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THE FRONTIERS COLLECTION

T he Q uantum Interv iew s

Maximilian Schlosshauer

ELEGANCE

AND ENIGMA

E d itor

Series Editors: Avshalom C. Elitzur Bar-Ilan University, Unit of Interdisciplinary Studies, 52900 Ramat-Gan, Israel email: avshalom.elitzur@weizmann.ac.il Laura Mersini-Houghton Dept. Physics, University of North Carolina, Chapel Hill, NC 27599-3255, USA email: mersini@physics.unc.edu Maximilian Schlosshauer

Mark P. Silverman Trinity College, Dept. Physics, Hartford CT 06106, USA email: mark.silverman@trincoll.edu Ru¨diger Vaas University of Giessen, Center for Philosophy and Foundations of Science, 35394 Giessen, Germany email: ruediger.vaas@t-online.de

Gaiberger Straße 38, 69151 Waldhilsbach, Germany email: zeh@uni-heidelberg.de

ISSN 1612-

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Springer-Verlag Berlin Heidelberg 2011

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and Quantum Information

H. Dieter Zeh

Ļe truth about the world, he said, is that anything is possible. Had you not seen it all from birth and thereby bled it of its strangeness it would appear to you for what it is, a hat trick in a medicine show, a fevered dream, a trance bepopulate with chimeras having neither analogue nor precedent, an itinerant carnival, a migratory tentshow whose ultimate destination af- ter many a pitch in many a muddled ŀeld is unspeakable and calamitous beyond reckoning.

Ļe universe is no narrow thing and the order within it is not constrained by any latitude in its conception to repeat what exists in one part in any other part. Even in this world more things exist without our knowledge than with it and the order in creation which you see is that which you have put there, like a string in a maze, so that you shall not lose your way. For existence has its own order and that no man’s mind can compass, that mind itself being but a fact among others.

— CśŞřōŏ MŏCōŞŠŔť, Blood Meridian

P R E FAC E

WŔť ōŜŜŞśōŏŔ the foundations of quantum mechanics, this vast and fascinat- ing subject area, through interviews? Why not a proper textbook instead? After all, many of the classic titles, such as Max Jammer’s Ļe Philosophy of Quantum Mechanics and Bernard d’Espagnat’s Conceptual Foundations of Quantum Mechanics , have aged visibly. A complete, up-to-date account of the ŀeld—one that would also pay appro- priate attention to recent developments like quantum information, experiments, and reconstructions of quantum theory—is arguably lacking. I’m well aware of this situ- ation. So it is not as if the idea of writing a textbook hadn’t ever occurred to me, or as if I simply shied away from the effort, however enormous I suspect the investment would have to be. Opting for the interview format instead is, as I see it, neither a cheap cop-out nor merely a temporary substitute. Rather, it is a uniquely effective way of laying out the ŀeld of quantum foundations as it stands today. It won’t be news to you that this ŀeld is no cut-and-dried solid-state physics: just attend any conference devoted to quantum foundations, and you’ll know that the debates at such events have the zeal of a political convention. How could a single author do the ŀeld full justice without coloring her story? I do think it could be done, but you’d have to be a card- carrying member of the Party of Utterly Neutral Quantum Scholars (“Punqs”) not to be accused of supporting, however subtly, the line of a particular foundational program or mindset. Ļe interview approach has diversity built in from the outset. It allows you to perceive the subject through the eyes of the ŀeld’s leading practitioners. You won’t need to go through stacks of research papers to get a representative cross-section of views, or trust any one author to faithfully reproduce all the shades of gray. Last but not least, interviews lend themselves to an informal and personal style. After all, we read books for enjoyment. Ļey shouldn’t be a slog.

Of course, if not handled judiciously, the interview approach can also go astray. One obvious make-or-break issue is the choice of questions. Ļere’s always a danger of bias, of putting spin on the questions. Here are some of the goals I set out.

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Ť ŜŞőŒōŏő

I wanted the questions to cover a wide range of topics, so that this book would provide a comprehensive rełection on the ŀeld. Ļere are, of course, the standard themes: interpretations of quantum theory, the measurement problem, quantum states, probabilities, issues of nonlocality and completeness, and the like. But in- cluded as well are questions on newer areas, such as quantum information and re- constructions, and about interdisciplinary aspects, such as the role of philosophy and the implications of our quest for a uniŀed theory. Another goal was to phrase the questions in reasonably broad terms, because I didn’t want the respondents to get caught up in technical details, nor did I want to unduly restrict the range of possible answers. At the same time, I tried to keep each question focused on a well-deŀned topic, so that answers can be compared side by side and don’t turn into blanket statements and clichéd generalizations. I also wanted to leave room for personal stories among all the heavy going. How did people originally become interested in quantum foundations? What would it take for them to embrace a rivaling view? What role do they attribute to individual temperament when it comes to the choice of foundational agenda? Ļose kind of questions. Picking participants for the interviews can be treacherous territory as well. On a practical level, we may tend to choose people we already know well, thereby running the risk of inadvertently excluding the up-and-coming talent or the recluse. On a psychological level, we may gravitate toward people who share our own worldview. I tried my best to assemble a cast that would do justice to the diversity of the ŀeld. Ļe interviewees for this book come in all foundational stripes: agnostics, infor- mationalists, Bohrians, Everettians, Bohmians, Bayesians, collapsists, ensemblists, reconstructionists—you name it. Ļey come from physics, philosophy, and math- ematics departments, and they range in age from the budding young academic to the distinguished emeritus professor who might have shaken hands with Einstein and Bohr. A serious lack of diversity, however, occurs in the gender department, as all participants are men. Another rełection of how regrettably male-dominated the world of physics (and the philosophy of physics) is! I lament this situation as much as you do, and if you’d like to suggest suitable female participants for a future edition of this book, please let me know.

A few words on how the book is organized. Biographical sketches introduce the participants at the beginning of the book. I put the same seventeen questions to each of the seventeen participants (the identical numbers are pure coincidence). All interviews were conducted in writing. Answers were limited to about one page in length, on average , and nothing has been omitted here. A minimal background in quantum mechanics should be all you need for this book. Ļere’s a glossary at the end of the book (page ȁȈȄ) that explains some of the technical terms repeatedly appearing in the interviews. Have a look there if you’re new to the ŀeld. As for the grouping of the interview answers, there are two obvious alternatives: by respondent or by question. Organization by respondent emphasizes autobiographi- cal coherence, allowing respondents to build a continuous argument. Organization

Ťŕŕ ŜŞőŒōŏő

cial thanks go to Chris Fuchs and David Mermin, who, besides their own answers, contributed a number of helpful comments and suggestions. An essential part in making this book a reality was played by Angela Lahee, editor at Springer. Angela is the kind of editor the doomsayers tell us no longer exists. For several years now, she’s been a trusted friend and conŀdante who is always willing to share her expertise and lend a sympathetic ear. Right from the moment when I ŀrst put the idea of this book to her, Angela threw her wholehearted support behind it. Her feedback accompanied the making of the book from start to ŀnish. In particular, she provided thoughtful comments on the interview questions and on a draft of some of the chapter introductions. At the end of the day, what really enables us to do what we do is the nourish- ment we get by being around the people dearest to our hearts. I’m most grateful to my wife, Kari, and to my son, Eli, who was born last year, for all the love and hap- piness we share every day. And thank you, Kari, for all your untiring patience and encouragement while your man is working on yet another weighty tome.

March ȁǿȀȀ MōŤŕřŕŘŕōŚ SŏŔŘśşşŔōšőŞ

C O N T E N T S

Ļe Participants Ȁ Prologue ȀȈ Ȁ ⋅ First Encounters ȁȂ ȁ ⋅ Big Issues ȂȈ Ȃ ⋅ My Favorite Interpretation ȄȈ ȃ ⋅ Quantum States ȇȆ Ȅ ⋅ Randomness ȀǿȈ ȅ ⋅ Quantum Probabilities ȀȁȄ Ȇ ⋅ Ļe Measurement Problem ȀȃȀ ȇ ⋅ Bell’s Inequalities ȀȅȀ Ȉ ⋅ Quantum Information ȀȇȀ

Ȁǿ ⋅ Reconstructions ȀȈȈ

ȀȀ ⋅ Ļe Experiment of My Dreams ȁȀȄ Ȁȁ ⋅ Switching Sides ȁȁȆ ȀȂ ⋅ Beliefs and Values ȁȂȆ Ȁȃ ⋅ Ļe Role of Philosophy ȁȃȈ ȀȄ ⋅ Uniŀcation ȁȅȀ Ȁȅ ⋅ Ļe Next Big Bang ȁȆȂ ȀȆ ⋅ Dear Oracle ȁȇȄ Epilogue ȁȈȀ Glossary ȁȈȄ Index ȂǿȄ

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Ȁ

ȁ ŠŔő ŜōŞŠŕŏŕŜōŚŠş: œšŕŐś ŎōŏŏŕōœōŘšŜŜŕ

GšŕŐś BōŏŏŕōœōŘšŜŜŕ is a senior lecturer in philosophy at the University of Ab- erdeen, U.K. Born in Milan, Italy, in ȀȈȅȄ, he started out studying physics at ETH in Zurich, Switzerland, but switched over to mathematics and earned his diploma degree. He then did a master’s degree in history and philosophy of science at Cam- bridge (under Michael Redhead). He went on to get his Ph.D. in philosophy, also at Cambridge. His thesis, supervised by Jeremy Butterŀeld, examined modal interpre- tations of quantum mechanics. After stints as British Academy Postdoctoral Fellow at Balliol College, Oxford, and junior lecturer in philosophy of physics at Oxford, in ȁǿǿǿ Guido became assistant professor in philosophy at the University of Cali- fornia at Berkeley. He then worked as an Alexander von Humboldt Fellow at the University of Freiburg, Germany, and at IGPP, Freiburg. He was also a researcher at IHPST , Paris. Seeing that there was yet another continent to explore, in ȁǿǿȅ he voyaged to Sydney, Australia, to take up a position as senior research fellow at the Centre for Time. He returned to Europe in ȁǿǿȈ to settle in Aberdeen, where he now lives in a small part of an Aberdeenshire castle with his wife, Jennifer Bailey, his daughter, Katie, and two dogs, Yabby and Stella. He teaches various undergraduate and postgraduate courses at Aberdeen, mainly in philosophy of science. He is the coauthor, with Antony Valentini, of Quantum Ļeory at the Crossroads: Reconsidering the dzǻǴǹ Solvay Conference (Cambridge, ȁǿǿȈ). He is an associate member at IHPST, and a trustee and secretary at the Archive for Mathematical Sciences and Philosophy. Guido’s principal spare-time interest is music. At the tender age of two years, he exclaimed in his playpen, “Mahler ŀnished, put on Schubert!” Participants of a conference in Oxford will also forever remember Guido as the third slave in Ļe Magic Flute that his wife was conducting. He had also prepared the English translation of the opera (“Papageno, frisch hinauf: ende deinen Lebenslauf !” turned into “Papageno, up the tree: put an end to your CV!”). Sometime he will need to ŀnish writing that cello sonata.

ȃ ŠŔő ŜōŞŠŕŏŕŜōŚŠş: ŖőŒŒŞőť ŎšŎ

JőŒŒŞő ť BšŎ is a Distinguished Professor with the Department of Philosophy and the Institute for Physical Science and Technology at the University of Mary- land, College Park. Born in Cape Town, South Africa, in ȀȈȃȁ, he started out study- ing chemical and electrical engineering at the University of Cape Town, but then switched to pure science and majored in math and physics. He got his graduate ed- ucation from Birkbeck College, University of London, where in ȀȈȅȅ he earned a Ph.D. in mathematical physics. His dissertation, advised by David Bohm, was titled “Ļe Problem of Measurement in Quantum Mechanics.” Ironically, Jeff ’s ŀrst job was as a postdoc in the chemistry department at the Uni- versity of Minnesota, where he was working with Alden Mead, a physical chemist. Ļe chemistry department was just across the Mall from Ford Hall, where Herbert Feigl ran the Center for Philosophy of Science with Grover Maxwell, and Jeff was able to get a two-year postdoc with Feigl at the Center. His ŀrst teaching position was at Yale, initially a joint physics–philosophy appointment, then just in philoso- phy. From ȀȈȆȀ to ȀȈȇȅ Jeff held a position at the University of Western Ontario. He has been at the University of Maryland since ȀȈȇȅ, with stints as visiting professor at various places (Princeton in ȀȈȇȈ, Yale in ȀȈȈȂ, University of California–Irvine in ȀȈȈȃ, University of California–San Diego in ȀȈȈȈ). Jeff is the author of Ļe Inter- pretation of Quantum Mechanics (Reidel, ȀȈȆȃ) and Interpreting the Quantum World (Cambridge, ȀȈȈȆ; revised paperback edition, ȀȈȈȈ), which won the Lakatos Award in ȀȈȈȇ. In ȁǿǿȄ he was awarded the Kirwan Faculty Research and Scholarship Prize. Foundations of Physics recently published a Festschrift in Jeff ’s honor, and his work has been celebrated in the collection Physical Ļeory and Its Interpretation: Essays in Honor of Jeffrey Bub (Springer, ȁǿǿȅ, edited by W. Demopoulos and I. Pitowsky). He lives in Washington, DC , in a narrow, three-story Victorian with his wife, Robin. In the summer, they live in their house in Quinson, a small village in the Alpes d’Haute Provence, where they enjoy hiking the hills.

ŠŔő ŜōŞŠŕŏŕŜōŚŠş: ōŞŠŔšŞ ŒŕŚő Ȅ

AŞŠŔšŞ FŕŚő is a professor of philosophy and an adjunct professor of history and physics at the University of Washington, Seattle. Born in Lowell, Massachusetts, in ȀȈȂȆ, he earned a bachelor’s degree in mathematics from the University of Chicago, where he also studied physics, systems theory, and philosophy. Undecided about whether to pursue math or philosophy, he did both, starting with a master’s degree in mathematics from Illinois Institute of Technology. He then earned a Ph.D. in philosophy from the University of Chicago. His dissertation was on the quantum theory of measurement. In physics, he worked with Gregor Wenzel from the Fermi Institute, and in philosophy with Henry Mehlberg, who was his supervisor. He was a postdoc in history and philosophy of science at Cambridge University. He has taught at the University of Illinois, Cornell, Northwestern, and now at the University of Washington. He served as president of the Philosophy of Sci- ence Association and the American Philosophical Association. He has also been a Guggenheim Fellow, a fellow at the Center for Advanced Study in the Behavioral Sciences, and the recipient of a senior fellowship from the National Endowment for the Humanities. He teaches undergraduates and graduate students in logic and philosophy of science and supervises graduate students in physics and in history of science. He enjoys doing public education about science and has given lectures at the Adler Planetarium in Chicago and other outlets. He is the author of Ļe Shaky Game: Einstein, Realism, and the Quantum Ļeory (Chicago, ȀȈȇȅ; Japanese transla- tion, ȀȈȈȁ; second edition, ȀȈȈȅ) and coeditor, with James T. Cushing and Shelly Goldstein, of Bohmian Mechanics and Quantum Ļeory: An Appraisal (Kluwer, ȀȈȈȅ). He lives with his wife, Micky Forbes, in Port Townsend, a small town on a penin- sula across Puget Sound from Seattle. Ocean on three sides, and mountains to the east and west. Ļey have a cat, Qiao, a beautiful tabby. His son Dana, a mathemati- cian, and daughter Sharon, a physician, live on the East Coast. Arthur enjoys hiking, and he and Micky spend time in France (Paris or the Vaucluse) whenever possible.