimental sampling coordinates on a geometry determined independently from the later event records. The resulting analysis identifies several mutually reinforcing features: strong inequality among the participation of mathematically accessible Bell outcomes; a corresponding hierarchy of geometric information cost; concentration of experimental variation into reduced effective dimensions; agreement between predicted geometric support and held-out experimental records; preservation of Bell-inequality violation on the reduced realization geometry; compatibility with no-signalling; and robustness tests against multinomial, temporal, detector, and other alternative explanations. The interpretation is not a restoration of Bell-local hidden variables and does not require superluminal signalling. Bell's theorem continues to exclude the relevant locally factorized descriptions. Instead, the observed correlations are interpreted as local measurements of a common non-separable relational structure. Spatial separation does not imply relational or informational separability. This distinction also reframes the quantum measurement problem. Wave-function collapse need not be regarded as the event that creates the physical structure responsible for an outcome. The framework instead proposes that admissibility constrains participation prior to observation, with measurement revealing a realization from an already structured physical domain. A definitive replacement for the standard measurement postulate will require discriminating predictions beyond those already shared with conventional quantum mechanics. The broader implication concerns the theory of motion itself. Newtonian mechanics, Hamiltonian dynamics, Maxwell's equations, Fourier transport, Navier–Stokes flow, relativity, and quantum mechanics employ very different evolution equations, yet all operate upon structured physical domains. The admissibility framework proposes that their deeper commonality lies not in a single dynamical equation but in an ontological ordering: the laws of motion govern how reality evolves; admissibility governs the structure within which reality can become realized. This has consequences beyond quantum foundations. In engineering and computational physics, enormous mathematical state spaces are routinely explored through CFD, finite-element simulation, optimization, Monte Carlo methods, and digital twins. If physically significant behavior occupies a substantially smaller admissible geometry, computation can be redirected from exhaustive exploration toward the structures in which realizable behavior is concentrated. This provides a theoretical basis for reduced-order physical prediction, targeted simulation, transport localization, and deterministic Physics Intelligence. In AI infrastructure and high-performance computing, the same principle suggests that workloads, thermal transport, energy utilization, routing, scheduling, and hardware states need not be treated as equally relevant points within enormous combinatorial spaces. Identifying admissible low-dimensional structure before exhaustive computation offers a route toward reduced search complexity, improved orchestration, higher computational and energy efficiency, and physics-informed predictive control. In aerospace, energy, advanced manufacturing, climate systems, and data-center infrastructure, the framework similarly motivates identifying the geometry of physically participating transport before expending computational resources resolving the entire mathematically accessible domain. The work therefore proposes a common principle extending from fundamental physics to computational engineering: Mathematical possibility is larger than physical realization. Uncertainty may remain genuine at the level of an individual event without implying an absence of antecedent structure. The results presented here provide empirical evidence consistent with the proposition that physical realization is organized by relational geometry prior to probability, while identifying the further tests required to establish whether this ordering constitutes a fundamental law of quantum and classical motion. The next decisive tests include prospective Bell predictions, multipartite and contextuality experiments, Lorentz-covariant formulation, recovery of the classical limit, and extension to quantum field theory and curved spacetime. Nature need not explore everything mathematics permits. Motion is the realization of what physical structure admits. That description captures the actual architecture of the finished 130-page work: it preserves Bell's theorem rather than attempting to evade it, explicitly separates mathematical accessibility from physical participation, and treats the NIST comparison as evidence for the proposed framework while acknowledging that a distinctive prediction beyond standard quantum mechanics remains necessary for a physical replacement of collapse. Principal Implications What This Means for Bell’s Inequality Bell's theorem remains intact. The present framework does not attempt to restore Bell-local hidden variables, weaken Bell's result, or explain the observed correlations through faster-than-light signalling. Bell establishes that the correlations observed between Alice and Bob cannot be reproduced by the relevant locally factorized description. The question developed here begins after that result: if Bell-local separability fails, what physical structure carries the correlations? The proposed answer is relational geometry. Alice and Bob remain spatially separated and perform local measurements, but their outcomes arise from different interrogations of a common non-separable quantum structure. Spatial separation therefore does not imply informational or relational separability. The correlation need not be interpreted as something transmitted from Alice to Bob at the moment of measurement; it can instead be understood as a property of the joint structure to which both measurements belong. The NIST analysis gives this interpretation a quantitative form. The characterized entangled state determines the relational geometry before the subsequent detector records are used for evaluation. Alice's and Bob's measurement settings sample different regions of that geometry. The resulting Bell outcomes exhibit strongly unequal participation, while Bell violation and no-signalling are preserved. The proposed implication is therefore: Bell violation demonstrates the failure of Bell-local factorization; admissibility proposes a geometry for the non-separable physical structure that remains. This reframes the familiar intuition of "action at a distance." The deeper question may not be how information crosses the distance between Alice and Bob, but why spatial distance should require the relational information defining their joint state to become separable in the first place. What This Means for Wave-Function Collapse The framework does not discard wave-function collapse as an operational rule for updating the quantum state after an observation. It changes the proposed ontological role assigned to collapse. In the conventional collapse-centered picture, the quantum state contains multiple possible outcomes and measurement marks the transition to a definite result. This leaves a foundational question: what physically distinguishes the outcome that becomes real from those that do not? Admissibility inserts a structural stage before observation. The accessible quantum
In studies of entanglement, finding out if a state is entangled and quantifying the amount of entanglement contained in a state are related but different questions. Similarly in studies of causality, finding out the causal structures compatible with a model and quantifying the causal strengths are related but different questions. Recently much research have been directed towards the first question but considerably less attention is paid to the second one. In this paper we propose axioms for all reasonable quantitative measures of causality to obey. The axioms apply to a broad family of operational probabilistic theories with and without definite causal structure. For quantum models, we study causality measures based on one-shot quantum capacities in detail. These measures are used to define the notions of quantum signalling and quantum causality measures in order to quantify quantum causality.
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.