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Anton Zeilinger heads up the IQOQI lab in Vienna. Photograph by Mark Mahaney.
To enter the somewhat formidable Neo-Renaissance building at Boltzmanngasse 3 in Vienna, you must pass through a small door sawed from the original cathedrallike entrance. When I first visited this past March, it was chilly and overcast in the late afternoon. Atop several tall stories of scaffolding there were two men who would hardly have been visible from the street were it not for their sunrise-orange jumpsuits. As I was about to pass through the nested entrance, I heard a sudden rush of wind and felt a mist of winter drizzle. I glanced up. The veiled workers were power-washing away the building’s façade, down to the century-old brick underneath.
In 1908 Karl Kupelwieser, Ludwig Wittgenstein’s uncle, donated the money to construct this building and turn Austria- Hungary into the principal destination for the study of radium. Above the doorway the edifice still bears the name of this founding purpose. But since 2005 this has been home of the Institut für Quantenoptik und Quanteninformation (IQOQI, pronounced “ee-ko-kee”), a center devoted to the foundations of quantum mechanics. The IQOQI, which includes a sister facility to the southwest in the valley town of Innsbruck, was initially realized in 2003 at the behest of the Austrian Academy of Sciences. However, the institute’s conception several years earlier was predominantly due to one man: Anton Zeilinger. This past January, Zeilinger became the first ever recipient of the Isaac Newton Medal for his pioneering contributions to physics as the head of one of the most successful quantum optics groups in the world. Over the past two decades, he and his colleagues have done as much as anyone else to test quantum mechanics. And since its inception more than 80 years ago, quantum mechanics has possibly weathered more scrutiny than any theory ever devised. Quantum mechanics appears correct, and now Zeilinger and his group have started experimenting with what the theory means.
Some physicists still find quantum mechanics unpalatable, if not unbelievable, because of what it implies about the world beyond our senses. The theory’s mathematics is simple enough to be taught to undergraduates, but the physical implications of that mathematics give rise to deep philosophical questions that remain unresolved. Quantum mechanics fundamentally concerns the way in which we observers connect to the universe we observe. The theory implies that when we measure particles and atoms, at least one of two long-held physical principles is untenable: Distant events do not affect one other, and properties we wish to observe exist before our measurements. One of these, locality or realism, must be fundamentally incorrect.
For more than 70 years, innumerable physicists have tried to disentangle the meaning of quantum mechanics through debate. Now Zeilinger and his collaborators have performed a series of experiments that, while neatly agreeing with the theory’s predictions, are reinvigorating these historical dialogues. In Vienna experiments are testing whether quantum mechanics permits a fundamental physical reality. A new way of understanding an already powerful theory is beginning to take shape, one that could change the way we understand the world around us. Do we create what we observe through the act of our observations?
Most of us would agree that there exists a world outside our minds. At the classical level of our perceptions, this belief is almost certainly correct. If your couch is blue, you will observe it as such whether drunk, in high spirits, or depressed; the color is surely independent of the majority of your mental states. If you discovered your couch were suddenly red, you could be sure there was a cause. The classical world is real, and not only in your head. Solipsism hasn’t really been a viable philosophical doctrine for decades, if not centuries.
But none of us perceives the world as it exists fundamentally. We do not observe the tiniest bits of matter, nor the forces that move them, individually through our senses. We evolved to experience the world in bulk, our faculties registering the net effect of trillions upon trillions of particles or atoms moving in concert. We are crude measurers. So divorced are we from the activity beneath our experience that physicists became relatively assured of the existence of atoms only about a century ago.
Physicists attribute a fundamental reality to what they do not directly perceive. Particles and atoms have observable effects that are well described by theories like quantum mechanics. Single atoms have been “seen” in measurements and presumably exist whether or not we observe them individually. The properties that define particles—mass, spin, etc.—are also thought to exist before we measure them. In physics this is how reality is defined; particles and atoms have measurable properties that exist prior to measurement. This is nothing stranger than your blue couch.
As a physical example, light consists of particles known as photons that each have a property called polarization. Measuring polarization is usually something like telling time; the property can be thought of like the direction of a second hand on a clock. For unpolarized light, the second hand can face any direction as with a normal clock; for polarized light the hand will face in only one or a few directions, as if the clock were broken. That photons can be polarized is, in fact, what allows some sunglasses to eliminate glare—the glasses block certain polarizations and let others through. In Vienna the polarization of light is also being used to test reality.
For a few months in 2006, Simon Gröblacher, who had started his PhD not long before, spent his Saturdays testing realism. Time in the labs at the IQOQI is precious, and during the week other experiments with priority were already underway. Zeilinger and the rest of their collaborators weren’t too worried that this kind of experiment would get scooped. They were content to let Gröblacher test reality in the lab’s spare time.
It was after 2 pm when I first met Gröblacher, and he had just woken up; they are installing an elevator in his lab and so he works nights. He had told me to come to the top floor of the IQOQI building to find him. I made my way up the broad granite steps, and on the final landing I heard shouts from a half-open door. There was a raucous game of foosball in the lounge. When Gröblacher saw me, someone else grabbed the handles.
The lab where Gröblacher performed the first experiment on realism is on the second floor of the Universität Wien physics department, which connects to the IQOQI through a third-floor bridge. The original experiment has given way to another, but, Gröblacher tells me, the setup looks roughly the same.
In the middle of the cramped space is a floating metal surface, about the size of a banquet table, latticed with drill holes. A forest of black optical equipment, like monocles atop tiny poles, seems to grow out of the table. Beam splitters resemble exact, glass die. In the center is an encased crystal that is not visible, and on the ends sit idle lasers.
Gröblacher walked me through the tabletop obstacle course: The laser light passes through a series of polarizers and filters, hits the crystal, and splits into two beams of single-file photons. Detectors in both beams measure the polarization of each photon, which are related to one another. The data is tested against two theories: one that preserved realism but allowed strange effects from anywhere out there in the universe, and quantum mechanics.
The whole experiment would fit snugly in a child’s bedroom, and as I looked at the table, I refrained from asking my first instinctual questions. “This is it? This is where you tested realism?” I already knew how unfair these questions were. It had taken a few months of tests, and almost two years for Zeilinger’s group to understand how this experiment tests realism. Before that, it had been more than 80 years since physicists began to argue about what quantum mechanics had to do with reality at all.
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