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arXiv:2012.05229

Quantum Buzzwords

James B. Hartle

Published Dec 9, 2020• 1 citation
arXivHTMLPDF

Abstract

Many scientists seeking to understand the quantum mechanics of measurement situations (Copenhagen quantum theory) agree on its overwhelmingly successful algorithms to predict the outcomes of laboratory measurements but disagree on what these algorithms mean and how they are to be interpreted. Some of these problems are briefly described and resolutions suggested from the decoherent (or consistent) histories quantum mechanics of closed systems like the Universe.

DOI
10.48550/arxiv.2012.05229

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Many scientists seeking to understand the quantum mechanics of measurement situations (Copenhagen quantum theory) agree on its overwhelmingly successful algorithms to predict the outcomes of laboratory measurements but disagree on what these algorithms mean and how they are to be interpreted. Some of these problems are briefly described and resolutions suggested from the decoherent (or consistent) histories quantum mechanics of closed systems like the Universe.
When quantum mechanics was developed in the '20s of the last century another revolution in physics was just starting. It began with the discovery that the universe is expanding. For a long time quantum mechanics and cosmology developed independently of one another. Yet the very discovery of the expansion would eventually draw the two subjects together because it implied the big bang where quantum mechanics wasimportant for cosmology and for understanding and predicting our observations of the universe today. Textbook (Copenhagen) formulations of quantum mechanics are inadequate for cosmology for at least four reasons: 1) They predict the outcomes of measurements made by observers. But in the very early universe no measurements were being made and no observers were around to make them. 2) Observers were outside of the system being measured. But we are interested in a theory of the whole universe where everything, including observers, are inside. 3) Copenhagen quantum mechanics could not retrodict the past. But retrodicting the past to understand how the universe began is the main task of cosmology. 4) Copenhagen quantum mechanics required a fixed classical spacetime geometry not least to give meaning to the time in the Schrödinger equation. But in the very early universe spacetime is fluctuating quantum mechanically (quantum gravity) and without definite value. A formulation of quantum mechanics general enough for cosmology was started by Everett and developed by many. That effort has given us a more general framework that is adequate for cosmology --- decoherent (or consistent) histories quantum theory in the context of semiclassical quantum gravity. Copenhagen quantum theory is an approximation to this more general quantum framework that is appropriate for measurement situations. We discuss whether further generalization may still be required.
We sketch a quantum mechanical framework for the universe as a whole. Within that framework we propose a program for describing the ultimate origin in quantum cosmology of the quasiclassical domain of familiar experience and for characterizing the process of measurement. Predictions in quantum mechanics are made from probabilities for sets of alternative histories. Probabilities can be assigned only to sets of histories that approximately decohere. Decoherence is defined and the mechanism of decoherence is reviewed. Decoherence requires a sufficiently coarse-grained description of alternative histories of the universe. A quasiclassical domain consists of a branching set of alternative decohering histories, described by a coarse graining that is maximally refined consistent with decoherence, with individual branches that exhibit a high level of classical correlation in time. A quasiclassical domain is emergent in the universe as a consequence of the initial condition and the action function of the elementary particles. It is an important question whether all the quasiclassical domains are roughly equivalent or whether there are various essentially inequivalent ones. A measurement is a correlation with variables in a quasiclassical domain. An observer (or information gathering and utilizing system) is a complex adaptive system that has evolved to exploit the relative predictability of a quasiclassical domain. We suggest that resolution of many of the problems of interpretation presented by quantum mechanics is to be accomplished, not by further scrutiny of the subject as it applies to reproducible laboratory situations, but rather by an examination of alternative histories of the universe, stemming from its initial condition, and a study of the problem of quasiclassical domains.
The interpretation of measurements in quantum mechanics is still under discussion (Healey [84], Bell [9], Redhead [136]). Besides the Copenhagen interpretation we have the many-worlds interpretations (Everett interpretations), the modal interpretations, the decoherence interpretations, the interpretations in terms of (nonlocal) hidden variables, the quantum logical interpretations. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
This paper presents a philosophical analysis of quantum mechanics, the measurement problem, the observer problem, decoherence, and quantum interpretation. It argues that the central difficulty in quantum mechanics is not only technical but interpretive: the theory has extraordinary predictive power without a single settled account of what is happening physically or ontologically. The paper frames this as an answerability problem, asking how interpretations should be judged when formal success outruns explanatory closure. It separates several meanings of observation—interaction, coupling, record formation, information update, and conscious awareness—and shows how collapsing these layers creates confusion in both physics and philosophy. It then compares major interpretations including Copenhagen, Many-Worlds, Bohmian mechanics, objective collapse, QBism, relational quantum mechanics, and consistent histories, treating them as different ways of stabilizing explanatory burden around an underconstrained formal core. The paper also argues that decoherence is crucial for understanding record stability and the appearance of classicality, but does not by itself solve the single-outcome problem. Overall, the paper offers a clear framework for thinking about quantum foundations, scientific explanation, ontology, observation, and philosophy of science without overstating what current theory can responsibly claim.