To say that quantum theory is about describing how atoms behave would be like saying that all Hemingway ever did was show us how to write terse prose. Quantum theory, more than any other physical theory, seems to rub against what we have traditionally come to see as the mission of science: namely, to provide a tangible description of an objectively existing external reality. But rather than telling us what exists, quantum theory talks only about measurements and observation--and not even about what we will observe, but only about the probabilities of observing this or that result.
Many people, Einstein included, have felt that something must be missing from this picture--that a satisfactory, complete physical theory ought to be more than an instrument for computing probabilities of something so observer-focused as measurement outcomes. Much of the persistent and heated debate about the meaning of quantum theory has centered on this issue. Over the course of decades, people have responded to Einstein's challenge in radically different ways. Personally, I've always found it intriguing how a theory can be so concisely formulated and inexhaustibly successful while fitting pretty much any worldview, from deep-seated realism to full-blown positivism. Perhaps this observation contains a lesson in itself.
Last year, I interviewed a bunch of physicists, philosophers, and mathematicians--many of whom are FQXi members--about the mysteries of quantum theory. I put the same set of questions to each of my interviewees, who are some of the most original thinkers working on quantum theory today. The answers, collected in my new book Elegance and Enigma: The Quantum Interviews, turned out to be marvels of bold thought and irresistible wit. They are deeply personal, providing rare glimpses into what motivates a group of scholars, all working off the same theory, to seek out drastically different approaches to the theory's interpretation.
My first question asked how my interviewees became enamored with quantum theory. (Go to the end of this article to see the full list of questions.) It's a question close to my heart, because I wouldn't be a physicist today hadn't it been for a chance encounter, in my last two years of high school, with Heisenberg's and Schrödinger's philosophical writings about quantum theory. Many of my interviewees told similar stories of decisive events: an eye-opening seminar they attended, or a book they had been given or picked up, or a radio broadcast they had heard (sometimes still as teenagers). Many had accepted, without giving it much thought, the standard presentation of quantum theory, only to be suddenly plunged into a sense of acute discomfort by something they happened to hear or read. They have never been the same since.
The first half of my interview questions focused on the core foundational problems of quantum theory. What is the best interpretation of the theory? How are we to understand the concept of measurement? What is the meaning of probabilities? Does quantum theory imply that nature is indeterministic? The second half of the questions looked at the bigger picture. What experiments may bring decisive progress to our understanding of quantum theory? What input may come from philosophy and from the search for a unified theory? How important are personal beliefs and values? What does the future hold?
The interview answers were a stark reminder of how little consensus has been reached in the century since quantum theory's birth. They testified to a persistent disagreement about what the central problems are, how to address them, and about how much or little we ought to worry.
Take the infamous "measurement problem" as an example. It has its roots in an apparent clash between two ways in which measurement may appear in quantum theory. First, measurement is introduced axiomatically, as a primitive notion: quantum theory gives us a recipe for computing probabilities of measurement results, but without in turn reducing the act of measurement to an explicit account of the physical going-ons inside the measurement apparatus, like we would expect in classical physics. On the other hand, nothing prevents us from using the quantum formalism to describe these going-ons in the same way we describe the going-ons in any other physical system. But in such a description, the apparatus ends up in a strangely suspended state without any definitive measurement result.
So the measurement problem amounts to several different possible concerns. Should we regard the axiomatic notion of measurement as inadequate and instead seek a deeper explanation of the measurement process? Should we worry about the indefinite apparatus state? Is there an inconsistency between this state and how measurement-as-axiom operates?
The interviews not only showed that everybody has a different opinion on how to answer these questions and whether the measurement problem is, as I put it in my interview question, a "serious roadblock or dissolvable pseudo-issue." They also showed that these opinions were strongly correlated with interpretive attitudes toward the quantum formalism as a whole. Those, such as Christopher Fuchs, a researcher at Perimeter Institute in Waterloo, Canada, and David Mermin, a professor emeritus of physics at Cornell University, who view quantum theory a man-made tool to help us structure and predict our experiences, tended to dismiss the measurement problem. Those, such as GianCarlo Ghirardi, a professor emeritus of physics at the University of Trieste, Italy, and Tim Maudlin, a philosopher at New York University, who believe a satisfactory physical theory ought to provide an observer-independent account of physical reality, were more likely to view the measurement problem as a real difficulty for quantum theory, calling for urgent remedy.
As far as interpretations of quantum theory are concerned, pretty much every possible interpretive flavor was represented among my interviewees. And some people were self-proclaimed agnostics. Lucien Hardy, a physicist at Perimeter Institute, was particularly blunt: "I do not believe any of the currently available interpretive programs." And some interviewees didn't think my question made sense to begin with. "The question is completely backward," Fuchs retorted. "It acts as if there is this thing called quantum mechanics, displayed and available for everyone to see as they walk by it--kind of like a lump of something on a sidewalk. The job of interpretation is to find the right spray to cover up any offending smells." Jeff Bub, a philosopher at the University of Maryland, College Park, had related concerns. "The program of interpreting quantum mechanics tends to treat the theory like a problem child in the family of theories and propose therapy," he said. "The aim is to get quantum mechanics to conform to some ideal of classical comprehensibility. If this is what it means to 'make the best sense of quantum mechanics,' then I think the exercise is misguided."
Over the past two decades or so, we have witnessed what has been called the "second quantum revolution." One development is quantum information theory. It has given us a completely new view on quantum theory as a theory phrased in terms of the processing and communication of information in physical systems. Generations of physicists raised on Heisenberg's uncertainty principle came away with the impression that quantum mechanics is about imposing all kinds of limits on what we can do in this world--like how we can't simultaneously determine the position and momentum of a particle with full accuracy. Quantum information theory, if nothing else, has turned the tables by showing that in a world governed by quantum mechanics, we can do lots of things we can't do in a classical world, like have completely secure communication or solve certain computational problems faster than any classical algorithm could ever do.
The question, of course, is whether quantum information theory has done anything to alleviate conceptual concerns about quantum theory. For Bub, "thinking about quantum mechanics from an information-theoretic standpoint has radically transformed the field of quantum foundations." Those who see the task of physics as formulating theories that give an account of what exists tended to be more critical. "The notion that quantum information theory or quantum computational theory could contribute to the foundational questions has always puzzled me," said Maudlin. "I have no concept of how one could turn the usual project on its head and derive or explain physics from information theory." Whatever view one takes, for Tony Leggett, a Nobel Prize-winning physicist at the University of Illinois at Urbana-Champaign, quantum information theory is having a practical, political benefit: "It is now rather widely accepted that an active interest in the foundations of quantum mechanics does not disqualify one from being a 'proper' physicist."
What might be next major development in the foundations of quantum mechanics? Some interviewees thought it will be the experimental demonstration that, as Leggett put it, "quantum mechanics is not the whole truth about the physical world"--in other words, that we will find a deeper, more general theory, with quantum mechanics simply reduced to an approximation. Daniel Greenberger, at City College of the CUNY, however, isn't so sure of the prospects. "I think looking for the order in the universe is a noble enterprise, and I like to be part of it, but I am highly skeptical of the outcome," he said. "Finding the 'theory of everything' is a pretty tall order for creatures who understand almost nothing."
So, now that I have seen all the answers--all three hundred pages of them--what are my overall observations and conclusions about the state of quantum theory? Too many things to mention come to mind, and anyway I wouldn't want to bias your own reading. But one observation has been robust and is worth mentioning. What the interview answers suggest is that what's happening today is not so much one interpretation fighting another, but rather a sharp contrast, in mindset and approach, between two camps, each encompassing a group of interpretations. The first camp wants to exorcise the observer from the theory and embed quantum theory into a realist interpretive framework with an explicit ontology (that is, with an explicit account of what *is*). The second camp looks at the quantum formalism as a tool for representing an observer's knowledge, an attitude that in many cases goes along with a desire to understand why we have this formalism to begin with and what particular features of nature make it so successful.
I closed the interviews by asking my interviewees what single question about the foundations of quantum mechanics they would want to put to an omniscient being. But not everyone took the bait, and some gave the question a new spin. "There are no omniscient beings," Fuchs said. "I believe this is one of the greatest lessons of quantum theory. For there to be an omniscient being, the world would have to be written from beginning to end like a completed book. But if there is no such thing as the universe in any completed and waiting-to-be-discovered sense, then there is no completed book to be read, no omniscient being." Greenberger didn't quite warm up to my question either. "Would you really want to live in a universe that was so simple that you could understand it, even if God himself tried to explain it to you?"
Caslav Brukner, a physicist at the University of Vienna, was even more curt. "Who cares about the foundations of quantum mechanics when offered an exclusive opportunity for posing a single question to an omniscient being?"
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You can check out free samples of the book here, and order a copy here.
THE PARTICIPANTS
Guido Bacciagaluppi, Caslav Brukner, Jeffrey Bub, Arthur Fine, Christopher Fuchs, GianCarlo Ghirardi, Shelly Goldstein, Daniel Greenberger, Lucien Hardy, Anthony Leggett, Tim Maudlin, David Mermin, Lee Smolin, Antony Valentini, David Wallace, Anton Zeilinger, and Wojciech Zurek.
THE QUESTIONS
1. What first stimulated your interest in the foundations of quantum mechanics?
2. What are the most pressing problems in the foundations of quantum mechanics today?
3. What interpretive program can make the best sense of quantum mechanics, and why?
4. What are quantum states?
5. Does quantum mechanics imply irreducible randomness in nature?
6. Quantum probabilities: subjective or objective?
7. The quantum measurement problem: serious roadblock or dissolvable pseudo-issue?
8. What do the experimentally observed violations of Bell's inequalities tell us about nature?
9. What contributions to the foundations of quantum mechanics have, or will, come from quantum information theory? What notion of information could serve as a rigorous basis for progress in foundations?
10. How can the foundations of quantum mechanics benefit from approaches that reconstruct quantum mechanics from fundamental principles? Can reconstruction reduce the need for interpretation?
11. If you could choose one experiment, regardless of its current technical feasibility, to help answer a foundational question, which one would it be?
12. If you have a preferred interpretation of quantum mechanics, what would it take to make you switch sides?
13. How do personal beliefs and values influence one's choice of interpretation?
14. What is the role of philosophy in advancing our understanding of the foundations of quantum mechanics?
15. What new input and perspectives for the foundations of quantum mechanics may come from the interplay between quantum theory and gravity/relativity, and from the search for a unified theory?
16. Where would you put your money when it comes to predicting the next major development in the foundations of quantum mechanics?
17. What single question about the foundations of quantum mechanics would you put to an omniscient being?