Traditional formulations of quantum mechanics rely on an unanalysed concept of measurement. Quantum systems are treated as evolving via the unitary Schrodinger evolution, except when they are measured or observed; then, all components of the state are discarded except the one corresponding to the actual measurement result. The component which remains is then regarded as the new state of the system and again is evolved forwards according to the unitary evolution. The measurement problem is the problem of explaining why this two-stage procedure employing a primitive concept of measurement works so well.
Bell 2004 contains a number of exceptionally clear discussions of the measurement problem. Bohr 1935 contains the first explicit claim that measurement plays a fundamental role in quantum theory.
What is quantum mechanics about? The most natural way to interpret quantum mechanics realistically as a theory about the world might seem to be what is called wave function ontology: the view according to which the wave function mathematically represents in a complete way fundamentally all there is in the world. Erwin Schroedinger was one of the first proponents of such a view, but he dismissed it after he realized it led to macroscopic superpositions (if the wave function evolves in (...) time according to the equations that has his name). The Many-Worlds interpretation1 accepts the existence of such macroscopic superpositions but takes it that they can never be observed. Superposed objects and superposed observers split together in different worlds of the type of the one we appear to live in. For these who, like Schroedinger, think that macroscopic superpositions are a problem, the common wisdom is that there are two alternative views: "Either the wave function, as given by the Schroedinger equation, is not everything, or is not right" [Bell 1987]. The deBroglie-Bohm theory, now commonly known as Bohmian Mechanics, takes the first option: the description provided by a Schroedinger-evolving wave function is supplemented by the information provided by the configuration of the particles. The second possibility consists in assuming that, while the wave function provides the complete description of the system, its temporal evolution is not given by the Schroedinger equation. Rather, the usual Schroedinger evolution is interrupted by random and sudden "collapses". The most promising theory of this kind is the GRW theory, named after the scientists that developed it: Gian Carlo Ghirardi, Alberto Rimini and Tullio Weber.. It seems tempting to think that in GRW we can take the wave function ontologically seriously and avoid the problem of macroscopic superpositions just allowing for quantum jumps. In this paper we will argue that such "bare" wave function ontology is not possible, neither for GRW nor for any other quantum theory: quantum mechanics cannot be about the wave function simpliciter. That is, we need more structure than the one provided by the wave function. As a response, quantum theories about the wave function can be supplemented with structure, without taking it as an additional ontology. We argue in reply that such "dressed-up" versions of wave function ontology are not sensible, since they compromise the acceptability of the theory as a satisfactory fundamental physical theory. Therefore we maintain that: 1- Strictly speaking, it is not possible to interpret quantum theories as theories about the wave function; 2- Even if the wave function is supplemented by additional non-ontological structures, there are reasons not to take the resulting theory seriously. Moreover, we will argue that any of the traditional responses to the measurement problem of quantum mechanics (Bohmian mechanics, GRW and Many-Worlds), contrarily to what commonly believed, share a common structure. That is, we maintain that: 3- All quantum theories should be regarded as theories in which physical objects are constituted by a primitive ontology. The primitive ontology is mathematically represented in the theory by a mathematical entity in three-dimensional space, or space-time. (shrink)
For a long time it was believed that it was impossible to be realist about quantum mechanics. It took quite a while for the researchers in the foundations of physics, beginning with John Stuart Bell [Bell 1987], to convince others that such an alleged impossibility had no foundation. Nowadays there are several quantum theories that can be interpreted realistically, among which Bohmian mechanics, the GRW theory, and the many-worlds theory. The debate, though, is far from being over: in what respect (...) should we be realist regarding these theories? Two different proposals have been made: on the one hand, there are those who insist on a direct ontological interpretation of the wave function as representing physical bodies, and on the other hand there are those who claim that quantum mechanics is not really about the wave function. In this paper we will present and discuss one proposal of the latter kind that focuses on the notion of primitive ontology. (shrink)
A major disagreement between different views about the foundations of quantum mechanics concerns whether for a theory to be intelligible as a fundamental physical theory it must involve a ‘primitive ontology’ (PO), i.e. variables describing the distribution of matter in four-dimensional space–time. In this article, we illustrate the value of having a PO. We do so by focussing on the role that the PO plays for extracting predictions from a given theory and discuss valid and invalid derivations of predictions. To (...) this end, we investigate a number of examples based on toy models built from the elements of familiar interpretations of quantum theory.11 Introduction2 The GRWm and GRWf Theories2.1 The GRW process2.2 GRWm2.3 GRWf3 Predictions and Primitive Ontology3.1 Calibration functions3.2 Taking the PO seriously3.3 Examples from the literature3.4 The main theorem about operators in the GRW formalism3.5 The GRW formalism4 A Set of Examples4.1 Bohmian mechanics4.2 Bohmian trajectories and GRW collapses4.2.1 Bohm’s law and GRW’s law4.2.2 Bohm’s law and a modified GRW law4.2.3 Trajectories from the GRW wave function4.2.4 Configuration jumps and GRW law4.2.5 Another way of configuration jumps and GRW law4.3 MBM: Bohm-like trajectories from the master equation4.3.1 Empirical equivalence of MBM with GRWm and GRWf4.4 Master equation and matter density4.5 Master equation and flashes5 Conclusions. (shrink)
Bohmian mechanics is a quantum theory with a clear ontology. To make clear what we mean by this, we shall proceed by recalling first what are the problems of quantum mechanics. We shall then briefly sketch the basics of Bohmian mechanics and indicate how Bohmian mechanics solves these problems and clarifies the status and the role of of the quantum formalism.
The measurement problem in quantum mechanics is presented in a completely non-technical way by means of the results of some very simple experiments. These experimental results themselves, rather than the formalism of quantum theory, are shown to be extremely hard to incorporate in a sensible state-space picture of the world. A novel twist is then added which makes the problem even harder than it appears to be in other presentations of the measurement problem.
The aim of this paper is to give a systematic account of the so-called “measurement problem” in the frame of the standard interpretation of quantum mechanics. It is argued that there is not one but five distinct formulations of this problem. Each of them depends on what is assumed to be a “satisfactory” description of the measurement process in the frame of the standard interpretation. Moreover, the paper points out that each of these formulations refers not to a unique problem, (...) but to a set of sub-problems. (shrink)
A field-theoretic version of Wigner’s friend (1961) illustrates how the quantum measurement problem arises for field theory. Similarly, considering spacelike separate measurements of entangled fields by observers akin to Wigner’s friend shows the sense in which relativistic constraints make the measurement problem particularly difficult to resolve in the context of a relativistic field theory. We will consider proposals by Wigner (1961), Bloch (1967), Helwig and Kraus (1970), and Bell (1984) for resolving the measurement problem for quantum field theory. We will (...) conclude by considering the possibility of giving up rich dynamical explanation in the context of a many-maps formulation of relativistic quantum field theory. (shrink)
There are two versions of the putative connection between consciousness and the measurement problem of quantum mechanics : consciousness as the cause of state vector reduction, and state vector reduction as the physical basis of consciousness. In this article, these controversial ideas are neither accepted uncritically, nor rejected from the outset in the name of some prejudice about objective knowledge. Instead, their origin is sought in our most cherished (but disputable) beliefs about the place of mind and consciousness in the (...) world. It is first pointed out that these common beliefs about mind and consciousness arise from reification of situated first-person experience. Then, situatedness is shown to be a constitutive part of any exhaustive treatment of quantum measurements. It turns out that the alleged connection between consciousness and the measurement problem is a symptom of (i) the ineliminability of our being situated from the end-product of science, and (ii) our difficulty to express correctly this being situated. (shrink)
We consider the problem of measurement using the Lindblad equation, which allows the introduction of time in the interaction between the measured system and the measurement apparatus. We use analytic results, valid for weak system-environment coupling, obtained for a two-level system in contact with a measurer (Markovian interaction) and a thermal bath (non-Markovian interaction), where the measured observable may or may not commute with the system-environment interaction. Analysing the behavior of the coherence, which tends to a value asymptotically close to (...) zero, we obtain an expression for the time of measurement which depends only on the system-measurer coupling, and which does not depend on whether the observable commutes with the system-bath interaction. The behavior of the coherences in the case of strong system-environment coupling, found numerically, indicates that an increase in this coupling decreases the measurement time, thus allowing our expression to be considered the upper limit for the duration of the process. (shrink)
The quantum theory of de Broglie and Bohm solves the measurement problem, but the hypothetical corpuscles play no role in the argument. The solution finds a more natural home in the Everett interpretation.
The quantum theory of de Broglie and Bohm solves the measurement problem, but the hypothetical corpuscles play no role in the argument. The solution ﬁnds a more natural home in the Everett interpretation.
Carlo Rovelli's relational interpretation of quantum mechanics holds that a system's states or the values of its physical quantities as normally conceived only exist relative to a cut between a system and an observer or measuring instrument. Furthermore, on Rovelli's account, the appearance of determinate observations from pure quantum superpositions happens only relative to the interaction of the system and observer. Jeffrey Barrett () has pointed out that certain relational interpretations suffer from what we might call the ‘determinacy problem', but (...) Barrett misclassifies Rovelli's interpretation by lumping it in with Mermin's view, as Rovelli's view is quite different and has resources to escape the particular criticisms that Barrett makes of Mermin's view. Rovelli's interpretation still leaves us with a paradox having to do with the determinacy of measurement outcomes, which can be accepted only if we are willing to give up on certain elements of the ‘absolute’ view of the world. (shrink)
Philosophical debate on the measurement problem of quantum mechanics has, for the most part, been confined to the non-relativistic version of the theory. Quantizing quantum field theory, or making quantum mechanics relativistic, yields a conceptual framework capable of dealing with the creation and annihilation of an indefinite number of particles in interaction with fields, i.e. quantum systems with an infinite number of degrees of freedom. I show that a solution to the standard measurement problem is available if we exploit the (...) properties of the infinite quantum models available in this broader conceptual framework. (shrink)
It is assumed that experiments yield results that are not isomorphic with reality, but represent a distorted image of reality. Reality is related to observation via a communication channel of finite capacity. Quantum uncertainties are due to the bound on the amount of information available. Use is made of recent results from information and communication theories.
Does physics describe anything that can meaningfully be called “independent reality,” or is it merely operational? Most physicists implicitly favor an intermediate standpoint, which takes quantum physics into account, but which nevertheless strongly holds fast to quite strictly realistic ideas about apparently “obvious facts” concerning the macro-objects. Part 1 of this article, which is a survey of recent measurement theories, shows that, when made explicit, the standpoint in question cannot be upheld. Part 2 brings forward a proposal for making minimal (...) changes to this standpoint in such a way as to remove such objections. The “empirical reality” thus constructed is a notion that, to some extent,does ultimately refer to the human means of apprehension and of data processing. It nevertheless cannot be said that it reduces to a mere name just labelling a “set of recipes that never fail.” It is shown that our usual notion of macroscopic causality must be endowed with similar features. (shrink)
Decoherence results from the dissipative interaction between a quantum system and its environment. As the system and environment become entangled, the reduced density operator describing the system "decoheres" into a mixture (with the interference terms damped out). This formal result prompts some to exclaim that the measurement problem is solved. I will scrutinize this claim by examining how modal and relative-state interpretations can use decoherence. Although decoherence cannot rescue these interpretations from general metaphysical difficulties, decoherence may help these interpretations to (...) pick out a preferred basis. I will explore whether decoherence solves nagging technical problems associated with selecting a preferred basis. (shrink)
The central problem in the interpretation of the quantum theory is how to understand the superposition of the eigenstates of an observable. To a considerable extent scientific practice here, especially as codified in versions of Bohr's Copenhagen interpretation, follows an interpretive principle that I have elsewhere called the Rule of Silence (Ref.1). That rule admonishes us not to talk about the values of an observable unless the state of the system is an eigenstate, or a mixture of eigenstates, of the (...) observable in question. With regard to the rule of silence, as in other matters bearing on the interpretation of the quantum theory, Einstein was one of the first to realize that there can be difficulties. They appear as soon as we look at something like an explosion; i.e., the interaction between a micro and a macrosystem that involves the amplification of a microphenomenon to macroscopic scale (Ref.2). John Bell describes the difficulty over the rule of silence this way. (shrink)
The London and Bauer monograph occupies a central place in the debate concerning the quantum measurement problem. Gavroglu has previously noted the influence of Husserlian phenomenology on London's scientific work. However, he has not explored the full extent of this influence in the monograph itself. I begin this paper by outlining the important role played by the monograph in the debate. In effect, it acted as a kind of 'lens' through which the standard, or Copenhagen, 'solution' to the measurement problem (...) came to be perceived and, as such, it was robustly criticized, most notably by Putnam and Shimony. I then spell out the Husserlian understanding of consciousness in order to illuminate the traces of this understanding within the London and Bauer text. This, in turn, yields a new perspective on this 'solution' to the measurement problem, one that I believe has not been articulated before and, furthermore, which is immune to the criticisms of Putnam and Shimony. (shrink)
Some aspects of the problem of measurement in quantum theory are treated. We stress that the problem is both physical and conceptual, that the physical problem has been solved and the conceptual one is inherent in quantum theory. We also deal with some remarks made by Wigner concerning physics and the explanation of life, and present alternative positions on the mind-matter relationship within a deterministic framework, as we see them.
A new realislic local model of light propagation and detection is described. The authors propose a novel stochastic model of low-intensity photon detection in which background noise is added to a part of the photon prior to absorption. In this model, in agreement with Planck, there is no quantization of the propagating field. The model has some similarities to theories advanced by E. Santos and T. Marshall in the last decade, but also has substantial deviations from these. A mechanism, conserving (...) energy and momentum, is proposed by which a sudden collapse of the wave-packet is avoided. The experimental Bell inequality violation of Aspect. Grangier and Roger [Phys. Rev. Lett.47, 460 (1981)]is discussed. The authors have carried out a computer simulation of a radio frequency (RF) analogue of the Einstein-Podolsky-Rosen thought experiment to illustrate how the manipulation of certain factors, especially signal to noise ratio, detector threshold and characteristics of the noise, enables the same Bell inequality to be either satisfied or violated by a realistic local model. Building on arguments by Santos. [Phys. Rev. A46. 3646 (1992)],the appropriateness of this Bell lest is discussed. Neither the authors' stochastic-optical model, nor their RF analogue, involves an enhancement assumption of the type defined by Clauser and Horne [Phys. Rev. D10, 526 (1974)]. (shrink)
An analysis of the two routes through which one may disentangle a quantum system from a measuring apparatus, hence protect the state vector of a single quantum system from being disturbed by the measurement, reveals that the argument from protected measurement to the reality of the state vector of a single quantum system is valid but unsound. From this negative result I draw some lessons on the available "interpretations" of quantum theory and on the debate on the quantum measurement problem.
The integration of recent work on decoherence into a so-called modal interpretation offers a promising new approach to the measurement problem in quantum mechanics. In this paper I explain and develop this approach in the context of the interactive interpretation presented in Healey (1989). I begin by questioning a number of assumptions which are standardly made in setting up the measurement problem, and I conclude that no satisfactory solution can afford to ignore the influence of the environment. Further, I argue (...) that there are good reasons to believe that on a modal interpretation environmental interactions rapidly ensure that a quantummechanically describable apparatus indeed records a definite result following a measurement interaction. (shrink)
A model for the quantum measurement of the electronic current in a Josephson junction is presented and analyzed. The model is similar to a Stern-Gerlach apparatus, relying on the deflection of a spin-polarized particle beam by the magnetic field created by the Josephson current. The aim is (1) to explore, with the help of a simple model, some general ideas about the nature of the information which can be obtained by measurements upon a quantum system and (2) to find new (...) approaches for obtaining information about the nature of the states of a macroscopic quantum system. In the case of sufficiently strong coupling between the system and the apparatus, we find that the model provides in principle a standard ideal measurement of the value of the instantaneous Josephson current. In the case of weak coupling, where the measurement is not ideal, we show that the scattering of neutrons from a junction can in principle be used to measure the average value of the Josephson current, thereby allowing an experimental distinction to be made between an eigenstate of relative phase and one of relative Cooper pair number. The possibility of the latter type of measurement suggests an experimental approach to answer a question of fundamental interest, namely whether two isolated superconductors (or superfluids) possess a definite relative phase or a definite relative number of superconducting (or super/lowing) particles. (shrink)
A geometric approach to quantum mechanics with unitary evolution and non-unitary collapse processes is developed. In this approach the Schrödinger evolution of a quantum system is a geodesic motion on the space of states of the system furnished with an appropriate Riemannian metric. The measuring device is modeled by a perturbation of the metric. The process of measurement is identified with a geodesic motion of state of the system in the perturbed metric. Under the assumption of random fluctuations of the (...) perturbed metric, the Born rule for probabilities of collapse is derived. The approach is applied to a two-level quantum system to obtain a simple geometric interpretation of quantum commutators, the uncertainty principle and Planck’s constant. In light of this, a lucid analysis of the double-slit experiment with collapse and an experiment on a pair of entangled particles is presented. (shrink)
The paper puts forward the proposal to do relativistic quantum theory without a position operator and without a position probability amplitude. The proposed scheme employs space and time in a fundamental manner and treats them equitably as in special relativity by defining the state vectors as functions of configuration spacetime. From a discussion of the conceptual structure and of the problem of measurement of quantum theory, there emerges an understanding which shows that the absence of a satisfactory position probability density (...) in important cases like that of the photon, in the usual scheme of quantum theory, poses a fundamental difficulty, but that the scheme proposed here is free from the same difficulty and promises, instead, to lead in a natural manner to a fundamentally more satisfactory description of the observations. (shrink)
I examine recent arguments based on functionalism that claim to show that Bohm's theory fails to solve the measurement problem, or if it does so, it is only because it reduces to a form of the many-worlds theory. While these arguments reveal some interesting features of Bohm's theory, I contend that they do not undermine the distinctive Bohmian solution to the measurement problem. ‡I would like to thank Harvey Brown, Martin Thomson-Jones, and David Wallace for helpful discussions. †To contact the (...) author, please write to: Department of Philosophy, University of Miami, P.O. Box 248054, Coral Gables, FL 33124–4670; e-mail: firstname.lastname@example.org. (shrink)
Abraham Stone recently has published an argument purporting to show that David Bohm's interpretation of quantum mechanics fails to solve the measurement problem. Stone's analysis is not correct, as he has failed to take account of the conditions under which the theorems he cites are proven. An explicit presentation of a Bohmian measurement illustrates the flaw in his reasoning.
Work on the central problems of the philosophy of science has led the author to attempt to create an intelligible version of quantum theory. The basic idea is that probabilistic transitions occur when new stationary or particle states arise as a result of inelastic collisions.
A new version of quantum theory is proposed, according to which probabilistic events occur whenever new statioinary or bound states are created as a result of inelastic collisions. The new theory recovers the experimental success of orthodox quantum theory, but differs form the orthodox theory for as yet unperformed experiments.
It is usually taken for granted that orthodox quantum theory poses a serious problem for scientific realism, in that the theory is empirically extraordinarily successful, and yet has instrumentalism built into it. This paper stand this view on its head. I argue that orthodox quantum theory suffers from a number of serious (if not always noticed) defects precisely because of its inbuilt instrumentalism. This defective character of orthdoox quantum theory thus undermines instrumentalism, and supports scientific realism. I go on to (...) consider whether there is here the basis of a general argument against instrumentalism. (shrink)
Because it fails to solve the wave-particle problem, orthodox quantum theory is obliged to be about observables and not quantum beables. As a result the theory is imprecise, ambiguous, ad hoc, lacking in explanatory power, restricted in scope and resistant to unification. A new version of quantum theory is needed that is about quantum beables.
Are speical relativity and probabilism compatible? Dieks argues that they are. But the possible universe he specifies, designed to exemplify both probabilism and special relativity, either incorporates a universal "now" (and is thus incompatible with special relativity), or amounts to a many world universe (which I have discussed, and rejected as too ad hoc to be taken seriously), or fails to have any one definite overall Minkowskian-type space-time structure (and thus differs drastically from special relativity as ordinarily understood). Probabilism and (...) special relativity appear to be incompatible after all. What is at issue is not whether "the flow of time" can be reconciled with special relativity, but rather whether explicitly probabilistic versions of quantum theory should be rejected because of incompatibility with special relativity. (shrink)
In this paper I put forward a new micro realistic, fundamentally probabilistic, propensiton version of quantum theory. According to this theory, the entities of the quantum domain - electrons, photons, atoms - are neither particles nor fields, but a new kind of fundamentally probabilistic entity, the propensiton - entities which interact with one another probabilistically. This version of quantum theory leaves the Schroedinger equation unchanged, but reinterprets it to specify how propensitons evolve when no probabilistic transitions occur. Probabilisitic transitions occur (...) when new "particles" are created as a result of inelastic interactions. All measurements are just special cases of this. This propensiton version of quantum theory, I argue, solves the wave/particle dilemma, is free of conceptual problems that plague orthodox quantum theory, recovers all the empirical success of orthodox quantum theory, and at the same time yields as yet untested predictions that differ from those of orthodox quantum theory. (shrink)
In this paper I expound an argument which seems to establish that probabilism and special relativity are incompatible. I examine the argument critically, and consider its implications for interpretative problems of quantum theory, and for theoretical physics as a whole.
A fully micro realistic, propensity version of quantum theory is proposed, according to which fundamental physical entities - neither particles nor fields - have physical characteristics which determine probabilistically how they interact with one another (rather than with measuring instruments). The version of quantum "smearon" theory proposed here does not modify the equations of orthodox quantum theory: rather, it gives a radically new interpretation to these equations. It is argued that (i) there are strong general reasons for preferring quantum "smearon" (...) theory to orthodox quantum theory; (ii) the proposed change in physical interpretation leads quantum "smearon" theory to make experimental predictions subtly different from those of orthodox quantum theory. Some possible crucial experiments are considered. (shrink)
This paper investigates the possibiity of developing a fully micro realistic version of elementary quantum mechanics. I argue that it is highly desirable to develop such a version of quantum mechanics, and that the failure of all current versions and interpretations of quantum mechanics to constitute micro realistic theories is at the root of many of the interpretative problems associated with quantum mechanics, in particular the problem of measurement. I put forward a propensity micro realistic version of quantum mechanics, and (...) suggest how it might be possible to discriminate, on expermental grounds, between this theory and other versions of quantum mechanics. (shrink)
In this paper, possible objections to the propensity microrealistic version of quantum mechanics proposed in Part I are answered. This version of quantum mechanics is compared with the statistical, particle microrealistic viewpoint, and a crucial experiment is proposed designed to distinguish between these to microrealistic versions of quantum mechanics.
It is argued that the so-called minimal statistical interpretation of quantum mechanics does not completely resolve the measurement problem in that this view is unable to show that quantjum mechanics can dispense with classical physics when it comes to a treatment of the measuring interaction. It is suggested that the view that quantum mechanics applies to individual systems should not be too hastily abandoned, in that this view gives perhaps the best hope of leading to a version of quantum mechanics (...) which does provide a complete solution to the measurement problem. (shrink)
It is argued that Robinson's attempt to show that alpha particle emission contradicts orthodox quantum mechanics does not succeed. However, the possibility remains that alpha particle emission does contradict quantum mechanics.