What is reality?

As a child, Anton Zeilinger used to pull the heads and limbs off his sister's dolls. "I liked taking things apart," he explained in an interview with the Institute of Physics (see video below). This childhood tendency grew up into scientific curiosity; Zeilinger is now a well-respected physicist, the head of a quantum optics group at the Institut fur Quantenoptik and Quanteninformation in Vienna.

We generally believe that Zeilinger can satisfy his scientific curiosity through experiments, either by doing his own or by learning of the results of others' work. When we ask nature a question, she will answer truthfully, as long as we ask the question honestly and know how to interpret her response. Our observations, then, can create a picture of reality, mapping the facts of the world "out there" in one-to-one correspondence with ideas in the mind "in here."




In a talk at the recent Quantum to Cosmos Festival at the Perimeter Institute in Waterloo, Ontario, Zeilinger raised a question that might seem, at first glance, naïve.

"What do we really describe in physics?" he asked. "Do we describe reality? Is it out there?"

Classical physicists would have said yes, resoundingly. Studying physics reveals nature's workings, providing an explicit map of reality's subtleties. Those subtleties would be there whether we figured out how to question and extract them.

But in the early part of this century, quantum mechanics put that happy belief on the chopping block. Quantum mechanics, for all its ability to describe the atomic and subatomic world, blurs the distinction between the observer and the observed. As a result, it calls into question the essence of scientific curiosity and inquiry.

According to quantum mechanics, an electron's position is a smattering of possibilities. It's likely to be found, perhaps, within a certain boundary, and less likely to be found outside it. When we ask the electron where it is, this smattering will collapse into a definite value.

But what about the electron before we observe it? What is the reality of the electron? Einstein believed that the electron must know where it is. And Heisenberg's uncertainty principle only makes this little gedankenexperiment more preposterous for classicists: once we know the electron's position, we can know nothing about its momentum.

Einstein believed that this was evidence that quantum mechanics was in some way incomplete. His formal argument is known as EPR, for his collaborators, Podolsky and Rosen. Joshua Roebke writes in an article in SEED on Zeilinger's work:

The EPR paper begins by asserting that there’s a real world outside theories… EPR argued that objects must have preexisting values for measurable quantities and that this implied that certain elements of reality could not be determined by quantum mechanics.

Quantum entanglement is one particular case that wreaks havoc on Einstein. In his talk at Quantum to Cosmos, Zeilinger explained the effect:

If you have a pair of dice that are quantum entangled—you can't buy them yet but I'm sure in a hundred years you can buy them as a Christmas present—a pair of quantum dice would be such that if you throw one die here and one die there they always show the same number. Now this can only be if they have a common cause, or if they are talking to each other somehow.

Zeilinger studies entanglement with photons; in lieu of the face of a die, the photon's polarization is the property in question. Separate two entangled photons by a galaxy, then have an observer measure the polarization of each. They will see the same polarization.

In 1997, Zeilinger demonstrated this effect in the lab; it was hailed as "teleportation," with information (the photon's polarization) being beamed instantly from one particle to another. In 2007, he demonstrated in spectacularly across two of the Canary Islands. It's as if one photon, the moment it was measured, sent an instantaneous, light-speed-limit-breaking signal to the other, telling it what polarization to have; accepting this non-locality would allow physicists like Einstein to hold onto realism, the idea that the photons must have a determined polarization before they are measured.

A physicist named Anthony Legget formulated this possibility into a testable theory, which he brought to colleagues who brought it to Zeilinger's lab. To Legget's chagrin (quantum entanglement upsets him nearly as much as it upsets Einstein), fastidious experiments proved that this wasn't the case (for further reading, see the full text of "Reality Tests"):

It took [Zeilinger and his colleagues] 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.

"What this tells us also in a deeper way is that there are situations where what we observe in experiment is not some reality which was there before," Zeilinger explained in his talk at Quantum to Cosmos. "Our experiment creates reality in a sense. What is then reality, really? What are we describing now with physical theories?"

In his talk, Zeilinger suggests that physics sorely needs new ideas that can comprehend these facts. "I mean, quantum mechanics is a hundred years old. Relativity is a hudnred years old. A new breakthrough is due," he said.
For Zeilinger, that new idea that makes everything clear might be the unification of these two big theories, something physicists have been grappling with for several decades now. Or it might involve another sort of unification altogether.

"Maybe we have to unify the idea of reality and information, which is my own personal theory," he said.