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An Entangled Drama

David Mermin

THE AGE OF ENTANGLEMENT: When Quantum Physics Was Reborn. Louisa Gilder. xvi + 443 pp. Knopf, 2008. $27.50.

Quantum mechanics is notoriously strange. Starting with Max Planck in 1900, people who struggled to understand the behavior of matter at the atomic level realized that it defied commonsense understanding. Even after the creation of the quantum theory in its modern form in 1925, the strangeness remained. In recent years, physicists, philosophers and information scientists have focused a lot of attention on the peculiar quantum relationship between physical systems known as entanglement.

Albert%20Einstein%20and%20his%20friend%20Paul%20EhrenfestClick to Enlarge ImageAttention was drawn to entanglement by a 1935 critique by Albert Einstein, Boris Podolsky and Nathan Rosen. They pointed out that, according to the quantum theory, two spatially separated physical systems that had interacted in the past could be so strongly correlated that it would be possible to predict the result of measuring one or the other of two “complementary” properties of the first system from a prior measurement of one or the other of two corresponding properties of the second system.

What makes this peculiar is that the quantum theory refuses to assign joint values to both of two complementary properties, justifying the refusal by the fact that any measurement needed to reveal one of the hypothetical values must necessarily disturb the system enough to alter uncontrollably the other hypothetical value. But in the situation described in the Einstein-Podolsky-Rosen (EPR) paper, the measurement acts only on the second system, which no longer interacts with the first. Having thus undermined the rationale for the nonexistence of joint complementary properties, the authors conclude that quantum mechanics offers an incomplete description of physical reality.

Those few quantum physicists who took the time to look away from their amazing experiments and spectacular calculations were generally unimpressed by this metaphysical point. Wolfgang Pauli wrote to Werner Heisenberg that “Einstein has once again expressed himself publicly on quantum mechanics. . . . [T]his is a catastrophe every time it happens.” Niels Bohr wrote a paper that almost everybody agreed showed that the EPR paper was wrong, though opinions have differed ever since on what the mistake was that Bohr identified.

Erwin Schrödinger was one of the very few contemporaries immediately to see the power behind the EPR argument, which he expanded on in interesting ways, giving the name entanglement to the strong quantum correlations that the EPR paper had exploited. Nearly 30 years were to pass before John Bell proved clearly, explicitly and unambiguously that the quantum mechanical description of physical reality could not be completed in the manner advocated by Einstein, Podolsky and Rosen.

In The Age of Entanglement, Louisa Gilder has produced a highly entertaining story of what led up to these events and of what followed them. Her tale stretches from Einstein’s proposal of light quanta three decades before the EPR paper to a conference in Vienna on quantum foundations seven decades after. The book reads like a good novel, and I found it just as hard to put down.

Unlike historians of science, Gilder tells the story of entanglement primarily through the unusual device of imaginary conversations. As she explains in a prefatory note to the reader, she stitched most of these together from the verbatim content of letters, memoirs, reminiscences, biographies or, as the story gets closer to the present, her own interviews with contemporary physicists—all carefully documented in 57 pages of endnotes.

Gilder skillfully blends these disparate sources into entertaining, coherent exchanges, embellishing them with her own vivid imagination. For example: “[Heisenberg] took a bite of cheese and looked over at Wolfgang Pauli, who was lying in the grass like the dead. . . . ‘Pass the cheese,’ said Pauli, without moving.” Or: “Einstein’s brow furrowed, and he looked a little teacherly as his glasses slid down his nose.” Helped by a little willing suspension of disbelief on the part of the reader, the result is a surprisingly effective re-creation of some of the most subtle intellectual history of the 20th century.

I was at first a bit put off by this puppeteering of some of the great thinkers of the 20th century, until I realized that if it had been a film—even a “docudrama”—it would have bothered me less. Gilder may have invented a new literary form: the docunovel. Whatever you want to call it, the book is a grippingly readable “history” of science.

And it is not just a lively retelling of an old, familiar story. I was surprised by Gilder’s version of the celebrated exchange between Bohr and Einstein at the 1930 Solvay Conference over Einstein’s “photon box” experiment. Bohr always viewed this as Einstein’s final attack on Heisenberg’s uncertainty principle, which Bohr famously managed to refute, after many anxious hours, by invoking general relativity against its own creator. This is the prevailing view of the encounter and had always been mine.

Gilder, however, bases her narrative on Don Howard’s 1989 analysis of a 1931 letter to Bohr from Paul Ehrenfest. The letter explains that Einstein had no wish to attack the uncertainty principle. Instead, his point was to demonstrate that one or the other of two complementary properties of a distant photon emitted by the box could be learned, by making one or the other of two measurements on the now faraway box: the EPR idea, five years before the article. I looked up Howard’s paper (which turned out to be based on a lecture that I had attended and promptly forgotten) and now find it quite convincing.

Gilder’s dramatization of Bohr’s misunderstanding is a lot of fun. She shows Einstein trying to get through to Bohr, who is rushing off to the misconstrual that shapes the story to this day: “‘I grant you the consistency of the uncertainty principle,’ Einstein said, laughing a little. ‘I’m not done with the experiment, Bohr.’” Then, as Bohr persists in barking up the wrong tree: “‘Wait, Bohr,’ said Einstein. ‘The problem is not yet complete.’” And, still later: “‘Bohr! Let me finish.’” Fiction, to be sure, but wonderful to imagine, and with a grain of truth at its center.

I have two reservations. Although Gilder’s title makes it clear that this is a history of entanglement, and not of the broader field of quantum physics, readers who don’t know much about the subject are likely to get the impression (reinforced by her subtitle) that entanglement is the pot of gold at the end of that rainbow first dimly perceived by Planck (1900), Einstein (1905) and Bohr (1913)—a pot of gold that Einstein, Podolsky and Rosen (1935), David Bohm (1951), John Bell (1964) and Alain Aspect (1981) finally revealed for all to see.

But entangled states (not so named) played a central role in the theory of the chemical bond in 1927, less than two years after the appearance of quantum mechanics. And although Gilder portrays Bohr as oblivious to the charms of entanglement, I read Bohr’s prompt rejoinder to the EPR paper as saying in part (although not in that language) that a measured system is entangled with the measurement apparatus in just the EPR manner. So in Bohr’s view, there is really nothing very new here.

Entanglement (but without the hoopla) has been a central part of quantum mechanics right from its beginning, and there is far more to quantum mechanics than entanglement. The story Gilder tells so charmingly is a Whig history of 20th-century physics, told from the perspective of the 21st-century quantum information community.

My second criticism is not a debatable matter of emphasis. When Gilder gets down to specific points of physics (and to her credit as an expositor it is not often necessary for her to do so), she sometimes gets them wrong. There can be harmless slips, as when she contrasts momentum—“just one number”—with position, which is specified by three numbers. (Momentum also has three components.)

But there can also be serious obstacles to understanding.* A two-page small-print appendix sets forth a version (based on an article by your reviewer, who is otherwise accurately portrayed as a minor character in the late-20th-century story of entanglement) of a wonderful form of Bell’s theorem that Daniel Greenberger, Michael Horne and Anton Zeilinger discovered in the late 1980s. The argument considers six hypothetical properties associated with two complementary choices for what to measure on each of three different systems.

Unfortunately Gilder uses only three symbols to represent the values of these six properties, failing to distinguish, for each of the three systems, between the two values associated with the two complementary choices. This renders unintelligible her otherwise excellent explanation of why all six properties cannot have joint values, and it will frustrate the efforts of any reader ambitious enough to try to follow her exposition.

I hope there will be a second edition in which these and some other mistakes are repaired, because Gilder is a fine storyteller who brings to life one of the great scientific adventures of our time.

*Erratum: The next-to-last paragraph of this review states incorrectly that author Louisa Gilder fails to distinguish between two sets of symbols. She does in fact make a distinction, explicitly calling the reader’s attention to her use of italics for one set of capital letters and roman type for the other. April 2, 2009

N. David Mermin is Horace White Professor of Physics Emeritus at Cornell University. He is the author of, among other books, Quantum Computer Science: An Introduction (Cambridge University Press, 2007) and It’s About Time: Understanding Einstein’s Relativity (Princeton University Press, 2005).

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