Paradoxes of Time Travel

Paradoxes of Time Travel Ryan Wasserman

OXFORD UNIVERSITY PRESS

Contents Preface 1. Introduction 1. Time Travel 2. ...And Not 3. Possibility 4. Paradoxes 2. Temporal Paradoxes 1. Two Debates in the Philosophy of Time 1.1 The ontology of time 1.2 The reality of tense 2. Eternalism and Time Travel 3. Presentism and Time Travel 3.1 The no destination argument 3.2 The definitional argument 3.3 The annihilation argument 4. The Growing Block and Time Travel 5. Special Relativity and Time Travel 6. General Relativity and Time Travel 3. Paradoxes of Freedom I 1. Stories of Self-Defeat 2. Rewriting History 3. The Branches of Time 3.1 The branching timeline model 3.2 The inconsistency objection 3.3 The immutability objection 3.4 The irrelevance objection 4. Traveling in Hypertime 4.1 The hypertime model 4.2 Three advantages to the hypertime model 4.3 Three objections to the hypertime model 5. The A-Model of Past-Alteration

4. Paradoxes of Freedom II 1. On Lewis’s Way Out 1.1 Clarifications 1.2 Applications 1.3 Reactions 2. Killing Baby Suzy 2.1 Vihvelin on retrosuicide 2.2 Vranas on retrosuicide 2.3 Sider on retrosuicide 3. The Problem with Banana Peels 3.1 Horwich on coincidences 3.2 Smith on fallacious reasoning 3.3 Smith on tomato rolls 4. Paradox without Freedom? 5. Causal Paradoxes 1. Preliminaries 1.1 Causal Loops 1.2 Backward Causation 2. The Bootstrapping Paradox 3. The Ex Nihilo Paradox 4. The Restoration Paradox 5. The Frequency Paradox 6. The Counterfactual Paradox 6.1 Lewis’s theory 6.2 Tooley’s argument 6.3 Tichy’s hat and Morgenbesser’s coin 6.4 A unified solution 6. Paradoxes of Identity 1. Two Puzzles about Sameness and Difference 2. Perdurantism and Self-Visitation 2.1 Perdurantism and persistence 2.2 Perdurantism, change, and self-visitation 2.3 The problem with temporal parts 2.4 The problem with person parts 3. Endurantism and Self-Visitation 3.1 Endurantism and persistence

3.2 Endurantism, change, and self-visitation 3.3 From relativism to compatibilism 3.4 Endurantism and indiscernibility 4. Mereology and Multi-Visitation 4.1 The identification account 4.2 The composition account 4.3 The constitution account 4.4 The elimination account 5. The Strange Tale of Adam the Atom References Index

Preface

One cannot help but wonder. H. G. Wells, The Time Machine In the fall of 2005, I taught my very first introductory philosophy course. That course included a unit on freedom and determinism and I decided to conclude that section by spending a day on the grandfather paradox. That meeting ended up being so much fun that I added two extra days on the topic the next time I taught the class. Among other things, we discussed David Lewis’s definition of time travel, Robert Heinlein’s tales of causal loops, and Doc Brown’s explanation of branching timelines from the Back to the Future movies. Student feedback on these topics was so positive that I eventually expanded the material into an entire course on time travel. The notes from that course went on to provide the basis of the book that you now hold. This book retains many features of my original classroom lectures. I have tried my best to introduce all of the topics in an entertaining way, and to provide as much background material as is required. I have also included many of the examples and illustrations that have proven helpful in class. My hope is that the resulting work is accessible to all students of philosophy, and that teachers will find the text useful in their own philosophy courses. I would like to thank all of the students who have discussed these topics with me over the years—your feedback has helped me to improve this work in many different ways. I am also grateful to one current student—Trevan Strean—for helping with the illustrations in this book. I would also like to thank Western Washington University for providing me with a teaching sabbatical and research support. Another prominent feature of this book is its heavy use of examples from the science fiction genre. As will become clear, almost all of the philosophical issues raised by time travel have their roots in science fiction, and I have done my best to trace out the history of these ideas. I have been greatly aided in this task by Paul J. Nahin’s excellent book Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction, and Michael Main’s comprehensive website . Both of these works have provided me with end-

less examples and inspiration. I am also very grateful to the library staff at Western Washington University who helped me track down some truly obscure publications. My hope is that this research will help make this book of interest to science fiction fans, as well as philosophers. Still, the main audience for this book will be professional philosophers— especially those with research interests in contemporary analytic metaphysics. Philosophers in this area will already be aware of some of the important issues raised by time travel—issues having to do with the nature of time, freedom, causation, and identity. However, philosophers in this area also know that, until this point, there has been no comprehensive study of these topics. That is the primary goal of this book—to survey, systematize, and expand upon the philosophical literature on time travel. I would like to thank all of the philosophers who have helped me in this task by providing comments on earlier drafts of this work. This includes Thomas Hall, Dan Howard-Snyder, Frances Howard-Snyder, Hud Hudson, David Manley, Ned Markosian, Gerald Marsch, Tim Maudlin, Jeff Russell, Michelle Saint, Neal Tognazzini, James van Cleve, and two anonymous readers for Oxford University Press. I would also like to thank Andreas Riemann, who proofread the material on special relativity, and Jonathan Bennett and Steffi Lewis, who provided me with unpublished materials relating to David Lewis’s work on time travel. Finally, I gratefully acknowledge Springer, Wiley, Oxford University Press, Cambridge University Press, and Hud Hudson for giving me permission to include portions of the following works in this book: “Van Inwagen on Time Travel and Changing the Past,” in Dean Zimmerman, ed., Oxford Studies in Metaphysics, Volume 5. Oxford: Oxford University Press, 2010; “Personal Identity, Indeterminacy, and Obligation,” in Georg Gasser and Matthias Stefan, eds., Personal Identity: Simple or Complex? Cambridge: Cambridge University Press, 2013; “Theories of Persistence,” Philosophical Studies, 173 (2016): 243–50; and “Vagueness and the Laws of Metaphysics,” Philosophy and Phenomenological Research (forthcoming). Lastly, and most importantly, I would like to thank my family—Christine, Ben, and Zoë. You have been a constant source of encouragement, support, and perspective throughout this process. I couldn’t have done it without you.