com Everywhere and Everywhen www.com This page intentionally left blank www.com Everywhere and Everywhen Adventures in Physics and Philosophy Nick Huggett 1 2010 www.com 3 Oxford University Press, Inc., publishes works that further Oxford University’s objective of excellence in research, scholarship, and education. Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With offices in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Copyright © 2010 by Oxford University Press, Inc. Published by Oxford University Press, Inc. 198 Madison Avenue, New York, NY 10016 www.com Oxford is a registered trademark of Oxford University Press.
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, electronic, mechanical, photocopying, recording, or otherwise, without the prior permission of Oxford University Press. Library of Congress Cataloging-in-Publication Data Huggett, Nick. Everywhere and everywhen : adventures in physics and philosophy / Nick Huggett.01—dc22 2009016318 9 8 7 6 5 4 3 2 1 Printed in the United States of America on acid-free paper www.com For my paradoxical twins, Kai and Ivor www.com This page intentionally left blank www.com Preface I remember when I discovered that you could be a philosopher of physics.
I was in the library of my school studying brochures for university admis- sions when I came across the Physics and Philosophy program at Oxford. It made perfect sense to me at the time; physics was my best subject and I’d developed an interest in philosophy in a fairly typical teenage intellectual way. So I applied and got in. In hindsight, though, I’m not quite sure what I thought the philosoph- ical study of physics was.
I certainly didn’t give a very good answer to that question in my admission interview! That was something I learned later, during my studies. (In fact, according to Rom Harré, a founder of the program at Oxford, the intention was not to produce philosophers of physics, but future leaders, well grounded in reasoning, ethics, and the sciences.) Still, I found that my youthful intuition was reliable, and after I completed my undergraduate studies I went on to Rutgers University in New Jersey, where I was lucky enough to work with some of the best and most generous philosophers of physics I have ever met. Best of all, afterward I found a job where I could research and teach my subject, at the University of Illinois in Chicago. So now my only problem is explaining to people—parents, friends, neighbors on planes and at dinner parties, and especially physicists and philosophers—what it is that I do.
Something to do with the ethics of science? That’s an important topic, but generally not part of philosophy of physics. Or perhaps the connections between Buddhism and quantum physics? That idea was popularized in the 1970s by Fritjof Capra and Gary Zukav, but it’s not what most philosophers of physics are interested in. Or is it an attempt to tell physicists what must be the case by pure speculation, regardless of the facts of experiment? Or perhaps to show that physics is nothing but a social fabrication, that truth about the physical world is not objective but whatever physicists decide. Well, the sociological dimensions of science are interesting, and some people do take a very hard line, but most philosophers of physics take very seriously physics’ ability to get at objective truth—and they think that it is the ultimate standard, not pure speculation.
So this book is to explain to all those people some of the ways in which physics and philosophy can be in fruitful dialogue—it is that dialogue that www.com viii Preface engages philosophers of physics (and, as we’ll see, many physicists). We shall see that indeed the traffic is two-way: while physics has important lessons for philosophy, the kind of investigations at which philosophers excel are necessary in science, and some of the most important advances in physics have required philosophical contemplation. To be more specific, we will see three main kinds of interaction. First, there are cases in which philosophical questions can be for- mulated in a precise way in physics and then addressed with resources of physics.
For instance, could space have an edge? Second, there are cases in which ideas used in physics turn out to be conceptually unclear or incompatible with new knowledge in physics. What physics requires here is a careful analysis of the concepts and an understanding of how they are used. That kind of work is philosophical, though it is often done by physicists—‘philosopher-scientists’, as Einstein was described. For instance, what is it for events to be “simultaneous”? And third and finally, the fact that we are physical beings living in a physical world of a specific type has profound consequences for the way we experience the world.
Having an understanding of these consequences is crucial for a clear philosophical view of a variety of problems. How, for instance, do we perceive left versus right handedness? To see these things in more detail, naturally we’ll need to introduce some physics and philosophy. You’ll notice some difference between the two here. The physics will be presented largely as the materials for our discussion, while I will be showing you how to think about the physics like a philosopher.
When you read a popular book on physics, the goal usually is to explain recent developments in terms that are accessible to the layperson; the details themselves take years of study even for very smart people. The best books of this kind do a great job of explicating the fundamental ideas and implications, but of course they don’t make you a physicist. I have a rather different ambition for this book. This is not a book that just seeks to explain recent developments in philosophy of physics—though we will talk about some of them—but one that aims to help the reader really think philosophically about physics and the physical world.
Having taken ten years in higher education to become a philosopher, I hesitate to say that this book will make the reader a philosopher of physics, but I do hope it will show the way and allow a first step in that direction. To put it another way, the book doesn’t just report on philosophy, it does it too, and I hope that example will be useful. As a result this book will be demanding in places. Philosophy involves patient reasoning, canvassing of different possible positions, step-by-step argument, and to-and-fro.
Sometimes it takes effort to keep the logic of the topic clear. However, I’ve picked pieces of physics and of philosophy that are suitable for a general audience (I’ve taught these topics to a lot of undergraduates of very different abilities, so I have a pretty good sense of what is digestible). The bottom line is that I’ve picked topics that should www.com Preface ix be fully comprehensible to anyone who is prepared to apply some careful thought. I offer the following deal: in return for carefully thinking through some sometimes challenging (but always interesting, I hope!) arguments, the reader will start to learn to be a philosopher of physics.
Here’s the plan of action. In the first chapter I will give an example of a philosophical problem so that the reader can see right away what kinds of concerns and what kinds of approaches drive the book. We’ll also fill in some of the physics background that we need, and some important philosophical concepts (just what is a ‘law’ of physics?). Chapters 2–3 discuss Zeno’s paradoxes, which challenge the very idea of motion.
For instance, an arrow cannot move during any instant because an instant has no duration. But if it doesn’t move at any time then how does it move at all? Chapters 4–6 concern the overall ‘shape’ of space, for instance whether it has an edge, whether it is ‘closed up’ on itself, and whether it has more than three dimensions, and if not why? Chapters 7–8 continue the discussion of the shape of space in a different direction, investigating its geometry. Is it flat? What would it mean if it weren’t? How could we tell? And generally, what does it mean to say space is curved? Chapter 9 com- pletes the discussion of space by asking the obvious remaining question– what is it? Chapters 10–11 take up the issue of time. Time seems so different from space: for instance, we certainly experience time differently than we do space, as something ‘moving’.
How could physics account for this fact? Chapters 12–13 are devoted to another puzzling aspect of time: is time travel possible? We’ll see possibilities and restrictions, and see ultimately how it is a coherent possibility. Chapters 14–15 explain and investigate Einstein’s relativity, which cru- cially changes our understanding of space and time. The presentation is a little more rigorous than most popular presentations, but all that is involved is a simple, if unfamiliar, geometry. We will be able to show why moving bodies contract and why moving clocks slow down, and understand what this means.
Chapters 16–18 are devoted to some issues that have been of particular interest to me. First is the question of what it is for an object to be left rather than right; what is it about a left hand, or left-handed glove, or left-handed screw that makes it left rather than right? After all, all these things are very, very similar to their mirror images. And then there is the question of identity and indistinguishability in physics. The particles of physics are exactly alike, so does it make any difference if they swap their locations, say? Are they like money in the bank or are they like people? A note on citations.
To maintain an informal style I have gathered annotated references at the end of each chapter instead of inserting citations in the main text. I have also omitted certain more technical www.com x Preface works, while crediting authors. I hope they will understand my aims and forgive me. I’ve had a lot of help with this book, first and foremost from all my students who have read it or discussed the issues.
Among them I should especially thank my research assistants David Lee, Poonam Merai, and Darrell Wu, and my TAs Rashi Agarwal (especially for ‘Mr. Toody’), Maria Balcells (especially for ‘spatter’), Isaac Thotz, and Aleks Zarnitsyn. I also had feedback and assistance from a number of friends and colleagues, particularly Craig Callender, Stanley Fish, David Hilbert, Rachel Hilbert, Tom Imbo, Jon Jarrett, David Malament, Tim Maudlin, Chris Wüthrich, and Eric Zaslow. Because of the informal style, I have not credited anyone with specific contributions; they know how they helped, and I hope that suffices.
Of these I wish to thank Tim Maudlin especially, for teaching me much of what I know of these topics in the first place, and for extremely generous and helpful comments on the manuscript. His contributions are too numerous to credit individually, but they certainly made this a much better book. With such assistance, I can sincerely say that any remaining faults in this book are mine alone. (And of course, none of the above necessarily agree with my arguments, or their conclusions.) Chapters 17–18 are based closely on an unpublished essay that I wrote with Tom Imbo, itself based on research carried out by him and his grad- uate students Randall Espinoza and Mirza Satriawan.
I am very grateful for all I have learned from Tom during the time we have worked together, and for his permission to use this work here. I am also grateful for the support I have received from Oxford Uni- versity Press. Peter Ohlin has been very encouraging, and Stephanie Attia did a great job polishing the text.