*Many Worlds?*. 2010, with Simon Saunders, Jonathan Barrett, Adrian Kent*The Emergent Multiverse*. 2012

What would it mean to apply quantum theory,
without restriction and without invoking 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.

The Everett interpretation of quantum mechanics takes the apparent craziness seriously, and asks, ‘what would it be like if particles really were in two places at once, if cats really were alive and dead at the same time’? The answer, it turns out, is that if the world were like that—if it were as quantum theory claims—it would be a world that, at the macroscopic level, was constantly branching into copies—hence the more sensationalist name for the Everett interpretation, the ‘many worlds theory’. But really, the interpretation is not sensationalist at all: it simply takes quantum theory seriously, literally, as a description of the world. Once dismissed as absurd, it is now accepted by many physicists as the best way to make coherent sense of quantum theory.

David Wallace offers a clear and up-to-date survey of work on the Everett interpretation in physics and in philosophy of science, and at the same time provides a self-contained and thoroughly modern account of it—an account which is accessible to readers who have previously studied quantum theory at undergraduate level, and which will shape the future direction of research by leading experts in the field.