Showing papers on "Consistent histories published in 1992"

Consistent interpretations of quantum mechanics

Abstract: Within the last decade, significant progress has been made towards a consistent and complete reformulation of the Copenhagen interpretation (an interpretation consisting in a formulation of the experimental aspects of physics in terms of the basic formalism; it is consistent if free from internal contradiction and complete if it provides precise predictions for all experiments). The main steps involved decoherence (the transition from linear superpositions of macroscopic states to a mixing), Griffiths histories describing the evolution of quantum properties, a convenient logical structure for dealing with histories, and also some progress in semiclassical physics, which was made possible by new methods. The main outcome is a theory of phenomena, viz., the classically meaningful properties of a macroscopic system. It shows in particular how and when determinism is valid. This theory can be used to give a deductive form to measurement theory, which now covers some cases that were initially devised as counterexamples against the Copenhagen interpretation. These theories are described, together with their applications to some key experiments and some of their consequences concerning epistemology.

Space-time quantum mechanics and the quantum mechanics of space-time

Abstract: These are the author's lectures at the 1992 Les Houches Summer School, "Gravitation and Quantizations". They develop a generalized sum-over-histories quantum mechanics for quantum cosmology that does not require either a preferred notion of time or a definition of measurement. The "post-Everett" quantum mechanics of closed systems is reviewed. Generalized quantum theories are defined by three elements (1) the set of fine-grained histories of the closed system which are its most refined possible description, (2) the allowed coarse grainings which are partitions of the fine-grained histories into classes, and (3) a decoherence functional which measures interference between coarse grained histories. Probabilities are assigned to sets of alternative coarse-grained histories that decohere as a consequence of the closed system's dynamics and initial condition. Generalized sum-over histories quantum theories are constructed for non-relativistic quantum mechanics, abelian gauge theories, a single relativistic world line, and for general relativity. For relativity the fine-grained histories are four-metrics and matter fields. Coarse grainings are four-dimensional diffeomorphism invariant partitions of these. The decoherence function is expressed in sum-over-histories form. The quantum mechanics of spacetime is thus expressed in fully spacetime form. The coarse-grainings are most general notion of alternative for quantum theory expressible in spacetime terms. Hamiltonian quantum mechanics of matter fields with its notion of unitarily evolving state on a spacelike surface is recovered as an approximation to this generalized quantum mechanics appropriate for those initial conditions and coarse-grainings such that spacetime geometry

Investigating decoherence in a simple system.

Abstract: I present the results of some simple calculations designed to study the loss of quantum coherence. The relevant physical issues are briefly reviewed, and then a very simple "toy" model is analyzed. Exact solutions are found using numerical techniques. The type of decoherence exhibited by the model can be changed by varying a coupling strength. I study the system from two points of view. One, the Schmidt paths approach, is closely related to the conventional approach of studying decoherence by checking the form of the density matrix. The consistent histories approach is also used, and the relationship between the two approaches is explored.

Does quantum mechanics obey the correspondence principle? Is it complete?

Abstract: This elementary review paper presents the compelling evidence which supports the notion that quantum mechanics is much too simple a theory to adequately describe a complex world. Rigorous arguments based on algorithmic complexity theory are used to show that both the quantum Arnol’d cat and a broad category of finite, bounded, undriven, quantum systems do not obey the correspondence principle, implying that quantum mechanics is also not complete. An experiment, well within current laboratory capability, is proposed which can expose the inability of quantum mechanics to adequately describe macroscopic chaos. In its final section, this paper describes a theoretical framework that provides a proper setting for interpreting these surprising results.

Quantum Physics and Observed Reality: A Critical Interpretation of Quantum Mechanics

Abstract: Introductory remarks and exposition observability and existence of properties the two-model representation of quantum mechanics time-dependence of the wave function and multiple events other application modes quasi-properties and psi-structures general discussion uncertainties and uncertainty relations critical notes on the Einstein-Podolsky-Rosen argument conclusion.

The Quantum Mechanics of Closed Systems

Abstract: A pedagogical introduction is given to the quantum mechanics of closed systems, most generally the universe as a whole. Quantum mechanics aims at predicting the probabilities of alternative coarse-grained time histories of a closed system. Not every set of alternative coarse-grained histories that can be described may be consistently assigned probabilities because of quantum mechanical interference between individual histories of the set. In the quantum mechanics of closed systems, containing both observer and observed, probabilities are assigned to those sets of alternative histories for which there is negligible interference between individual histories as a consequence of the system's initial condition and dynamics. Such sets of histories are said to decohere. Typical mechanisms of decoherence that are widespread in our universe are illustrated. Copenhagen quantum mechanics is an approximation to the more general quantum framework of closed subsystems. It is appropriate when there is an approximately isolated subsystem that is a participant in a measurement situation in which (among other things) the decoherence of alternative registrations of the apparatus can be idealized as exact. Since the quantum mechanics of closed systems does not posit the existence of the quasiclassical domain of everyday experience, the domain of the approximate aplicability of classical physics must be explained. We describe how a quasiclassical domain described by averages of densities of approximately conserved quantities could be an emergent feature of an initial condition of the universe that implies the approximate classical behavior of spacetime on accessible scales.

Quantum Chaos and Semiclassical Mechanics

Abstract: This paper discusses the problem of finding and defining chaos in quantum mechanics. While chaotic time evolution appears to be ubiquitous in classical mechanics, it is apparently absent in quantum mechanics in part because for a bound, isolated quantum system, the evolution of its state is multiply periodic. This has led a number of investigators to search for semiclassical signatures of chaos. Here I am concerned with the status of semiclassical mechanics as a distinct third theory of the asymptotic domain between classical and quantum mechanics. I discuss in some detail the meaning of such crucial locutions as the "classical counterpart to a quantum system" and a quantum system's "underlying classical motion". A proper elucidation of these concepts requires a semiclassical association between phase space surfaces and wave-functions. This significance of this association is discussed in some detail.

An Interpretation of the formalism of quantum mechanics in terms of epistemological realism

Abstract: We present an alternative to the Copenhagen interpretation of the formalism of nonrelativistic quantum mechanics. The basic difference is that the new inter- pretation is formulated in the language of epistemological realism. It involves a change in some basic physical concepts. Elementary particles are considered as extended objects and nonlocal effects are included. The role of the new concepts in the problems of measurement and of the Einstein-Podolsky-Rosen correlations is described. Experiments to distinguish the proposed interpretation from the Copenhagen one are pointed out.

Quantum monadology: A world model to interpret quantum mechanics and relativity

Abstract: In order to give a new insight to fundamental problems of quantum mechanics, relativity and mind, we propose a world model suggested from the monadology of Leibniz. The world is assumed to consist of "monads" which have their individuality and whose primary attribute is a space-time frame and not a position in spacetime. Each monad has freedom to change its frame. Accompanying this change, the world time is put forward, and the world state jumps off the unitary evolution. This model explains not only the measurement process of quantum mechanics but also the "passing now" and the origin of free will.

New formulation of quantum mechanics

Abstract: A reformulation of the mathematical foundations of quantum mechanics is presented. This new framework is based on the concepts of measurement, generalized action, and a unique universal influence function. The main axiom is that the probability of a measurement outcome is the sum (or integral) of the influences between pairs of alternatives that result in the outcome when the measurement is executed. The framework provides answers to various puzzling questions of traditional quantum mechanics. Moreover, it gives a realistic model that extends the usual quantum mechanical formalism.

Contextual quantum process theory

Abstract: A logically complete interpretation of quantum mechanics is given in terms of a theory of quantum processes.

Groups and probabilities at the foundations of quantum mechanics

Abstract: The current state of the foundations of quantum mechanics is discussed. The analysis takes as its starting point the theory of probability amplitudes, which is intimately related to the group-theoretic approach. A detailed examination is presented of the relationships of classical and quantum theories, the transition to the classical limit, the different forms of uncertainty relations, and the properties of quantum structures determined by the Clebsch–Gordan coefficients. Possible future generalizations are examined, including those involving quantum algebras.

Conceptual problems in quantum mechanics

Abstract: This review is devoted to a discussion of the interpretation of quantum mechanics. The heuristic role and limitations of the principle of observability and of operationalism are discussed. It is shown that the probabilistic approach to quantum mechanics is essential as a way of reconciling the conflicting concepts of particle and wave. The reason why the reduction of the wave packet is not a physical process, but a logical act is explained. The discussion of the paradoxes of quantum mechanics covers many well known examples and includes the Aharonov-Bohm effect and interference between two independent laser beams. It is suggested that the causality principle does not reduce to determinism, but has certain other manifestations too. This is illustrated by the fact that Newtonian mechanics was at one time considered as abstract and impenetrable, in contrast to the 'natural', but eventually fruitless mechanics of Descartes. It is shown that a classical foundation cannot be provided for quantum mechanics, i.e., it is impossible to introduce hidden variables into quantum mechanics. Mathematical manipulation is reduced to the essential minimum, and many examples are provided to illustrate the discussion. Outstanding contributors to physics are extensively quoted. The review is intended for readers with higher education in both the natural sciences and the humanities, who are interested in conceptual problems in modern science.

Quantum probabilities, operators of state preparation, and the principle of superposition

Abstract: An integrated view concerning the probabilistic organization of quantum mechanics is first obtained by systematic confrontation of the Kolmogorov formulation of the abstract theory of probabilities with the quantum mechanical representationand its factual counterparts. Because these factual counterparts possess a peculiar space-time structure stemming from the operations by which the observer produces the studied states (operations of state preparation) and the qualifications of these (operations of measurement), the approach brings forth “probability-trees,” complex constructs with treelike space-time support. Though it is strictly entailed by confrontation with the abstract theory of probabilities as it now stands, the construct of a quantum mechanical probability treetransgresses this theory. It indicates the possibility of an extended abstract theory of probabilities: Quantum mechanics appears to be neither a “normal” probabilistic theory nor an “abnormal” one, but a pioneering particular realization of afuture extended abstract theory of probabilities. The integrated perception of the probabilistic organization of quantum mechanics removes the current identifications of spectral decompositions of one state vector, with superpositions of several state vectors. This leads to the definition of operators of state preparation and of the calculus with these and to a clear understanding of the physical significance of the principle of superposition. Furthermore, a complement to the quantum theory of measurements is obtained.

The subject matter of quantum mechanics

Abstract: This is a philosophical paper. It deals with the interpretation of quantum mechanics, i.e., with reality, the objects of quantum mechanics, probabilities, etc. It is important to distinguish between real things and physical systems. A physical theory is a collection of rules for predictions on the outcome of measurements. Contrary to general belief “prediction” and “possible and actual” are key concepts in physics, as well as the concept of probability, being the most general empirically testable prediction. The Copenhagen interpretation is nothing but a “minimal semantics” of quantum mechanics, dealing with possibilities rather than with facts. Quantum mechanical realism is the futile attempt to confine physics to the description of facts. We answer the old question whether probability is about single events or about series of events: it can be about either, if it is correctly interpreted as a representative of the abstract ensemble. Quantum mechanics is only interesting if it is the most general theory of all possible systems. But this is where the hard problems arise: measurement, reality, indeterminism, etc. These problems can be solved if we accept seriously the key role of prediction and possibility, and abandon the ontology of classical physics.

Phase space representation of quantum mechanics in terms of coherent states

Abstract: The functional representation of the Hilbert space in terms of coherent states is reconsidered with the purpose of studying the connection between quantum states and the corresponding distributions of classical statistical mechanics. The statistical predictions of both theories are compared and the relevance of these results for the conventional interpretation of quantum mechanics is discussed.

Measurement and probability in quantum mechanics

Abstract: Measurement-theoretical foundations of quantum probabilities are investigated in the form of measurement statistics and a statistical ensemble interpretation of quantum mechanics.

Quantum thermodynamics: new light upon the physicl meaning of entropy and the origin of irreversibility

Abstract: What is the physical significance of entropy? What is the physical origin of irreversibility? Do entropy and irreversibility exist only for complex and macroscopic systems? The bulk of the physics community accepts and teaches that all these fundamental questions are rationalized within statistical mechanics. Indeed, for everyday laboratory physics, the mathematical formalism of statistical mechanics (canonical and grandcanonica1, Boltzmann, Bose-Einstein and Fermi-Dirac distributions) allows a successful description of the thermodynamic equilibrium properties of matter, including entropy values. But an ever growing handful of physicists (Schrodinger among the first) have realized that, even in its explanation of the meaning of entropy, statistical mechanics is impaired by ambiguities and logical inconsistencies. They have started to search for a better theory to eliminate these stumbling blocks while maintaining the mathematical formalism that has been so successful in so many applications. This handful of upstreamers must not be confused with the many schools of physicists that have thrived on the more renowned incompleteness of statistical mechanics, namely, the lack of a quantitative (and the weakness of the qualitative) explanation of the origin of irreversibility. In these studies the thrust is provided by the discovery that the macroscopic dynamics of certain complex systems may be modeled using a few-degrees-of-freedom nonlinear Hamiltonian with singularities that give rise to bifurcations and chaotic behavior. These results have generated successful ways to describe irreversible behavior, but their link to the origin of irreversibility is still only heuristic (what is the connection between the nonlinear model Hamiltonian and the true full Hamiitonian?) and does not provide yet a rigorous resolution of the century-old paradox of the conflict between the irreversibiiity of macroscopic behavior, and the reversibility of the laws of mechanics. To resolve both the problem of the meaning of entropy and that of the origin of irreversibility we have built entropy and irreversibility into the laws of mechanics. The result is a theory that we call quantum thermodynamics that has all the necessary properties to combine mechanics and thermodynamics uniting all the successful results of both theories, eliminating the logical inconsistencies of statistical mechanics and the paradox on irreversibiiity, and providing an entirely new perspective on the microscopic origin of irreversibility, nonlinearity and therefore chaotic behavior. The mathematical formalism of quantum thermodynamics differs from that of statistical mechanics mainly in the equation of motion which is nonlinear but has solutions identical to those of the Schrodinger equation for all the states for which statistical mechanics reduces to quantum mechanics. The physical meaning of the formalism of quantum thermodynamics differs more drastically from that of statistical mechanics. The significance of the state operator of quantum thermodynamics is entirely different from that of the density operator of statistical mechanics, even though the two are mathematically equivalent. Indeed they obey different equations of motion. In particular, quantum thermodynamics is concerned only with those systems for which quantum mechanics would describe the states with vectors in Hilbert space or, equivalently, projection operators. Using a well known jargon, we can say that quantum thermodynamics like quantum mechanics is concerned only with pure quantum states. However, it postulates that the set of pure quantum states of a system is much broader than contemplated by quantum mechanics. Pure quantum states must be described by operators defined by all the features of projection operators except the condition of idempotence. As a result, an operator that within statistical mechanics would describe a mixed quantum state (that is, the average state of a statistical mixture of identical systems in different pure quantum states) in quantum thermodynamics describes a pure quantum state, a state that neither quantum mechanics non statistical mechanics would contemplate. Conceptually, the increased richness of pure quantum states is a new revolutionary postulate of quantum physics. But from the point of view of the statistical mechanics practitioners the new theory is not as traumatic as it seems. Whenever one uses a nonidempotent density operator to describe a thermodynamic equilibrium state one simply has to reinterpret it as one of the new pure quantum states. One even saves the usual ad hoc arguments on thermal baths and reservoirs that are usually required in statistical mechanics to justify the use of a nonidempotent density operator to describe the state of a system. In this paper we discuss the background and formalism of quantum thermodynamics including its nonlinear equation of motion and the main general results. Our objective is to show in a not-too-technical manner that this theory provides indeed a complete and coherent resolution of the century-old dilemma on the meaning of entropy and the origin of irreversibility. As a byproduct, we discuss a long set of criteria that a theory should meet in order to afford the same claim.

The Interpretation of quantum cosmological models

Abstract: We consider the problem of extracting physical predictions from the wave function of the universe in quantum cosmological models. We state the features of quantum cosmology an interpretational scheme should confront. We discuss the Everett interpretation, and extensions of it, and their application to quantum cosmology. We review the steps that are normally taken in the process of extracting predictions from solutions to the Wheeler-DeWitt equation for quantum cosmological models. Some difficulties and their possible resolution are discussed. We conclude that the usual wave function-based approach admits at best a rather heuristic interpretation, although it may in the future be justified by appeal to the decoherent histories approach.

Axioms for quantum theory

Abstract: The first three of these axioms describe quantum theory and classical mechanics as statistical theories from the very beginning. With these, it can be shown in which sense a more general than the conventional measure theoretic probability theory is used in quantum theory. One gets this generalization defining transition probabilities on pairs of events (not sets of pairs) as a fundamental, not derived, concept. A comparison with standard theories of stochastic processes gives a very general formulation of the non existence of quantum theories with hidden variables. The Cartesian product of probability spaces can be given a natural algebraic structure, the structure of an orthocomplemented, orthomodular, quasi-modular, not modular, not distributive lattice, which can be compared with the quantum logic (lattice of all closed subspaces of an infinite dimensional Hubert space). It is shown how our given system of axioms suggests generalized quantum theories, especially Schrodinger equations, for phase space amplitudes.

Homodyne detection of de broglie waves

Abstract: According to de Broglie, a quantum object is composed of a wave which guides a corpuscle, both existing objectively in the space and time. The Copenhagen interpretation of quantum mechanics is completely different. The wavefunction is considered as a mathematical tool to calculate probabilities which collapses whenever a measurement is performed. In this letter we propose an experiment which can distinguish between the two approaches.