[scroll down for the American Journal of Physics Review by Marcus Appleby]

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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.

American Journal of Physics, Vol. 77, No. 10, October 2009

The Age of Entanglement: When Quantum Physics was Reborn. Louisa Gilder.

Marcus Appleby, Reviewer.

To the world-at-large, Einstein’s name is indelibly associated with the theory of relativity. But to Einstein himself relativity was only “a sort of respite which I gave myself during my struggles with the quanta” (p. 54). It was a struggle he lost.

Shortly before his death, almost half a century after his 1905 photoelectric paper, he resignedly wrote “All these fifty years of conscious brooding have brought me no nearer to the question, ‘What are light quanta?’ Nowadays every Tom, Dick, and Harry thinks he knows it, but he is mistaken” (p. 328).

One way or another, the question is still with us. Of course now, as then, there is no shortage of Toms, Dicks, and Harrys who will confidently tell you the answer. But since they say different things, the skeptic is entitled to suspect that no one really knows. And that is the subject of Gilder’s book: The struggle to make sense of quantum mechanics.

Her subject is much broader than her title might suggest. When I picked up the book, I assumed it would be about the revolution in quantum information theory of the past 2 or 3 decades. But, in fact, in a book of over 400 pages, she devotes a mere 44 pages to recent developments, and even in that small space there is remarkably little about the practical details of quantum computation and cryptography.

Instead, her focus is on the ideas: Specifically the ideas over which Einstein, Bohr, and Schrödinger fought their battles and which continued to obsess Bohm, Bell, and a handful of others during the long years when an interest in such questions was liable to do serious damage to one’s career.

Experiments are not ignored. For example, she gives a long and fascinating “behind-the-scenes” account of early attempts to test the Bell inequalities. However, experiments are only of interest to her to the extent they illustrate or are otherwise directly related to her central theme: The quantum enigma, as it might be called. The same is true of her attitude to quantum information. Her interest in quantum computation and cryptography stems largely, if not entirely, from the fact that they show, contrary to what was long assumed, that the work of Einstein, Podolsky, Rosen, and Bell has important practical implications.

She tells the story extremely well. Her account is packed with a richness and depth of historical detail that is most unusual in a book intended for the general reader. I learned quite a few things I did not know before. But even when I did know what was coming next, the book continued to hold my interest in the way that an old and familiar story always can be found interesting if the telling of it is sufficiently good.

What is most distinctive about the book is the way she tries to tell the story from the inside, as it were. As she remarks, the impression physics textbooks give to students is of a “perfect sculpture sitting in a vacuum-sealed case, as if brains, only tenuously connected to bodies, had given birth to insights fully formed.” Gilder seeks to correct that impression. In her words:

As Tom Wolfe wrote at the beginning of The Electric Kool-Aid Acid Test: ‘I have tried not only to tell what the Pranksters did but to re-create the mental atmosphere or subjective reality of it. I don’t think their adventure can be understood without that.’ Wolfe was recounting a very different kind of mental history, but his point, I find, is even more true about the portentous history of science and intellect that unfolded as the age of entanglement.

I had some reservations about this at first, particularly her use of imaginary conversations between the main protagonists, which I feared would turn out to be nothing more than a cheap gimmick. But, in fact, it works very well. She does an excellent job of conveying a sense of the way in which physics actually progresses.

Gilder’s evocation of the subjective reality of physical thinking should go some way toward dispelling a serious and, I believe, dangerous fallacy. Based on what is, admittedly, some very restricted and unsystematic sampling, I have the impression that it is widely believed among the general public that physics is the production of (at best) a bunch of geeks or (at worst) a bunch of robots who have failed their Turing test. Try to tell them that physics at the highest level does, in its way, demand as much passion and imagination and creativity as music or literature and you are likely to be met with a look of blank incomprehension. I think this is, as I said, dangerous because in a democracy it is the general public who ultimately call the shots, and if physics is not strongly rooted in the wider culture it will, I fear, eventually wither and die. Of course, there is no shortage of popular science writers who will tell their readers that science is wonderful and exciting.

But telling people is one thing, making them really feel it is quite another. Gilder does the latter.

I believe her book could also be read with profit by all the politicians and government officials who, in Britain at least, seem to imagine that ideas can be squeezed out of creative thinkers by applying the same managerial techniques that are used in a juice-bottling factory.

It might make them realize that (as Einstein put it) “the state of mind which enables a man to do work of this kind is akin to that of the religious worshiper or the lover; the daily effort comes from no deliberate intention or program, but straight from the heart.”1 The activity is spontaneous, even anarchic. Try to bureaucratically control it and you will end by destroying it.

Last but not least I think her book can be read with profit by physicists. I believe that students would find it inspiring, and I think that even the old hands should find it enjoyable and even educative (as I have done).

I should perhaps conclude with a couple of cautionary notes. I said, and I think it is true, that students would find the book inspiring. But they should be warned that it is not a textbook on the interpretation of quantum mechanics. For example, she devotes 50 pages to Bohm but barely mentions other significant figures, such as Wheeler or Everett. I think that is fair enough given her focus on entanglement and the Bell inequalities, and given her wish to evoke the subjective inwardness of the process of scientific discovery by giving in-depth portraits of a few individuals. However it is important that a student who reads the book should not be under the misapprehension that it says everything important there is to say about the interpretation of quantum mechanics. Students should also be warned that the author is a little prone to careless mistakes. For instance, she says (or at least implies) that when, in the Bohr model of the hydrogen atom, an electron makes a jump, the frequency of the emitted photon is the mean of the frequencies of the initial and final orbits (p. 34). Students should be aware that, if they are puzzled by something she says, the fault may lie with her rather than with them.

1 Address by Albert Einstein on Max Planck’s 60th birthday (1918), online at <www.cs.ucla.edu/~slu/on_research/einstein_essay2.html>.

D. M. Appleby is a Senior Visiting Fellow at Queen Mary, University of London, and a visiting researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario. He has published widely on quantum information and the foundations of quantum mechanics.