The Emergent Multiverse presents a striking new account of the 'many worlds' approach to quantum theory. The point of science, it is generally accepted, is to tell us how the world works and what it is like. But quantum theory seems to fail to do this: taken literally as a theory of the world, it seems to make crazy claims: particles are in two places at once; cats are alive and dead at the same time. So physicists and philosophers have often been led either to give up on the idea that quantum theory describes reality, or to modify or augment the theory.
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.
Part I: The Plurality of Worlds
1: The Paradox of Measurement
2: The emergence of multiplicity
3: Chaos, decoherence, and branching
Part II: Probability in a Branching Universe
4: The Probability Puzzle
5: Symmetry, rationality, and the Born Rule
6: Everettian statistical inference
Part III: Quantum Mechanics, Everett style
7: Uncertainty, Possibility, and Identity
8: Spacetime and the Quantum State
9: The Direction of Branching and the Direction of Time
10: A Cornucopia of Everettian Consequences
A: Proof of the Branching-Decoherence Theorem
B: Classical decision theory
C: Formal proofs of decision-theoretic results
D: Proof of the Utility Equivalence Lemma
David Wallace was born in San Rafael, California, in 1976, but has been resident in the UK since 1977. He studied theoretical physics at Oxford University from 1994-2002, but upon realising his research interests lay mostly in conceptual and foundational aspects of physics, he moved across into philosophy of physics. For the last six years he has been Tutorial Fellow in Philosophy of Science at Balliol College, Oxford. He holds PhDs in physics and in philosophy, and his research interests span a wide range of issues on the boundary between philosophy and physics: symmetry and the gauge principle, the direction of time, the structure of quantum field theory, and of course the interpretation of quantum mechanics.
"The Emergent Multiverse is the most extensive, careful, and wide-ranging discussion of Hugh Everetts so-called Many Worlds interpretation of quantum theory in existence (at least on our branch of the multiverse), and is certain to become the locus classicus for all future discussions of the theory. Since the first obligation of a reviewer is to give guidance to potential readers, I will discharge that obligation first: if you have any interest in studying or trying to understand the Everett theory, you must get this book. You wont find a better discussion of both foundational issues and far-flung consequences of the theory anywhere. David Wallace has been brooding on the theory, and fielding objections to it, for over a decade. His considered views and responses are as careful and sophisticated as any on the market, and are equally attuned to physical and to philosophical issues."
– Tim Maudlin, Nous
"This book is an outstanding achievement. It presents the current state of the art in the Everett interpretation to a depth and level of sophistication that will be appreciated by the leading experts in the foundations of quantum theory (of whom Wallace is one) – and will educate them, and should chasten most of them. Yet, at the same time, the presentation is so clear and down-to-earth that this could serve as an introductory textbook for (say) undergraduates who are unfamiliar with any of the issues or even with quantum theory. This combination of relentlessly watertight argument with relentless common sense, however counter-intuitive the subject matter, is something Wallace is very good at. So much so that I think that even a philosophically-minded lay person, who would have to skip most of the technical discussion and equations, might nevertheless devour this book and learn a great deal from it"
– David Deutsch, Centre for Quantum Computation, The Clarendon Laboratory, University of Oxford
"it's well-written, it's often funny, it will expand your mental horizons, and it's not impossible that it will turn out to be one of the pivotal books of the twenty-first century. "
– Emmanuel Rayner
"Nobody has done more to defend, clarify and advance the Everett interpretation over the past dozen years than Wallace, and this book is the culmination of his work on this area. As those who have read Wallace's articles will expect, it is an excellent book, and should be required reading for anyone interested in the foundations of quantum mechanics."
– Peter J. Lewis, Notre Dame Philosophical Reviews