Even weirder quantum weirdness

It’s not often that one gets the chance to learn, or say, something new about reality itself. So when I came across an article entitled “An experimental test of non-local realism” (Nature, 19 April 2007, p 871), I couldn’t resist looking into it.

I don’t like giving away the conclusion before telling the whole story, but when talking about such basic matters, it’s best to be clear. What the authors, seven quantum physicists in Austria and Poland led by Anton Zeilinger, conclude from theory and experiment is that “any non-local extension of quantum theory has to be highly counterintuitive.”

By that understatement they mean that some of the most fundamental assumptions scientists and lay people make about reality are almost certainly wrong. At risk are such fundamentals as Aristotelian logic, including for example the idea that a proposition cannot be simultaneously true and false; or that the universe is deterministic; or that the present cannot change the past.

“We believe,” the authors write, “that our results lend strong support to the view that any future extension of quantum theory that is agreement with experiments must abandon certain features of realistic descriptions.”

It might be helpful to define some terms. By “realism” the authors mean a point of view that assumes that something exists, that events occur even if not observed, and that the outcomes of observations depend on pre-existing properties of objects, properties that themselves are independent of the observation.

“Local realism” is where Einstein famously dug in his heels. It assumes that events in “space-like separated regions” cannot influence each other—what he disparaged as “spooky” action at a distance. The current authors point out that a long series of increasingly refined experiments dating back to 1972 have definitively proven Einstein wrong on this. Local realism now rests alongside other quaint and outdated scientific ideas such as phlogiston and the aether. Physicists do believe in spooks, at least of this sort.

Still, some aspects of realism might be salvaged through a set of theories based on “non-local hidden variables.” Those theories (which, to give the game away again, the current authors have now tested and found to be wrong) make three assumptions:

--Realism: measurement outcomes are determined by pre-existing properties of particles.

--Hidden variables: the physical states of particles are statistical mixtures of sub-ensembles which themselves have definite properties (for example, polarization).

--Averaging: taken together, the sub-ensembles obey physical laws (in the case of polarization, Malus’ law, which states that the intensity of a polarized beam that has passed through a polarized filter depends on the cosine of the angle between the polarization of the beam and the filter).

The core idea here is that particles such as a pair of entangled photons speeding away from each other are composed of hidden parts which, taken together, carry properties, such as polarization, that show up if and when they are measured at a particular point in space and time.

The authors point out that theories based on these assumptions have been proposed to explain those “spooky” correlations between spatially separated but entangled, particles. These theories have been successful in that predictions based on them have matched the results of all relevant entanglement experiments prior to those the authors undertook.

So what did these authors do? They tested the class of non-local hidden variable theories by measuring the polarization of entangled photons. However, unlike previous experiments, their detectors did not lie in the same plane with respect to the photon source. This novel geometry was needed to test an inequality first derived by Anthony Leggett in 2003. The inequality is based on a very simple property of integers, that, when applied to the quantized properties of entangled particles, allows a thumbs-up, thumbs-down test of non-local realism.

It took some careful tweaking to minimize the noise in their measurements, but the researchers were able to generate entangled photon pairs and compare their polarizations within the correct geometry.

The result wasn't even a close call. They found that Leggett’s inequality was violated by nine standard deviations. If extraordinary conclusions demand extraordinary proof, there it is.

This result does not say as much about what reality is as what it is not. At the very least, particles can not be said to carry hidden information that could do away with that pesky action-at-a-distance spook. The properties of particles really are random until they are observed, and measuring one member of an entangled pair really does determine the corresponding properties of the other, no matter where or when they are measured.

More strongly, the authors suggest that the universe in which we live may not oblige our prejudices by, for example, following Aristotelian logic. The statement, “Their properties really are random until they are observed,” may be both true and false. Present events may be able to influence the past. And—yet another blow to Einstein—the universe may simply not be deterministic.

Although Zeilinger suggests that any or all of these implications of quantum weirdness may be the case, he advocates caution. Writing about Einstein and Schrödinger, both of whom had deep reservations about what quantum theory said about reality, Zeilinger writes, “Yet it is very much to their credit that they both clearly understood which radical changes in our view of the world (Weltanschauung) quantum mechanics in the end necessitates. Changes which might be so radical that it is certainly reasonable and understandable to thoroughly investigate all other possibilities before taking the leap.”

As to what those other possibilities might be, Zeilinger remains agnostic. He quotes the Nobel prizewinning physicist, Isidor Rabi, who said, “"The problem is that the [quantum] theory is too strong, too compelling. I feel we are missing a basic point. The next generation, as soon as they will have found that point, will knock on their heads and say: How could they have missed that?".

Whatever that head-banging enlightenment may be, it's still very much missing.

Robert Adler

For the institute