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Paradoxes of Time Travel Paradoxes of Time Travel Ryan Wasserman 1 3 Great Clarendon Street, Oxford, Ox2 6dP, United Kingdom Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries © Ryan Wasserman 2018 The moral rights of the author have been asserted First Edition published in 2018 Impression: 1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, by licence or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights department, Oxford University Press, at the address above You must not circulate this work in any other form and you must impose this same condition on any acquirer Published in the United States of America by Oxford University Press 198 Madison Avenue, New York, NY 10016, United States of America British Library Cataloguing in Publication data data available Library of Congress Control Number: 2017943946 ISBN 978–0–19–879333–5 Printed and bound by CPI Group (UK) Ltd, Croydon, CR0 4YY Links to third party websites are provided by Oxford in good faith and for information only. Oxford disclaims any responsibility for the materials contained in any third party website referenced in this work. To Ben and Zoë May you always be filled with wonder Contents Preface ix 1. Introduction 1 1. Time Travel 2 2. . . . And Not 8 3. Possibility 13 4. Paradoxes 18 2. Temporal Paradoxes 23 1. Two debates in the Philosophy of Time 24 1.1 The ontology of time 24 1.2 The reality of tense 27 2. Eternalism and Time Travel 30 3. Presentism and Time Travel 38 3.1 The no destination argument 39 3.2 The definitional argument 42 3.3 The annihilation argument 46 4. The Growing Block and Time Travel 49 5. Special Relativity and Time Travel 55 6. General Relativity and Time Travel 65 3. Paradoxes of Freedom I 70 1. Stories of Self-defeat 71 2. Rewriting History 74 3. The Branches of Time 78 3.1 The branching timeline model 79 3.2 The inconsistency objection 83 3.3 The immutability objection 85 3.4 The irrelevance objection 89 4. Traveling in Hypertime 90 4.1 The hypertime model 90 4.2 Three advantages to the hypertime model 94 4.3 Three objections to the hypertime model 96 5. The A-Model of Past-Alteration 99 4. Paradoxes of Freedom II 107 1. On Lewis’s Way Out 108 1.1 Clarifications 108 1.2 Applications 111 1.3 Reactions 114 viii Contents 2. Killing Baby Suzy 114 2.1 Vihvelin on retrosuicide 115 2.2 Vranas on retrosuicide 120 2.3 Sider on retrosuicide 124 3. The Problem with Banana Peels 130 3.1 Horwich on coincidences 131 3.2 Smith on fallacious reasoning 135 3.3 Smith on tomato rolls 137 4. Paradox without Freedom? 139 5. Causal Paradoxes 145 1. Preliminaries 146 1.1 Causal loops 146 1.2 Backward causation 150 2. The Bootstrapping Paradox 154 3. The Ex Nihilo Paradox 157 4. The Restoration Paradox 163 5. The Frequency Paradox 165 6. The Counterfactual Paradox 171 6.1 Lewis’s theory 172 6.2 Tooley’s argument 174 6.3 Tichý’s hat and Morgenbesser’s coin 176 6.4 A unified solution 179 6. Paradoxes of Identity 183 1. Two Puzzles about Sameness and difference 184 2. Perdurantism and Self-Visitation 187 2.1 Perdurantism and persistence 187 2.2 Perdurantism, change, and self-visitation 189 2.3 The problem with temporal parts 192 2.4 The problem with person parts 194 3. Endurantism and Self-Visitation 197 3.1 Endurantism and persistence 197 3.2 Endurantism, change, and self-visitation 199 3.3 From relativism to compatibilism 200 3.4 Endurantism and indiscernibility 203 4. Mereology and Multi-Visitation 209 4.1 The identification account 211 4.2 The composition account 214 4.3 The constitution account 216 4.4 The elimination account 217 5. The Strange Tale of Adam the Atom 218 References 225 Index 243 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 back- ground 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 especially grateful to three current students: Trevan Strean, who helped with the illustrations, dee Payton, who worked on the index, and Sean Nalty, who did proofreading. 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 <http://www.storypilot.com/>. Both of these works have provided me with endless 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,