Measurement Problem In Quantum Mechanics Research Articles

How to discover that the real is unreal

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.

Quantum logical solution to the measurement problem of quantum mechanics

In this paper I propose a reformulation and solution of the measurement problem of quantum mechanics. The reformulation depends on a quantum logical interpretation of quantum mechanics, broadly construed. The solution depends on a theorem about partial Boolean algebras which is proved here.

Many-Hilbert-spaces approach to the wave-function collapse

The many-Hilbert-spaces approach to the measurement problem in quantum mechanics is reviewed, and the notion of wave function collapse by measurement is formulated as a dephasing process between the two branch waves of an interfering particle. Following the approach originally proposed in Ref. 1, we introduce a “decoherence parameter,” which yields aquantitative description of the degree of coherence between the two branch waves of an interfering particle. By discussing the difference between the wave function collapse and the orthogonality of the apparatus' wave functions, we analyze critically two proposals, recently appeared in the literature, (2, 3) and argue that neither one describes a dephasing process. We conclude that the concept of “wave function collapse,” according to the conventional Copenhagen interpretation, is to be replaced by that of a statisticallydefined dephasing process.

Wave-function collapse by measurement and its simulation.

The many-Hilbert-space approach to the measurement problem in quantum mechanics is applied to a typical ``yes-no'' experiment relative to two branch routes corresponding to mutually exclusive propositions. First, we reformulate the notion of wave-function collapse by measurement as a dephasing process between the two branch waves of an interfering particle (from our own point of view as opposed to the conventional Copenhagen interpretation). In this way, the concept of ``wave-function collapse'' is replaced by that of a statistically defined dephasing process. One of the most important points of this paper is the introduction of an order parameter \ensuremath{\epsilon} that quantitatively describes the degree of decoherence. Its value ranges from \ensuremath{\epsilon}=0 (which describes the case in which the two waves are perfectly coherent) to \ensuremath{\epsilon}=1 (which describes the case in which coherence is totally lost); for this reason \ensuremath{\epsilon} is named the ``decoherence parameter.'' In terms of this parameter we formulate a definite criterion to judge whether an instrument works well or not as a measuring apparatus. Then, we study the interaction between a microscopic particle and a macroscopic system (a detector), by modeling the macrosystem with a linear array of complex \ensuremath{\delta} potentials, which undergo several kinds of statistical fluctuations. This leads us, under particular conditions, to the so-called wave-function collapse, which is attained in the limit \ensuremath{\epsilon}=1. We also examine in some detail which kind of elastic and/or inelastic collisions can give the wave-function collapse. Some connections with recent experimental results in neutron interferometry and quantum optics are also stressed.

Desiderata for a Modified Quantum Dynamics

A cluster of problems — the “quantum mechanical measurement problem”, the “problem of the reduction of the wave packet”, the “problem of the actualization of potentialities,” and the “Schrödinger Cat problem” — are raised by standard quantum dynamics when certain assumptions are made about the interpretation of the quantum mechanical formalism. Investigators who are unwilling to abandon these assumptions will be motivated to propose modifications of the quantum formalism. Among these, many (including Professor Ghirardi and Professor Pearle) have felt that the most promising locus of modification is quantum dynamics, and in their presentations (this Volume) they have suggested stochastic modifications of the standard deterministic and linear evolution of the quantum state.

Resolution of the measurement problem through decoherence of the quantum state

In the recent literature there have been several proposals to solve the quantum mechanical measurement problem by taking into account that in measurement interactions there are many unobserved degrees of freedom. Tracing out the unobserved degrees of freedom results in decoherence between components of the wave function. This effectively implies transition to a mixture, which is then taken to explain the “reduction of the wave packet”. It is here pointed out that such “solutions” are unsatisfactory as they stand, and must be supplemented by a new empirical interpretation of the formal state description of quantum mechanics.

Difficulty in detecting deviations from wave-function collapse.

We use the example of a Stern-Gerlach apparatus to show that the quantum-mechanical measurement problem ceases to be a physical problem for a measuring device with many degrees of freedom in the sense that deviations from wave-function collapse become virtually impossible to detect.

How to solve the measurement problem of quantum mechanics

A solution to the measurement problem of quantum mechanics is proposed within the framework of an intepretation according to which only quantum systems with an infinite number of degrees of freedom have determinate properties, i.e., determinate values for (some) observables of the theory. The important feature of the infinite case is the existence of many inequivalent irreducible Hilbert space representations of the algebra of observables, which leads, in effect, to a restriction on the superposition principle, and hence the possibility of defining (macro-) observables which commute with every observable. Such observables have determinate values which are not subject to quantum interference effects. A measurement process is schematized as an interaction between a microsystem and a macrosystem, idealized as an infinite quantum system, and it is shown that there exists a unitary transformation which transforms the initial pure state of the composite system in a finite time (the duration of the interaction) into the required mixture of disjoint states.

From Micro to Macro: A Solution to the Measurement Problem of Quantum Mechanics

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 want to 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.There is a qualitative difference between quantum systems with an infinite number of degrees of freedom and quantum systems with a finite number of degrees of freedom. In the infinite case, there exist many unitarily inequivalent irreducible Hilbert space representations of the algebra of observables of the system. In the finite case, there is only one such representation.

The insolubility proof of the quantum measurement problem

Modern insolubility proofs of the measurement problem in quantum mechanics not only differ in their complexity and degree of generality, but also reveal a lack of agreement concerning the fundamental question of what constitutes such a proof. A systematic reworking of the (incomplete) 1970 Fine theorem is presented, which is intended to go some way toward clarifying the issue.

Interaction between classical and quantum systems: A new approach to quantum measurement.I

We introduce an unconventional approach to the measurement problem in quantum mechanics: we treat the apparatus as a classical system belonging to the macroworld. To describe the quantum measurement process we must couple the classical apparatus to the quantum system. In this paper we explain how this is to be done: we embed the classical apparatus into a larger quantum-mechanical structure, making use of a superselection principle. The apparatus can now be coupled to the quantum system in a straightforward manner. We discuss what constraints should be placed on the coupling so that an interpretation of the interaction as a measurement results. We require that unambiguous information of the values of a quantum observable should be transferred to the variables of the apparatus. We also require that the apparatus should retain its classical identity. This latter requirement is formulated as a principle of integrity, in both weak and strong forms.

Liapounov function for the friedrichs model

Abstract The Friedrichs model of an unstable particle provides us with a non-trivial example of a dynamical system with a continuous spectrum. The non-unitary transformation operator which leads in our approach to a semi-group description of the time evolution of the system has been constructed in earlier papers. It is used here to evaluate the Liapounov function which plays the role of a microscopic entropy. The results, obtained without any recourse to perturbation expansions, validate resummation procedures introduced previously in a perturbative treatment. Various types of initial conditions are considered. It is shown explicitly why the distinction between pure states and mixtures is lost in the asymptotic time limit. The result is of relevance for the discussion of the quantum mechanical measurement problem.

Logical self reference, set theoretical paradoxes and the measurement problem in quantum mechanics

Logical self reference, set theoretical paradoxes and the measurement problem in quantum mechanics

Van Fraassen's Modal Model of Quantum Mechanics

Bas van Fraassen in [4] has recently tried to use modal logic to solve the measurement problem of quantum mechanics. His model is based on a method of expressing quantum states developed by Hugh Everett [1] called the “relative state formulation.” Unfortunately, Everett's mathematics cannot be generalized as van Fraassen requires. The difficulty itself is elementary enough. But a revision of van Fraassen's postulates (such as the one he adopted on p. 341 after he had been told of the problem) can save the mathematics only on pain of making the whole study irrelevant to the physics. I shall explain the dilemma in this note. In a later paper [5], van Fraassen avoids this dilemma by developing an abstract modal interpretation which does not rely on any expression for quantum states.

Mathematics of the Measurement Problem in Quantum Mechanics

The mathematical structure of the simplest nontrivial hidden variable theory is derived. The core of the resulting theory is an essentially unique nonlinear differential equation which gives a causal description of the continuous collapse of the wavefunction during a measurement. It is shown that the simplest collapse equation is the same as the equation given ad hoc by Bohm and Bub.

Time scales and the problem of measurement in quantum mechanics

A contribution is given to the attempts towards a solution of the measurement problem in quantum mechanics, in the spirit of some previous papers. In order to justify the possibility of a macroscopic description of the measuring apparatus, a relevant hypothesis is found to be the separation of two characteristic times: the decay time for the kernel of a generalized master equation and the characteristic time for the evolution of macroscopic quantities of the measuring device.

A Proposed Solution of the Measurement Problem in Quantum Mechanics by a Hidden Variable Theory

The measurement problem in quantum mechanics is re-examined and it is shown that it cannot really be solved in a satisfactory way, within the framework of the usual interpretation of the theory. We then discuss von Neumann's attempts to prove that quantum mechanics is incompatible with the introduction of hidden variables, and develop a more detailed form of Bell's argument, showing that von Neumann's analysis is invalid. Using certain ideas that are implicit in the differential-space theory of Wiener and Siegel, we go on to propose a new deterministic equation of motion, describing a kind of coupling of the measuring instrument to the observed system that explains in detail how the wave packet is reduced during a measurement in a continuous and causally determined way. By averaging over the hidden parameters, we then recover the usual statistical results of quantum mechanics as a special case. However, a more detailed analysis of the theory shows that new experimental and theoretical questions can now be raised, which go outside the framework of the quantum theory as it is now formulated. These questions are examined briefly.