Many worlds? Everett, quantum theory and reality. (Q2900383)

This book is a collection of papers originating in contributions to two conferences on occasion of the fiftieth anniversary of Hugh Everett's dissertation, one held in Oxford, organized by S. Saunders, and the other held at the Perimeter Institute, Waterloo, organized by J. Barrett, A. Kent, and D. Wallace. S. Saunders and D. Wallace argue Everetts proposal to be the only coherent way to understand quantum mechanics, whereas A. Kent argues that quantum theory cannot be deduced from Everett's proposal. J. Barrett's position is neutral. With this book the editors give a documentation of the present opinions, statements, and discussions among philosophers and physicists.NEWLINENEWLINENEWLINENEWLINEThe Everett approach is one of several attempts to reinstall a deterministic background of the world after the mechanistic picture was destroyed by the discovery of quanta and the emergence of quantum theory with uncertainty relations and probabilities not based on a lack of knowledge. The Everett approach culminates in the metaphysical assumption of a universal wave function, the pure state of the universe which develops causally according to the Schrödinger equation. What we can see and are able to operate on is merely the reduced state of our near surrounding as a small part of the universe. However, there seems to remain a problem with quantum measurements. Inevitable decoherence may destroy this picture. Everett himself has had the idea to assume that any measurement causes the world to split into as many worlds as different outcomes are possible such that in any split one single outcome is realized. Everett abandoned this idea in the final version of his dissertation thesis. But B. de Witt, later on, proclaimed this idea again as an inevitable consequence of Everett's approach. Whether many worlds are necessary or not is a main subject of essays in the present book, which are arranged in six parts after an introduction by \textit{Simon Saunders} (pp. 1 - 49) who gives a first impression of the impact of Everett*s approach on the philosophy of nature.NEWLINENEWLINENEWLINENEWLINEThe title of the first part (pp. 53--117) is ``Why Many Worlds?''. In his contribution \textit{David Wallace} explains the ontological claims of the Everett approach: ``The quantum state is all there is.'' -- ``Worlds' are mutually dynamical isolated structures instantiated within the quantum state, which are structurally and dynamically quasiclassical.'' -- Worlds exist because of decoherence. How in these quasiclassical worlds our everyday experience of the deterministic laws of classical physics emerge is the subject of \textit{Jim Hartle}'s essay. How macroscopic properties of many particle systems and thermodynamics fit into Everett's picture of the world is the subject of the contribution by \textit{Jonathan Halliwell}.NEWLINENEWLINENEWLINENEWLINEThe second part (pp. 121--178) is entitled ``Problems with Ontology''. \textit{Tim Maudlin} asks whether the monistic ontology of the Everett's approach is too poor to describe reality. He refers to Bohmian mechanics for the measurement problem. \textit{John Hawthorne} asks from a metaphysical point of view which structure of the world is driving people to believe in the Everettian interpretation and how to reconcile it with the macroscopic world. This essay is commented and replied by \textit{James Ladyman}. The concluding section is a transcript of a discussion among the speakers about the poposals of the latter two parts.NEWLINENEWLINENEWLINENEWLINEThe third part (pp. 78 -- 304) is entitled ``Proboability in the Everett Interpretation''. How probaability, chance, and statistics may be understood in Everett's unitary development of the universal wave function without decoherence is the subject of \textit{Simon Saunders}' contribution. \textit{David Papineau} states in his essay that the difficulties with probability in the orthodox interpretaion of quantum mechanics are much bigger than those in the Everettian. How to prove the Born rule in a many worlds theory using decision theory is considered by \textit{David Wallace}. Probabilities as limits of relative frequencies in series of experiments are reproducible and considered to be an evidence for quantum mechanics is stated by \textit{Hilary Greaves} and \textit{Wayne Myrvold} and they argue that this holds true in multiverse as in universe interpretations as well.NEWLINENEWLINENEWLINENEWLINEThe fourth part (pp. 307 -- 406) is entitled ``Critical Replies''. \textit{Adrian Kent} reviews critically the latter two essays in his contribution ``One world versus many'' and gives several arguments showing inconsistencies in Everett's interpretation. \textit{David Albert} adverts to differences between the emergences of probability in Everett's deterministic evolution of the universal wave function and classical mechanics. \textit{Huw Price} analyses critically arguments originating from D. Deutsch as reformulated by D. Wallace and further by S. Saunders and H. Greaves in favor of Everett's settings. He concludes that these arguments cannot prevent the lapse of Everett's view caused by the fact that all outcomes of a quantum measurement are treated as real. The concluding section is a transcript of a discussion among speakers about the poposals of the latter two parts.NEWLINENEWLINENEWLINENEWLINEThe fifth part (pp. 409 -- 517) is entitled ``Alternatives to Many Worlds''. \textit{Wojciech Hubert Zurek} explains the fundamental features of quantum measurement processes based on the quantum physics of open systems and quantum information. He highlights the correctness of the Everett's relative state formalism and states that it does not require for many worlds. \textit{Jeffrey Bub} and \textit{Itamar Pitowsky}, too, consider quantum measurements as dynamical processes which need not a dogmatic collapse postulate. Quantum states and phase points belong to different categories of concepts. The Hilbert space is a kinematic fundament of quantum theory. This view is critically commented by \textit{Christopher Timpson}. \textit{Rüdiger Schacht} considers the decision theoretic approach due to D. Deutsch and D. Wallace. He observes that it can be taken to be a quantum version of David Lewis' principal principle. The attempt of a rigorous application, however, leads to new demands. \textit{Antony Valentini} points out why the De Broglie--Bohm pilot wave theory cannot be considered as a many-worlds theory with an extra configuration appended. \textit{Harvey Brown} critically replies to A. Valentini.NEWLINENEWLINENEWLINENEWLINEThe sixth part (pp. 521--607) is entitled ``Not only Many Worlds''. \textit{Peter Byrne} gives a very nice overview on the life of Hugh Everett III from the stay at Princeton on, the development of his thesis, his relation to John Archibald Wheeler, and the impact of his idea on the scientific world. \textit{David Deutsch}'s contribution is a sceptical review about knowledge in natural sciences in which Everett's view is a discovery rather than an interpretation. \textit{Max Tegmark} remarks that only a small fraction of objections against Everett's many-worlds interpretation are specific to quantum mechanics. \textit{Lev Vaidman} considers the measurement problem in the time asymmetric many worlds interpretation and proposes a supplementation. The concluding section is a transcript of a discussion among the speakers about the poposals of the latter two parts.NEWLINENEWLINENEWLINENEWLINEThe commented collected works by \textit{H. Everett III} [(2012; Zbl 1270.81003)] together with the present book give an excellent insight to Everett's proposal, the history of the venture, and the impact on the philosophy of science during the latter fifty years.