N EWS F OCUS

Quantum Physics

fundamental issues without being embar- rassed,” Gohlike says. Nonetheless, scholars Elite Retreat Takes the Measure must brace themselves for dizzying discus- sion and some spirited ribbing when they Of a Weirdly Ordinary World gather in the cool quiet of the library. “Would you call that wave-function col- At a remote lodge in the rural Midwest, physicists, philosophers, and historians lapse?” asks one physicist as he explains how confront the stubborn mysteries of quantum mechanics he and colleagues apply quantum theory to the state of the entire universe at once. STILLWATER,MINNESOTA—Two men sit at a cists and philosophers since the theory was “You don’t call that wave-function col- long table, oblivious to the breakfast-time invented in the 1920s. The theory explains lapse,” says a historian. commotion. One moves a coffee cup from phenomena ranging from chemical bond- “Then we don’t have wave-function col- one side of a water glass to the other. “If I ing to the structure of neutron stars, and it lapse.” look here and don’t see the cup,” he says to may open the way to uncrackable quantum “They probably do,” another physicist the other, “then I know it must be there.” communications and superefficient quan- chimes in. It sounds like a “deep” exchange be- tum computers. Yet the strange quantum “He said they didn’t,” says a philosopher, tween swotty young philosophy majors. But rules clash with our most basic experience gesturing toward the historian. the fellow moving the cup has gray hair— of reality. In fact, no one knows how our “I know, but I think he’s in error about and a Nobel Prize in physics. Sliding the boring old “classical world,” in which a what he would say.” porcelain, Anthony Leggett of the University thing can be in only one place at a time, The scholars are debating a deceptively of Illinois, Urbana-Champaign, explains arises from the weirdness of quantum theo- simple question: What does it mean to make how scientists might try to see past the stric- ry, says Gerard Milburn, a physicist at the a measurement? tures of quantum mechanics, the bizarre the- University of Queensland in St. Lucia, According to quantum mechanics, a ory that governs the behavior of tiny objects Australia: “The fundamental question re- tiny object does not follow a continuous tra- and clashes with our everyday notions of re- mains, Why do we have classical behavior jectory. Instead, it must be described by os- ality. “None of the existing interpretations of in a quantum world?” cillating quantum waves that reveal only the quantum mechanics as a theory of the entire probability of finding it here or there. An world is satisfactory,” Leggett says to John Fast cars and philosophy object such as an electron may also be in Preskill, a physicist at the California Insti- Once home to the county poor two different places at once or spin in oppo- tute of Technology in Pasadena. farm, the two-story Out- site directions at the same time, a condition Leggett, Preskill, and 22 other physicists, ing Lodge at Pine Point known as a superposition. But when meas- philosophers, and historians have gathered reflects the eclectic uring an electron, an observer always finds here for the Seven Pines Symposium.* character of owner and it spinning one way or the other. Packed into a wonderfully rustic lodge in a symposium patron Even stranger, two separat- wooded state park, the scholars—all of them Lee Gohlike. None ed electrons can be men—will share their insights, suites of of the rooms have “entangled” so that rooms without telephones, and meals of roast Internet access, both spin both ways quail and pheasant at a long communal table. but one provides at once, but in oppo- Perhaps not since the famous Solvay Confer- a manual type- site directions from ences of the early 20th century, at which writer in good each other. If a meas- Niels Bohr and Albert Einstein debated the repair. In urement shows that one meaning of quantum theory in their free another, electron is spinning clock- time, has physics seemed so genteel. six por- wise, the observer instantly Each year, the symposium tackles anoth- traits of knows that the other is spin- er of physics’ enduring puzzles: the nature of Marilyn ning counterclockwise, even the vacuum, the concept of a field, the Monroe hang if it is a world away. meaning of time. The aim is not to resolve in two neat ranks. All this suggests that meas- the mysteries but to seed new lines of in- On the shelves of the ex- urement makes the quantum quiry, says Roger Stuewer, a historian at the pansive second-floor library, the “wave function” collapse into one University of Minnesota, Twin Cities, and Proceedings of the Royal Society of possible outcome or another. Yet chair of the symposium’s advisory board. London and The Philosophical Magazine that view—the so-called Copen- “When we get the best people together,” he mingle with Oscar Wilde’s comedies, Ein- hagen interpretation of quantum mechanics says, “ideas are planted that in some intangi- stein’s letters, and a book entitled The Mys- —has troubled physicists for nearly 80 ble way will influence what they do in the teries of Pittsburgh. years. It assumes that between the bizarre future.” Those germinating ideas are watered An amateur philosopher of science, realm of tiny objects and the commonsense with a mellow Zinfandel and nourished with Gohlike made his money restoring classic world of everyday things lies a fuzzy border. succulent steak. Mercedes-Benz racing cars. He befriended Measurement and wave-function collapse This year, attendees spent 5 days grap- historian Stuewer 20 years ago and several somehow bridge the divide. But why, re- pling with the implications of quantum years later proposed—and offered to pay searchers wonder, should measurement have mechanics, which have perplexed physi- for—the symposium. The invitation-only af- such unique power? The Copenhagen inter- fair provides “a kind of demilitarized zone pretation also tacks the classical world onto * The Seven Pines Symposium VIII, 5 to 9 May. where people are allowed to get into more the quantum realm like a sundeck on a sub-

1896 25 JUNE 2004 VOL 304 SCIENCE www.sciencemag.org N EWS F OCUS marine, without explaining why it even ex- there may be a hitch, he says. Contemplat- ists. If quantum mechanics governs every- ing the period just after the big bang, quan- thing, then why don’t voltmeters and people tum cosmologists routinely talk about the behave as oddly as atoms and photons? wave function of the entire universe. “If the wave function of the universe is only a Information rules characterization of somebody’s information, Like theologians reinterpreting scripture, then whose is it?” he asks. physicists and philosophers have With such questions unanswered sought to resolve their quandaries by and dinnertime approaching, the reading mind-bending new meanings scholars opt for a stopgap solution: into the rules of quantum mechanics. They head for the bar. Some favor a “many worlds” interpreta- tion, in which the wave function never More craziness ahead? collapses but merely splits into branches Yet, with martinis in hand, many at each potential measurement, leading to continue to talk quantum mechanics. multiple realities of which we experience Such is the pull of the riddles of only one. Other prefer so-called modal in- quantum theory—especially now, as terpretations that rely on undetectable “par- new experiments may allow physi- tial hidden variables” to ensure, for example, cists to probe the limits of the theory. that a voltmeter never reads 0 volts and 5 Driven in part by the need to volts at the same time. make reliable “qubits”—bits that can Whatever their philosophical leanings, be 0, 1, or 0 and 1 at the same time— physicists now generally agree that quan- for quantum computers, researchers tum mechanics itself makes it extremely are striving to put ever larger objects difficult to spot really big things in super- and systems into quantum super- positions. Large objects quickly entangle positions. In 2000 two teams independ- themselves with their surroundings, and ently coaxed billions of electrons to flow such “decoherence” obscures a super- simultaneously in opposite directions position in somewhat the same way that around tiny rings of a superconductor interference from a vacuum cleaner scram- (Science, 31 March 2000, p. 2395). Two bles the image on a television screen. years ago another team proposed a Strictly speaking, however, decoherence scheme to place a tiny mirror in two leaves superpositions intact, says historian places at once (Science, 11 October 2002, Michel Janssen of the University of Min- Mens sana. Fortified by country air, gourmet p. 342). Eventually, says the University of nesota. And even if they’re hidden, he says, meals, and an occasional nip, scholars at the Illinois’s Leggett, such experiments might re- two-places-at-once coffee cups defy our Seven Pines Symposium debate why physics veal that laws and forces beyond quantum sense of a single definite reality and leave us seems so strange—and daily reality doesn’t. mechanics intervene to prevent truly macro- mired in “an ontology of possibilia and an scopic objects from being in superpositions epistemology of radical deceit.” chanics is like grasping at minnows in a —a notion known as macrorealism. To avoid that dreary fate, participants at stream, however, and information is a par- The trick is to thoroughly isolate a siz- this year’s Seven Pines Symposium are tak- ticularly slippery concept. “What’s it mean able thing and push back the curtain of de- ing a different tack entirely. Spurred by re- to say that the universe is just informa- coherence far enough to put the object in cent efforts to develop quantum information tion?” says Philip Stamp, a physicist at the two places at once—or fail in a way that re- technologies, thinkers such as Jeffrey Bub, a University of British Columbia in Vancou- veals a new phenomenon. A coffee cup will philosopher at the University of Maryland, ver. “To me information has to be informa- likely always remain too large for such ex- College Park, hope to prove that quantum tion about something.” periments, but researchers are marching to- mechanics grows naturally out of the princi- Physicist Christopher Fuchs of Lucent ward the macro realm. “Ideas that weren’t ples of information theory. Technologies in Murray Hill, New Jersey, taken seriously 25 years ago are taken quite Last year Bub and colleagues showed has an answer: The wave function is an ob- seriously now,” Leggett says. that three restrictions on how information server’s information about the objective Even if some new theory lurks just over can be transmitted and copied can reproduce world. A measurement instantly refines that the quantum horizon, don’t expect it to re- from scratch the basic features of quantum information, which is all in the observer’s store sanity and comprehensibility to mechanics. They assume that the world can head, giving the illusion that the wave func- physics. Any subsequent theory will still be described by a type of mathematics tion collapses. In Fuchs’s view, the instanta- have to explain all the bizarre phenomena known as a C* (pronounced C-star) algebra. neous collapse is no more mysterious than that quantum mechanics already does, says Then they lay down the laws: no instanta- the clarity that suddenly emerges when you Robert Wald, a theoretical physicist at the neous transmission of information, no per- remember that you left your checkbook be- University of Chicago, so it, too, will be a fect copying of information, and no sharing side the toaster. “Quantum measurement is strange beast. “What’s going to come next is of information in a particular way called the refinement of expectations, period,” going to be just as crazy or crazier,” he says. “unconditional bit commitment.” Those re- Fuchs says. “The only mystery in quantum “You can count on that.” strictions produce a mathematical formalism mechanics is why there is a restriction” on Perhaps that’s a good thing. A theory that looks like quantum mechanics, Bub the completeness of knowledge. more comprehensible than quantum me- says. The result suggests that the basic stuff James Hartle, a physicist at the Univer- chanics wouldn’t require such a sympo- of the universe is information, he says. sity of California, Santa Barbara, says he sium. And that would spoil the fun.

CREDITS: PETERSON PAUL Trying to comprehend quantum me- likes the spirit of Fuchs’s approach. But –ADRIAN CHO

www.sciencemag.org SCIENCE VOL 304 25 JUNE 2004 1897