www.sciencemag.org SCIENCE VOL 325 17 JULY 2009 269

BOOKS ET AL.

Entanglement with a Twist
PHYSICS

Jonathan P. Dowling

When I picked up The Age of Entanglement, the first thing to catch my eye was a quote on the back dustjacket, “for a moment I almost thought I understood quantum mechanics.” I thought, “Oh boy, this could be trouble.” Recollections danced in my noggin of uncomfortable conversations on crowded airplanes that begin with “Oh, you are a quantum physicist?—Then you must have seen What the Bleep Do We Know!” But proceeding through the book, my fear was never realized. I instead found a witty, charming, and accurate account of the history of that bugaboo of physics—quantum entanglement.

When I was a graduate student in physics, I made the decision to spurn a respectable career in high-energy physics theory (if nowadays one can consider superstring theory to be respectable) and embraced a future in the foundations of quantum mechanics. As Louisa Gilder repeatedly points out, in the mid-1980s such a career move was considered the kiss of death. At that time a respected professor pointedly told me, “This foundations of quantum mechanics is crackpot stuff—you will never get at job.” My, how times have changed.

There are many books out there on the history or foundations of quantum mechanics. Some are more technical, others more historical, but none take the unique approach that Gilder has—to focus on the quantum weirdness of entanglement itself as her book’s unifying theme and to present it in an inviting and accessible way. The Age of Entanglement offers neither a technical nor a biographical account. Rather, as Gilder states up front, it provides a collection of reconstructed conversations among some of the 20th century’s greatest physicists. These conversations all revolve around the notion of quantum entanglement: the spooky, action-at-a-distance effect predicted by quantum theory but only slowly recognized as the theory’s defining feature and even more slowly shown to be experimentally verifiable.

Your opinion of the book will largely hinge on how you react to these reconstructed conversations. Concerning one such imagined conversation between Albert Einstein and Niels Bohr on a streetcar in Copenhagen, Gilder notes, “We know that the conversation…happened, because Bohr mentioned it in an interview….The content of the conversation is easy to gather from a look at what the three men were working on…around the same time.” Rather than provide dry quotations from original sources, Gilder decided to weave information from these sources into a series of imagined conversations. The author offers extensive documentation for these conversations in the notes, so they are not flights of fancy. Her technique leads to text such as, “ ‘If, however’—and here [Einstein] looked straight at Heisenberg, who was leaning forward in his chair, his pale hair shining in the dim room—‘as is obviously the case in modern atomic physics….’ ”

I suppose neither Einstein, Werner Heisenberg, nor anyone else recorded that Heisenberg’s pale hair was shining in the dim room, but it makes for a good story. For this protocol to work for me, I had to first execute Coleridge’s “willing suspension of disbelief” and then engage Tolkien’s “secondary belief.” That done, I was enthralled and found the book delightful.

Gilder skillfully relates the early discomfort physicists felt concerning some of the arcane predictions of quantum mechanics; how Einstein, Erwin Schrödinger, and others repeatedly distilled and titrated their misgiving until they were able in the 1930s to present the essence of their fears in the form of the Einstein-Rosen-Podolsky paradox; Schrödinger’s cat; and the now famous notion of quantum entanglement—spooky action-at-a-distance that quantum theory must contain. Much in these older “discussions” was familiar to me from other sources. What I found most gratifying were the studiously documented dialogs of later developments: Bell’s inequalities and the consequent experiments, which proved that nature is stranger than we can think. The details of the story of David Bohm and his trials, after constructing a nonlocal hidden variable theory, were new to me. The account of John Clauser and his cohorts in the race to demonstrate (by testing Bell’s theory) once and for all that this quantum weirdness did or did not exist was side-splitting. An old friend and collaborator, Clauser does in fact curse like a sailor, as Gilder often has him do. (He is a sailor, and I wonder whether the cursing or the sailing came first.) I was spellbound by the details of the struggles of Clauser and colleagues with the massive, punch-tape spewing, “coffin” contraption clanking away, day after day, in the bowels of Berkeley. It is tragic that this apparatus now lies in mothballs in the attic of LeConte Hall instead of on display at the Smithsonian.

Gilder wraps up The Age of Entanglement with conversations among younger quantum technologists such as Artur Ekert, Nicolas Gisin, Daniel Greenberger, Michael Horne, Terrence Rudolph, and Anton Zeilinger. As I read these pages, I wondered if I should feel slighted—there is no mention of me. Then I happened upon this description of a colleague and friend: “Meanwhile in the Sangre de Cristo Mountains of New Mexico, Paul Kwiat, an endearingly birdlike man in glasses and suspenders with boundless energy and encyclopedic knowledge, led his team in attempting various eavesdropping strategies on their Alice and Bob.” Thank goodness for small favors, I thought, smiling to myself.

The Age of Entanglement: When Quantum Physics Was Reborn
by Louisa Gilder
Alfred A. Knopf, New York,
2008. 462 pp. $27.50, C$32.
ISBN 9781400044177.

The reviewer is at the Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803–4001, USA.