Skip to main content

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.


  1. Surely "describe" is more leading than the founding fathers of QM had in mind. "What do we do when we create a model?" is closer, because creating a model leaves the possibility that what we have created is sui generis, with a relationship with our experience that is more like the proverbial relationship between a map and the territory. Anti-realist classical physicists of the 19th and early 20th Century used this way of talking as well as talking of descriptions.

    Speaking of "models" does not discard realism about Physical Theories, although it demotes realism from its central position in neo-modernist attitudes to Physics. Speaking of models is equally attuned to instrumentalist ideals, and accepts that the model may be *of* something, but, unless we insist that there is one fundamental type of Lego with which all fundamental models must be made, admits the possibility of many useful types of model.

    I take issue with your exclusive choice of "particles" as modeling material. Models constructed using fields are often a better way to approach Quantum Physics, with talking about the mathematical details of quantum field states generally much better than talking about "photons". In contrast, classical modeling is too often constrained to be in terms of classical particles or classical fields, when random fields are a more appropriately sophisticated level of mathematical models to compare with quantum fields.

    Pace Zeilinger, since he only reflects the constant beating of the straw man of classical particles by everyone else. The commonly cited paper by Bell, "The theory of local beables", which is generally supposed to rule out all field theories, has neither been tested adequately by experiment nor as theory.

    Simple-mindedly, in a random field model, it is only in the presence of a thermodynamically sophisticated macroscopic or mesoscopic experimental apparatus, which makes discrete thermodynamic transitions from a ready state to an excited state, that we observe anything of "whatever there may be" between classically separated parts of an experimental apparatus, so there are no statistics of construed properties, unless we observe and record discrete events that we have engineered. More technically, random field models are naturally contextual.

    My apologies that this is too long and too partial an account to be taken seriously. With apologies for what I hope is only gentle sarcasm, best wishes in your attempts, and almost everyone else's, to explain QM in terms of its relationships with properties of classical particles.

  2. Physics is above all a description of experience.

    It seems we cannot "abstract out" the process of experience from our description of the physical world, which is an abstraction from our experience.

  3. In the ERP paradox, how can we say we "know" the position of the "other" particle, when we cannot observe it at the same time as the first particle?

    If you know something that you cannot observe (are there any observable consequences that can confirm the "knowledge"), what does knowing mean?

    What if there are two observers, one for each particle, and one observes position and the other momentum? It would seem that for uncertainty to be preserved, there would have to be uncertainty of the time of the observations, so that it would be impossible to say they were simultaneous. Or else?

    I am probably missing something fairly obvious. Maybe someone could try to explain the paradox again, please? For the layperson??

  4. Hi, Second Anonymous. The "something fairly obvious" that you're missing may be in your first paragraph. The standard way of telling whether we are seeing two particles that are related is to say that if we observe two events at the same time, then they are related, and they are caused by a "particle pair". If we observe only one event, that may have been caused by half of a particle pair, but we didn't observe the other half, or it may have been a stray single particle, which makes the analysis significantly more delicate.

    To set up an EPR-Bell situation, which has been implemented many times but is admittedly distinct from the EPR experiment, which to my knowledge has not been implemented, one typically uses FOUR detectors, two at each end of a long baseline, then says that one has detected a "particle pair" if one observes two events at nearly the same time, at different ends of the long baseline. Setting the width of the window is largely pragmatic; careful reporting of an EPR-Bell experiment includes information about how wide that window is and about how the statistics of "simultaneous" events change as the window is changed.

    Better, however, I suggest, to talk of the detailed statistics of observed events, and take the statistics to be modeled by random fields (or quantum fields if we must), since it is only the statistics that we can experimentally confirm (in the slightly dubious manner of confirmation that shows that observed statistics are within some confidence level window of predicted probabilities), instead of talking about individual events as caused by individual or entangled particles.

    Hi, first anonymous, "Physics is above all a description of experience"? I suppose so, but the heat is generated when one asks what makes a "better" description. What then is the advantage of higher theoretical descriptions, in contrast to more directly phenomenological descriptions? Why is unification a merit if it's mostly about description? I think your second paragraph needs more cashing out; to place it in the literature, it seems similar to accepting that the relationship between experience and description is pragmatic, which can be found in the literature in many different formulations, as "bridge principles" (Lakatos), "operational definitions" (Bridgman), "rules of correspondence"(Margenau), to name a few that I've noted recently, which in more folksy language is just to note, that the map is not the territory, and cannot be, but can be very useful if we have learned the correspondences. "Maps" and "territories" is not a really adequate analogy for the higher mathematics of Physics, but it captures the possibility of useful similarity but ultimately different.

  5. Let's go for unification ! Even if we should not be to much eager.


Post a Comment

Popular Posts

How 4,000 Physicists Gave a Vegas Casino its Worst Week Ever

What happens when several thousand distinguished physicists, researchers, and students descend on the nation’s gambling capital for a conference? The answer is "a bad week for the casino"—but you'd never guess why.

Ask a Physicist: Phone Flash Sharpie Shock!

Lexie and Xavier, from Orlando, FL want to know:
"What's going on in this video? Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!"

The Science of Ice Cream: Part One

Even though it's been a warm couple of months already, it's officially summer. A delicious, science-filled way to beat the heat? Making homemade ice cream.

(We've since updated this article to include the science behind vegan ice cream. To learn more about ice cream science, check out The Science of Ice Cream, Redux)

Over at Physics@Home there's an easy recipe for homemade ice cream. But what kind of milk should you use to make ice cream? And do you really need to chill the ice cream base before making it? Why do ice cream recipes always call for salt on ice?