Preface

This is an introduction to the idea that our universe is just one of many universes, that it is part of a multiverse. This idea is very topical. A multiverse of one kind or another is seriously advocated by many philosophers. And similarly for physics: many physicists advocate a multiverse—usually of a different kind than that of the philosophers. So, the time seems ripe to assess the various versions of this idea. In this book, I will assess three versions of the multiverse. One is from philosophy, more specifically, from logic’s treatment of possibility. The other two are from physics, more specifically, from the Everettian interpretation of quantum theory, and from inflationary cosmology. I will discuss these in order, and then, in my final chapter, I’ll compare them and relate them to each other.

I should declare at the outset that the book has three main limitations. First, I completely set aside the treatment of the multiverse in countless novels, plays, and films. I agree that the parallel worlds (so-called) in, for example, the films Sliding Doors (1998) and Everything Everywhere All at Once (2022), raise plenty of philosophical issues. But in this book, I will pursue the issues in themselves, in an academic manner. I have no aptitude for literary matters, so I set aside (albeit with some regret) these entertaining examples.

Second, within physics and philosophy, I should mention that there are multiverse proposals other than the three I will assess. But among these, there are two that I will discuss during my assessments of my main three. The first of these is the multiverse of string theory (string theory is a speculative physical theory). This proposed multiverse, often called “the landscape,” gives the main rationale for the cosmological multiverse, and so I will discuss it in that context. The second is the “Pythagorean” proposal that all of reality is mathematical, and that for this reason our universe is one of many universes. I will discuss this toward the end of my assessment of the philosophical multiverse, which, as I mentioned, is based on logic’s treatment of possibility.

Third, this book is a philosopher’s introduction. I mean this both in the sense that I am a philosopher, not a physicist, and that my intended readership is people who are interested in philosophy and the philosophical aspects of physics—but who are newcomers to it. So, I endeavour to explain every term that is “in-house” for the disciplines of either physics or philosophy. For example, I will explain the terms “truth table” (in philosophy) and “wave function” (in physics). But the tone is brisk and breezy. References are few, and almost entirely confined to suggested readings at the end of each chapter. Thus, it will be evident throughout that my discussion is inconclusive, and my conclusions are tentative.

Of course, being inconclusive is hardly surprising. For a subject as large in every sense (and more importantly, multifaceted) as the multiverse, even a learned and exhaustive investigation must surely be inconclusive. No human mind could know all the relevant strands of evidence and argumentation, from all of physics and philosophy. And even if one did know all that, the process of weighing them against each other—for there are bound to be conflicts—would involve judgments that could no doubt be disputed.

So, while I admit that my discussion is inconclusive, I console myself with the thought that a brief and non-exhaustive investigation can serve a purpose. Indeed, I hope it can satisfy the curiosity of the newcomer reader and prompt their own thinking as well as a long and learned book can. (In chapter 1, I will say more in defence of this philosophical outlook: namely, that one should “dive in” and speculate, even when one’s evidence is minimal and perhaps defective.)

I am grateful to many people for their comments and corrections on drafts of this material and for their discussion. For very helpful comments and corrections on drafts of various chapters, I am especially grateful to Feraz Azhar, Guido Bacciagaluppi, Mark Burgman, Frank Cudek, Richard Dawid, Fred Muller, Ruward Mulder, Alex Oliver, Bryan Roberts, Simon Saunders, David Wallace, and Alastair Wilson. I am also grateful to participants in the 2022 Cambridge–LSE Philosophy of Physics “Bootcamp” online discussion group, especially Frank Cudek, Henrique Gomes, Josh Hunt, Brendan Kolb, Klaas Landsman, Joanna Luc, Tushar Menon, Ruward Mulder, Neil Dewar, and particularly Bryan Roberts. I am also grateful for the comments and corrections of two anonymous readers for the British Society for the Philosophy of Science. My thanks also go to the audiences at talks about various parts of this material, held in the mathematics, philosophy, and physics departments and societies at the University of Amsterdam, University of Cambridge, the University of Chile, King’s College London, the University of Oxford, the University of Salzburg, and the University of Vienna. Finally, it is a pleasure to thank some of my physicist colleagues for discussion of the Everettian and cosmological multiverses, and for their encouragement: John Barrow, Bernard Carr, George Ellis, Jonathan Halliwell, Jim Hartle, and Martin Rees.

I am very grateful to Helen Beebee, Oxford University Press, and the Firestone Library at Princeton University, for generous advice and for permission to reprint (in section 10 of chapter 3) a letter by David Lewis. This letter is reprinted in volume 1 of Philosophical Letters of David K. Lewis, edited by H. Beebee and A. R. J. Fisher, Oxford University Press, 2020.

The cartoon “Parallel Universes,” by Roz Chast, © 1984, is reproduced (see also section 10 of chapter 3) with the kind permission of The Wylie Agency (UK) Limited.

It is also a pleasure to thank both the editors of British Society for the Philosophy of Science Open, the society’s series of open-access books, and the editorial and production staff at The University of Calgary Press—especially Helen Hajnoczky, Melina Cusano, and copy editor Glen Hoffmann—for all their work and helpful advice.

This book is dedicated to my wife, Mari, for all her love and support.

JNB

Trinity College,

Cambridge

January 12, 2026