Everybody agrees that quantum physics is strange, and that the world view it implies is elusive. However, it is rarely considered that the theory might be opaque because the mathematical language it employs is inarticulate. Perhaps, if a mathematical language were constructed specifically to handle the theory's subject matter, the theory itself would be clarified. This article explores that possibility. It presents a simple but rigorous language for the description of dynamics, experiments, and experimental probabilities. This language is then used to answer a compelling question: What is the set of allowed experiments? If an experiment is allowed, then the sum of the probabilities of its outcomes must equal 1. If probabilities are non-additive, there are necessarily sets of outcomes whose total probability is not equal to 1. Such experiments are therefore not allowed. That being the case, in quantum physics, which experiments are allowed, and why are the rest disallowed? What prevents scientists from performing the disallowed experiments? By phrasing these questions within our mathematical language, we will uncover answers that are complete, conceptually simple, and clearly correct. This entails no magic or sleight of hand. To write a rigorous mathematical language, all unnecessary assumptions must be shed. In this way, the thicket of ad hoc assumptions that surrounds quantum physics will be cleared. Further, in developing the theory, the logical consequences of the necessary assumptions will be laid bare. Therefore, when a question can be phrased in such a language, one can reasonably expect a clear, simple answer. In this way we will dispel much of the mystery surrounding quantum measurements, and begin to understand why quantum probabilities have their peculiar representation as products of Hilbert space projection operators.
Everybody agrees that quantum physics is strange, and that the world view it implies is elusive. However, it is rarely considered that the theory might be opaque because the mathematical language it employs is inarticulate. Perhaps, if a mathematical language were constructed specifically to handle the theory's subject matter, the theory itself would be clarified. This article explores that possibility. It presents a simple but rigorous language for the description of dynamics, experiments, and experimental probabilities. This language is then used to answer a compelling question: What is the set of allowed experiments? If an experiment is allowed, then the sum of the probabilities of its outcomes must equal 1. If probabilities are non-additive, there are necessarily sets of outcomes whose total probability is not equal to 1. Such experiments are therefore not allowed. That being the case, in quantum physics, which experiments are allowed, and why are the rest disallowed? What prevents scientists from performing the disallowed experiments? By phrasing these questions within our mathematical language, we will uncover answers that are complete, conceptually simple, and clearly correct. This entails no magic or sleight of hand. To write a rigorous mathematical language, all unnecessary assumptions must be shed. In this way, the thicket of ad hoc assumptions that surrounds quantum physics will be cleared. Further, in developing the theory, the logical consequences of the necessary assumptions will be laid bare. Therefore, when a question can be phrased in such a language, one can reasonably expect a clear, simple answer. In this way we will dispel much of the mystery surrounding quantum measurements, and begin to understand why quantum probabilities have their peculiar representation as products of Hilbert space projection operators.
While it is widely acknowledged that quantum theory is strange, and that the world view it implies is elusive, there is considerably less consensus as to why this is the case. This article will explore the possibility that the theory may be opaque because the mathematical language it employs is inarticulate. To address this possibility, three interconnected mathematical theories will be developed, each specifically designed to handle aspects of quantum theory's subject matter. The first will describe dynamic systems in terms that are general enough to be applicable to any kind dynamics, including both classical and quantum. That description will then be used to construct a theory of experiments, with the aim of understanding what types of experiments scientific methodology allows. Finally, the theory of experiments will found a theory of experimental probabilities applicable to any collection of experiments, so long as their probabilities are internally consistent. The resulting over-arching mathematical language will then be applied to quantum theory. In the initial application, it will be used to explain the core nature and structure of quantum measurement and probability. Following that, it will be used to explore quantum theory's models. Like any theory written in a mathematical language, quantum theory derives its meaning from its class of models. To understand the class quantum models, two models of central importance will be described. The first will be a conceptually simple construction that follows almost immediately from the above theory of dynamics. It fully satisfies both quantum theory and the above mentioned theory of experimentation; additionally, its macroscopic objects obey classical mechanics. The second will utilize a minimal amount of mathematical structure to satisfy the same conditions.
Quantum theory is the soul of contemporary physics. It was discovered in an adventurous way, under the urge to solve the puzzles posed by atomic spectra and blackbody radiation. But after its invention, it immediately became clear that it was not just a theory of specific physical systems: it was rather a new language of universal applicability. Already in 1928, the theory had received solid mathematical foundations by Hilbert, von Neumann, and Nordheim, and this work was brought to completion in the monumental work of von Neumann, in the form that we still study nowadays. The theory is extraordinarily successful, and its predictions have been confirmed to an astonishing level of precision in a large spectrum of experiments. However, almost 90 years after von Neumann's book, quantum theory remains mysterious. Its mathematical formulation – based on Hilbert spaces and self-adjoint operators – is far from having an intuitive interpretation. The association of physical systems to Hilbert spaces whose unit vectors represent pure states, the representation of transformations by unitary operators and of observables by self-adjoint operators – all such postulates look artificial and ad hoc . A slightly more operational approach is provided by the C * -algebraic formulation of quantum theory – still, this formulation relies on the assumption that observables form an algebra , where the physical meanings of the multiplication and the sum are far from clear. In short, the postulates of quantum theory impose mathematical structures without providing any simple reason for this choice: the mathematics of Hilbert spaces is adopted as a magic blackbox that “works well” at producing experimental predictions. However, in a satisfactory axiomatization of a physical theory the mathematical structures should emerge as a consequence of postulates that have a direct physical interpretation. By this we mean postulates referring, e.g., to primitive notions like physical system, measurement, or process, rather than notions like, e.g., Hilbert space, C * -algebra, unit vector, or selfadjoint operator. The crucial question thus remains unanswered: why quantum theory? Which are the principles at the basis of the theory? A case that is often invoked in contrast is that of Special Relativity theory, which directly follows from the simple understandable principle of relativity.
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.
We attempt to contribute some novel points of view to the "foundations of quantum mechanics", using mathematical tools from "quantum probability theory" (such as the theory of operator algebras). We first introduce an abstract algebraic framework for the formulation of mathematical models of physical systems that is general enough to encompass classical and quantum mechanical models. After a short discussion on the properties of classical systems (realism and determinism), we move on to the study of a general class of quantum-mechanical models of physical systems and explain what some of the key problems in a quantum theory of observations and measurements are. In the last part of our essay, we attempt to elucidate the roles played by entanglement between a system and its environment and of information loss in understanding "decoherence" and "dephasing", which are key mechanisms in a quantum theory of measurements and experiments. We also discuss the problem of "time in quantum mechanics" and sketch an answer to the question when an experiment can be considered to have been completed successfully.
The newly revised edition of this well-known book categorizes prevalent views on measurement as described by quantum mechanics, discusses basics agreed to by many who on other issues disagree, and describes some of the problems. Although many paragraphs rest unchanged from the first (1991) edition, results found since 1991 are used to reorganize and sharpen the focus of the book, so that even the old paragraphs are now understood differently and more clearly. One advance in the second edition is a new and more general no-go theorem; another is separation of formal derivations from interpretation. The dozen or so definitions essential to following the book even casually are stated clearly. By this means the authors succeed pretty well in achieving their stated desire to make the book comprehensible to those who do not share their philosophic predispositions. While I differ in philosophy from the authors, attending to their discussion uncovered weaknesses in my own thinking and improved it. The mathematical language of quantum mechanics expresses a system under measurement by states (as rays in a Hilbert space) and a hamiltonian operator, distinct from the apparatus that produces outcomes, which is expressed by a positive operator-valued (POV) measure. In this way quantum language separates state preparation from state measurement. Often the Hilbert space is viewed as a tensor product of subspaces, allowing further conceptual separations. The authors take the stance that quantum mechanics calls for splitting the empirical world into parts, the most essential being (1) objective systems S (to be observed) and (2) apparatus A (preparation and registration devices that produces outcomes (or, as the authors write, definite pointer values) of the observed system). In some interpretations, one or both of an observer and an environment appear as additional parts. What is to be included in the system S versus the apparatus A is not specified by the language, but is open to the theoretician; a probe, for example, can be analysed as either part of the apparatus or as part of the system, as was discussed by von Neumann in work cited by the authors. With what should a theory of measurement be concerned? A broad division of approaches is defined by how one answers the question: should one try to do away with the cut between measuring apparatus and system? The authors phrase this as a choice of REFERENT, a term which in my mind puts them on shaky semantic ground; anyway The Quantum Theory of Measurement is concerned with implications of the `yes'. The book unfolds with the power of a tragedy, in which the logical consequences lead, step-by-step, to the statement and proof of theorem 6.2.1, which, roughly speaking, asserts the non-existence of a solution to the problem of getting definite pointer values from measuring apparatus that exhibits the superposition demanded by its description as a quantum system. Possible responses to this theorem are discussed, among them the many-worlds interpretation and a modification of quantum mechanics that elevates decoherence to a principle. Citations of the literature are thorough, and many points of view are crisply summarized. The book is indispensable to both those who work in the broad direction chosen by the authors and to those who wish to set any other direction in context.
The argument of this article is threefold. First, the article argues that, from its rise in the sixteenth century to our own time, the advancement of modern physics, as mathematical-experimental science, has been defined by the invention of new mathematical structures. Secondly, the article argues that quantum theory, specifically with quantum mechanics, gave this thesis a radically new meaning by virtue of the following two features: one the one hand, quantum phenomena themselves are defined by purely physical features, as essentially different from all previous physics; and on the other, quantum mechanics and quantum field theory are defined by purely mathematical postulates, which connect them to quantum phenomena strictly in terms of probabilities, without representing or otherwise relating to how these phenomena physically come about. While these two features may appear discordant, if not inconsistent, I argue that they are in accord with each other, at least in certain interpretations (including the one adopted here), designated as “reality without realism,” RWR, interpretations. This argument allows this article to offer a new perspective on a thorny problem of the relationships between continuity and discontinuity in quantum physics. In particular, rather than being concern only with the discreteness and continuity of quantum objects or phenomena, quantum mechanics and quantum field theory relate their continuous mathematics to the irreducibly discrete quantum phenomena, in terms of probabilistic predictions, while, at least in RWR interpretations, precluding a representation or even conception of how these phenomena come about. This subject is rarely, if ever discussed, apart from previous work by the present author. It will, however, be given a new dimension in this article which introduces, as one of its main contributions, a new principle: the mathematical complexity principle.