2009年3月27日 星期五

Sons of Atom

Sons of Atom

Published: March 26, 2009

The first quarter of the 20th century produced two theories, relativity and quantum mechanics, that are still changing our universe.

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Illustration by Jason Logan


When Quantum Physics Was Reborn

By Louisa Gilder

Illustrated. 443 pp. Alfred A. Knopf. $27.50

With special relativity, Albert Einstein upended the long-understood meaning of time, space and simultaneity. With general relativity, he swapped Newton’s law of gravity based on force for curved space­time, and cosmology became a science. Just after World War I, relativity made front-page news when astronomers saw the Sun bend starlight. Overnight, Einstein became famous as no physical scientist before or since, his theory the subject of poetry, painting and architecture.

Then, with the development of quantum mechanics in the 1920s, physics got ­really interesting. Quantum physics was a theory so powerful — and so powerfully weird — that nearly a century later, we’re still arguing about how to reconcile it with Einsteinian relativity and debating what it tells us about causality, locality and realism.

Relativity leads to a world far from every­day intuition. But relativity was still classical physics: classical in the sense that it was as causal, maybe even more so, as the physics of Newton. The relativist could defend the view that we could refine our local specification of the state of things now — that we could spell out what every last particle was up to — and then predict the future, as accurately as wanted. Back in the Enlightenment, Pierre-Simon de Laplace imagined a machine that could calculate the future. He didn’t know relativity, of course, but you could imagine a Laplace 2.0 (with relativity) that kept his predictive dream alive.

Quantum mechanics shattered that Laplacian vision. From 1925 to 1927, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Max Born and many others made the theory into a toolkit that could be used to calculate how copper conducted electricity, how nuclei fissioned, how transistors worked. Quantum mechanics was easy to use, but hard to understand. For example, two particles that interacted might subsequently fly to opposite sides of the solar system, and still act as if they were dependent. Measuring one near Pluto affected measurements as the other zipped by Mercury. Einstein viewed this inseparability, now known as “entanglement,” as the fatal mark of the incompleteness of quantum mechanics: he sought a successor theory that would be local, realist and therefore complete.

Looking back on the early 20th century, Bohr wistfully reflected that Einstein had done so much of relativity theory by himself, while quantum mechanics took a whole generation of physicists 30 years. Telling the quantum story up to 1927 has been an industry for the past 80 years. In the first half of her new book, “The Age of Entanglement,” Louisa Gilder does her level best to cope with this plethora of sources, characters and topics, with mixed results. She writes engagingly, using dialogue reconstructed from letters, papers and memoirs to capture the spirit of confrontation among the players. That’s good. But she seems ill at ease with the German sources and so is reliant on the secondary literature — some of which is well done, some not. That’s not so good.

But on Page 181, the clouds part and Gilder reveals a sparkling, original book. Leaving Copenhagen, Berlin and Göt­tingen behind, she recounts a history of the quantum physics that did not end in 1927. With a smaller, more contemporary cast of characters from Berkeley, Innsbruck, Harvard and CERN, the big accelerator outside Geneva, Gilder brings the reader into a mix of ideas and personalities handled with a verve reminiscent of Jeremy Bernstein’s scientific portraits in The New Yorker.

This second-half book begins with the story of David Bohm, a student of J. Robert Oppenheimer who dissented from political orthodoxy and paid for it with his career. Hauled before the House Un-American Activities Committee in 1949, he refused their bullying questions, lost his job, and fled to Brazil and then to Israel. Often ill, the isolated Bohm railed against the orthodox interpretation of quantum physics as well, and agitated for a theory he hoped would replace it. He had the sympathy of Einstein and Richard Feynman but somehow always orbited outside the action — his work, Wolfgang Pauli once said, like an uncashed check. Gilder movingly portrays Bohm’s lonely trajectory. Even Einstein turned away from Bohm in 1952, calling his approach “too cheap,” while Max Born later wrote back that Pauli had come up with a new idea that “slays Bohm not only philosophically but physically.”

Next comes the real center of her story, John Bell, a remarkable Irish theorist at CERN. Like Bohm, Bell resisted the too easy slide into orthodoxy that had made one interpretation of quantum physics into a canon law from which even questioning was greeted suspiciously. For several years, Bell worked through Bohm’s studies, isolating what was so troubling about quanta. Better yet, he derived predictions.

Bell’s theorem, stated in a 1964 paper: You cannot have a theory consistent with his experimental predictions of quantum mechanics and have that theory describe the world in a completely local way. To put it differently, we may be troubled by various aspects of quantum physics and hope it can be replaced by some other theory that will capture its predictions but go deeper, giving a local, un-entangled account. But Bell showed that if a certain measurable inequality was confirmed experimentally, it would follow that any successor theory to quantum physics you tried to write would itself exhibit one of the strangest features of quantum theory: it will still be non-local.

Bell’s prediction bore on correlations in properties between particles that had once been entangled — even if the particles flew far apart. Suddenly interpretations of quantum mechanics opened into something else: a laboratory test to demonstrate that local hidden variable theories could not exist. Experimentalists, not theorists, now had the floor, and Gilder beautifully evokes their world: equipment catalogs instead of books; piles of dry ice; messy clockwork; boiling metal. Gilder captures the vaulting ambition of this recent generation in joining engineering with the foundations of quantum theory — no easy task. Alongside the successes, she shows the frustration of contradictory results, the worries about whether these results reflected reality — or were just a stupid machine bug.

Some experimentalists wanted quantum mechanics to succeed. Others hoped it would crash and burn. These experiments seemed all at once to be playing for the highest stakes possible and yet might just confirm again what almost every physicist already accepted. Would the experiments kill the greatest theory, or wreck careers not yet begun?

Quantum physics survived Bell’s test. But in all the testing in those years since the mid-1960s, the nature — the weirdness — of quantum mechanics gained a clarity and force it had never had, even in the hands of Einstein and Bohr. Entanglement was here to stay: Bell’s inequality, powered by experiment, said so. What’s more, the oddness of entanglement makes a new kind of computing imaginable. Odd as it might seem, these foundational ideas of quantum mechanics have led governments, industries and militaries to explore how the entangled state of separated particles might accelerate computing to a staggering degree: instead of taking, say, a million steps to crack a secret password, the still-nascent quantum computer promises a solution in the square-root number of steps — in this case, a mere thousand steps.

What had been for generations a story of theoretical malcontents now intrigues spooks and start-ups. All this radiates from Louisa Gilder’s story. Quantum physics lives.

Peter Galison is a professor of the history of science at Harvard and the author of “Einstein’s Clocks, Poincaré’s Maps.” His film “Secrecy,” made with Robb Moss, had its premiere at the 2009 Sundance Film Festival.