Quantum theory is the glory and the scandal of modern physics. Its glory comes from the unprecedented number of phenomena explained and predictions confirmed under its aegis; its scandal is that to this day, just how we are to understand the vexed concepts of measurement and observation is obscure. In both its relativistic and nonrelativistic guises, the theory makes extraordinary claims about the world—about superconductors and Higgs particles, about transistor mechanisms and cosmological fluctuations—and yet it is often described as just providing a set of rules for experimental practice.
Adam Becker’s remarkable new book, What Is Real? The Unfinished Quest for the Meaning of Quantum Physics, takes on the puzzles of quantum mechanics with a historical approach that is at once accessible and scholarly. He begins with the founding fathers of quantum theory by revisiting the familiar tale of Niels Bohr versus Albert Einstein. In traditional tellings, Einstein stubbornly failed to understand the wisdom of Bohr’s Copenhagen interpretation; in revisionist versions, Bohr’s mesmerized disciples missed Einstein’s obvious insights. Becker engages deeply with the historical record and provides a convincing and nuanced account that largely vindicates Einstein without simplistically dismissing Bohr.
In the core of the book, Becker considers how questions about understanding and interpreting quantum theory—what we today call “quantum foundations”—developed in the period 1945–80. The story is mostly told through three figures: David Bohm, Hugh Everett, and John Bell. As Becker shows, all three physicists asked serious questions about the meaning of quantum theory and as a result faced both intellectual opposition and, disturbingly often, unprofessional pressure. Bohm was hounded by accusations of un-Americanism that were inextricably entwined with his quantum heresy. Everett was forced to rewrite his thesis on the many-worlds interpretation to preserve his adviser John Wheeler’s cordial relationship with Bohr. Bell’s ideas on nonlocality remained in obscurity for years, and Bell himself was shielded from professional repercussions only by his brilliance in more conventional particle-physics topics.
All in all, Becker makes a powerful case for three central claims. First, he argues that the Copenhagen interpretation is not a coherent viewpoint but a mixture of confused and contradictory instructions for making sense of quantum theory bound together by historical circumstance and political expedience. Second, he says that John von Neumann’s no-hidden-variables proof was used by people who didn’t understand it as an excuse to ignore viable alternatives to conventional quantum theory. Finally, Becker shows that to question the conceptual foundations of quantum theory was to risk your career. I was shocked to read that as late as the 1980s, Dieter Zeh’s work on decoherence theory had serious consequences for him and disastrous ones for his students.
Fortunately, things have changed. As Becker acknowledges, decoherence is now the philosophically orthodox interpretation of the transition from the classical to the quantum scale; it replaces the obscure Copenhagen recipe with a dynamical account of when the quantum can be effectively treated as classical. But Becker argues that decoherence only explains quantum mechanics in conjunction with Everett’s many-worlds idea, and that most physicists who use decoherence downplay or ignore the importance of the many-worlds theory.
I think he’s right in both cases. But Becker’s discussion of contemporary physics creates tension between his encouraging historical narrative and the more pessimistic lessons he wishes to draw. If physics has moved from an incoherent understanding of measurement to a coherent understanding whose full consequences are not recognized, that is enormous progress. Becker concludes, on the other hand, that we face a philosophical crisis in which the physics community still falls back on Copenhagen’s confusion while alternatives—Everett’s many worlds, Bohm’s pilot wave, theories of dynamical collapse—remain underappreciated and underexamined.
Becker’s conclusion hides real differences between those three approaches. Advocates of many worlds, like myself, are fairly content. Decoherence vindicates Everett, and physicists now discuss measurement in a way that makes sense. It’s a shame if other physicists don’t recognize the many-worlds consequences of their practice, but happily Everett’s interpretation is steadily gaining popularity. Advocates of dynamical collapse look to experiment: Their theories have the great virtue of testability, and the experimental state of the art gets closer each day to delivering triumph or rejection. Supporters of Bohm’s theory continue to despair at physics practice and complain, with reason, as misinterpretations of Bell’s theorem are used to reject their approach out of hand. But it is hard to see pilot-wave theory becoming mainstream as long as it fails to reproduce relativistic physics, a concern that Becker mentions but perhaps could have further explored.
Nonetheless, What Is Real? is a superb contribution both to popular understanding of quantum theory and to ongoing debates among experts. In one of many arresting anecdotes in the book, Becker discusses a line attributed to Bohr: Truth is complementary to clarity. That is, to achieve one is to forsake the other. In at least this respect, Becker convincingly refutes Bohr’s notion of complementarity—his book reconciles clarity and accuracy. It deserves wide attention and careful study.
David Wallace studied physics at Oxford University before moving into philosophy of physics. He is now a professor of philosophy at the University of Southern California. His research interests include foundations of statistical mechanics, quantum gravity, symmetry, and the quantum measurement problem—in particular the Everett interpretation.
