Many Worlds?: Everett, Quantum Theory, & Reality

by Simon Saunders, Jonathan Barrett, Adrian Kent

What would it mean to apply quantum theory, without restriction and without involving any notion of measurement and state reduction, to the whole universe? What would realism about the quantum state then imply?



This book brings together an illustrious team of philosophers and physicists to debate these questions. The contributors broadly agree on the need, or aspiration, for a realist theory that unites micro- and macro-worlds. But they disagree on what this implies. Some argue that if unitary quantum evolution has unrestricted application, and if the quantum state is taken to be something physically real, then this universe emerges from the quantum state as one of countless others, constantly

branching in time, all of which are real. The result, they argue, is many worlds quantum theory, also known as the Everett interpretation of quantum mechanics. No other realist interpretation of unitary quantum theory has ever been found.



Others argue in reply that this picture of many worlds is in no sense inherent to quantum theory, or fails to make physical sense, or is scientifically inadequate. The stuff of these worlds, what they are made of, is never adequately explained, nor are the worlds precisely defined; ordinary ideas about time and identity over time are compromised; no satisfactory role or substitute for probability can be found in many worlds theories; they can't explain experimental data; anyway, there are

attractive realist alternatives to many worlds.



Twenty original essays, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They consider questions of ontology - the existence of worlds; probability - whether and how probability can be related to the branching structure of the quantum state; alternatives to many worlds - whether there are one-world realist interpretations of quantum theory that leave quantum dynamics unchanged; and open questions even given many worlds, including the multiverse

concept as it has arisen elsewhere in modern cosmology. A comprehensive introduction lays out the main arguments of the book, which provides a state-of-the-art guide to many worlds quantum theory and its problems.

Genres: Physics, Philosophy, Science

635 pages, Kindle Edition

First published June 1, 2010

Reviews

[Jonathan Platter · Rating 4 out of 5]

This book is a collection of diverse essays by leading philosophers of science, metaphysicians, and physicists dealing with Everett's interpretation of quantum mechanics (QM). It is not an easy read, though some essays are easier than others, but it does address several important issues in the philosophy of quantum physics.

For the most part the contributors are sympathetic to the Everettian "many worlds" interpretation of QM. There are a few exceptions, most notably Tim Maudlin's and John Hawthorne's essays on ontology and metaphysics, respectively.

Two issues that are dealt with multiple times throughout are the "problem" of "preferred basis" for the understanding of measurement in Everett and the more recent concept of decoherence, which is itself partly a solution to the problem of preferred basis.

The issues and various perspectives in this book are too complex and multi-sided to summarize in this setting. The book is worth reading for anyone interested in metaphysics on the one hand (my personal reason for reading) or interested in Everett's interpretation of QM. Physicists would benefit from serious engagement with the philosophers and metaphysicians, and vice versa. A difficult but worthwhile read.

[Joseph Yue · Rating 2 out of 5]

Many-Worlds interpretation of quantum mechanics is very much like a contemporary Plato's cave, as in both cases what people look at are projections rather than real things. As Wallace argues in this volume, there is a sense in which the existence of real things may be recovered from the inherent structure of the quantum world, but if anything, structures are emphatically not a part of the formalism, the definition and classification of structures must be postulated. The famous retort that Many-Worlds envisions a rather extravagant parsimony turns out to be true not only in the number of worlds, but also in the number of extra metaphysical postulates in addition to the formalism. The cost for a simpler physical theory seems to be a much more complex metaphysical theory, perhaps this is a new meta uncertainty principle. In any case, as much as this volume is praised as a modern classic, no one actually cares, nor should anyone. The only significant achievement of this book is probably that it serves as a definite proof that Humeanism is at best a complete waste of time, and at worst, a spectacular scam.

[Christopher Hellstrom · Rating 4 out of 5]

This was too technical for me. But the parts I understood were thought provoking.