Creating our own reality

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.

[ ... ]

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.

~ From: The Reality Tests ~