The Reality Tests

Feature / by Joshua Roebke /

A team of physicists in Vienna has devised experiments that may answer one of the enduring riddles of science: Do we create the world just by looking at it?

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Leggett doesn’t believe quantum mechanics is correct, and there are few places for a person of such disbelief to now turn. But Leggett decided to find out what believing in quantum mechanics might require. He worked out what would happen if one took the idea of nonlocality in quantum mechanics seriously, by allowing for just about any possible outside influences on a detector set to register polarizations of light. Any unknown event might change what is measured. The only assumption Leggett made was that a natural form of realism hold true; photons should have measurable polarizations that exist before they are measured. With this he laboriously derived a new set of hidden variables theorems and inequalities as Bell once had. But whereas Bell’s work could not distinguish between realism and locality, Leggett’s did. The two could be tested.

When Aspelmeyer returned to Vienna, he grabbed the nearest theorist he could find, Tomasz Paterek, whom everyone calls “Tomek.” Tomek was at the IQOQI on fellowship from his native Poland and together, they enlisted Simon Gröblacher, Aspelmeyer’s student. With Leggett’s assistance, the three spent six months painfully checking his calculations. They even found a small error. Then they set about recasting the idea, with a few of the other resident theorists, into a form they could test. When they were done, they went to visit Anton Zeilinger. The experiment wouldn’t be too difficult, but understanding it would. It took them months to reach their tentative conclusion: If quantum mechanics described the data, then the lights’ polarizations didn’t exist before being measured. Realism in quantum mechanics would be untenable.

Anton Zeilinger stands in front of the door to his office. To his left is a glass cabinet that holds the numerous medals he has won for tests of quantum mechanics. Photograph by Mark Mahaney.

On my final morning in vienna, snow was tumbling like dryer sheets as I stared out the window of the IQOQI waiting to speak again with Zeilinger. Suddenly, there was a great flash of lightning and a long roll of thunder as snow continued to fall. I turned around to no one and Zeilinger’s assistant appeared. He now had time to talk.

Though less robust and more intimidating, Zeilinger bears a slight resemblance to the American Kris Kringle. Born in 1945, he is tall and stout with a beard and white mane of hair. He wears tailored jackets, though insists he is a hands-on kind of guy.

As a student in Vienna in the 1960s, Zeilinger never attended a single course in quantum mechanics, which may help to explain the way he has investigated it since—with the zeal of a late convert. In the past decade or so, Zeilinger and his many collaborators were the first to teleport light, use quantum cryptography for a bank transaction (with optical fibers in the sewers of Vienna), realize a one-way quantum computer, and achieve entanglement over large distances through the air, first across the Danube River and then between two of the Canary Islands. Zeilinger’s work had also previously shown the greatest distinction between quantum mechanics and local realism.

Zeilinger’s office is large and sparsely decorated. A few books lean on a lengthy, glass-fronted bookshelf. As he spoke, Zeilinger reclined in a black chair, and I leaned forward on a red couch. “Quantum mechanics is very fundamental, probably even more fundamental than we appreciate,” he said, “But to give up on realism altogether is certainly wrong. Going back to Einstein, to give up realism about the moon, that’s ridiculous. But on the quantum level we do have to give up realism.”

With eerie precision, the results of Gröblacher’s weekend experiments had followed the curve predicted by quantum mechanics. The data defied the predictions of Leggett’s model by three orders of magnitude. Though they could never observe it, the polarizations truly did not exist before being measured. For so fundamental a result, Zeilinger and his group needed to test quantum mechanics again. In a room atop the IQOQI building, another PhD student, Alessandro Fedrizzi, recreated the experiment using a laser found in a Blu-ray disk player.

Leggett’s theory was more powerful than Bell’s because it required that light’s polarization be measured not just like the second hand on a clock face, but over an entire sphere. In essence, there were an infinite number of clock faces on which the second hand could point. For the experimenters this meant that they had to account for an infinite number of possible measurement settings. So Zeilinger’s group rederived Leggett’s theory for a finite number of measurements. There were certain directions the polarization would more likely face in quantum mechanics. This test was more stringent. In mid-2007 Fedrizzi found that the new realism model was violated by 80 orders of magnitude; the group was even more assured that quantum mechanics was correct.

Leggett agrees with Zeilinger that realism is wrong in quantum mechanics, but when I asked him whether he now believes in the theory, he answered only “no” before demurring, “I’m in a small minority with that point of view and I wouldn’t stake my life on it.” For Leggett there are still enough loopholes to disbelieve. I asked him what could finally change his mind about quantum mechanics. Without hesitation, he said sending humans into space as detectors to test the theory. In space there is enough distance to exclude communication between the detectors (humans), and the lack of other particles should allow most entangled photons to reach the detectors unimpeded. Plus, each person can decide independently which photon polarizations to measure. If Leggett’s model were contradicted in space, he might believe. When I mentioned this to Prof. Zeilinger he said, “That will happen someday. There is no doubt in my mind. It is just a question of technology.” Alessandro Fedrizzi had already shown me a prototype of a realism experiment he is hoping to send up in a satellite. It’s a heavy, metallic slab the size of a dinner plate.

Brucker stands between two other theorists: Johannes Kofler (left) and Tomasz Paterek. Photograph by Mark Mahaney.

On markus aspelmeyer’s desk there are three tall empty boxes of Veuve Clicquot. Experimentalists at the IQOQI receive champagne for exceptional results, and on one of the boxes are written congratulations for Markus’s initiation of the realism test. ˇCaslav Brukner, who helped with the theory, keeps a squat box of Chinese plum wine on his desk facing Markus’s. When I asked about the wine, thinking it the theorists’ complementary tradition, he laughed and said he just needed a counterbalance. Brukner has an easy manner and has been with Zeilinger’s group almost continuously since arriving in Austria in 1991 after leaving then Yugoslavia.

Last year Brukner and his student Johannes Kofler decided to figure out why we do not perceive the quantum phenomena around us. If quantum mechanics holds universally for atoms, why do we not see directly its effects in bulk?

Most physicists believe that quantum effects get washed out when there are a large number of particles around. The particles are in constant interaction and their environment serves to “decohere” the quantum world—eliminate superpositions—to create the classical one we observe. Quantum mechanics has within it its own demise, and the process is too rapid to ever see. Zeilinger’s group, which has tested decoherence, does not believe there is a fundamental limit on the size of an object to observe superposition. Superpositions should exist even for objects we see, similar to the infamous example of Schrödinger’s cat. In fact, Gröblacher now spends his nights testing larger-scale quantum mechanics in which a small mirror is humanely substituted for a cat.

Brukner and Kofler had a simple idea. They wanted to find out what would happen if they assumed that a reality similar to the one we experience is true—every large object has only one value for each measurable property that does not change. In other words, you know your couch is blue, and you don’t expect to be able to alter it just by looking. This form of realism, “macrorealism,” was first posited by Leggett in the 1980s.

Late last year Brukner and Kofler showed that it does not matter how many particles are around, or how large an object is, quantum mechanics always holds true. The reason we see our world as we do is because of what we use to observe it. The human body is a just barely adequate measuring device. Quantum mechanics does not always wash itself out, but to observe its effects for larger and larger objects we would need more and more accurate measurement devices. We just do not have the sensitivity to observe the quantum effects around us. In essence we do create the classical world we perceive, and as Brukner said, “There could be other classical worlds completely different from ours.”

Zeilinger and his group have only just begun to consider the grand implications of all their work for reality and our world. Like others in their field, they had focused on entanglement and decoherence to construct our future information technology, such as quantum computers, and not for understanding reality. But the group’s work on these kinds of applications pushed up against quantum mechanics’ foundations. To repeat a famous dictum, “All information is physical.” How we get information from our world depends on how it is encoded. Quantum mechanics encodes information, and how we obtain this through measurement is how we study and construct our world.

I asked Dr. Zeilinger about this as I was about to leave his office. “In the history of physics, we have learned that there are distinctions that we really should not make, such as between space and time… It could very well be that the distinction we make between information and reality is wrong. This is not saying that everything is just information. But it is saying that we need a new concept that encompasses or includes both.” Zeilinger smiled as he finished: “I throw this out as a challenge to our philosophy friends.”

A few weeks later I was looking around on the IQOQI website when I noticed a job posting for a one-year fellowship at the institute. They were looking for a philosopher to collaborate with the group.

Originally published June 4, 2008

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