SundayReview

The Reality of Quantum Weirdness

FEB. 20, 2015

By EDWARD FRENKEL

IN Akira Kurosawa’s film “Rashomon,” a samurai has been murdered, but it’s not clear why or by whom. Various characters involved tell their versions of the events, but their accounts contradict one another. You can’t help wondering: Which story is true?

But the film also makes you consider a deeper question: Is there a true story, or is our belief in a definite, objective, observer-independent reality an illusion?

This very question, brought into sharper, scientific focus, has long been the subject of debate in quantum physics. Is there a fixed reality apart from our various observations of it? Or is reality nothing more than a kaleidoscope of infinite possibilities?

This month, a paper published online in the journal Nature Physics presents experimental research that supports the latter scenario — that there is a “Rashomon effect” not just in our descriptions of nature, but in nature itself.

Over the past hundred years, numerous experiments on elementary particles have upended the classical paradigm of a causal, deterministic universe. Consider, for example, the so-called double-slit experiment. We shoot a bunch of elementary particles — say, electrons — at a screen that can register their impact. But in front of the screen, we place a partial obstruction: a wall with two thin parallel vertical slits. We look at the resulting pattern of electrons on the screen. What do we see?

If the electrons were like little pellets (which is what classical physics would lead us to believe), then each of them would go through one slit or the other, and we would see a pattern of two distinct lumps on the screen, one lump behind each slit. But in fact we observe something entirely different: an interference pattern, as if two waves are colliding, creating ripples.

Astonishingly, this happens even if we shoot the electrons one by one, meaning that each electron somehow acts like a wave interfering with itself, as if it is simultaneously passing through both slits at once.

So an electron is a wave, not a particle? Not so fast. For if we place devices at the slits that “tag” the electrons according to which slit they go through (thus allowing us to know their whereabouts), there is no interference pattern. Instead, we see two lumps on the screen, as if the electrons, suddenly aware of being observed, decided to act like little pellets.

To test their commitment to being particles, we can tag them as they pass through the slits — but then, using another device, erase the tags before they hit the screen. If we do that, the electrons go back to their wavelike behavior, and the interference pattern miraculously reappears.

There is no end to the practical jokes we can pull on the poor electron! But with a weary smile, it always shows that the joke is on us. The electron appears to be a strange hybrid of a wave and a particle that’s neither here and there nor here or there. Like a well-trained actor, it plays the role it’s been called to perform. It’s as though it has resolved to prove the famous Bishop Berkeley maxim “to be is to be perceived.”

Is nature really this weird? Or is this apparent weirdness just a reflection of our imperfect knowledge of nature?

The answer depends on how you interpret the equations of quantum mechanics, the mathematical theory that has been developed to describe the interactions of elementary particles. The success of this theory is unparalleled: Its predictions, no matter how “spooky,” have been observed and verified with stunning precision. It has also been the basis of remarkable technological advances. So it is a powerful tool. But is it also a picture of reality?

Here, one of the biggest issues is the interpretation of the so-called wave function, which describes the state of a quantum system. For an individual particle like an electron, for example, the wave function provides information about the probabilities that the particle can be observed at particular locations, as well as the probabilities of the results of other measurements of the particle that you can make, such as measuring its momentum.

Does the wave function directly correspond to an objective, observer-independent physical reality, or does it simply represent an observer’s partial knowledge of it?

If the wave function is merely knowledge-based, then you can explain away odd quantum phenomena by saying that things appear to us this way only because our knowledge of the real state of affairs is insufficient. But the new paper in Nature Physics gives strong indications (as a result of experiments using beams of specially prepared photons to test certain statistical properties of quantum measurements) that this is not the case. If there is an objective reality at all, the paper demonstrates, then the wave function is in fact reality-based.

What this research implies is that we are not just hearing different “stories” about the electron, one of which may be true. Rather, there is one true story, but it has many facets, seemingly in contradiction, just like in “Rashomon.” There is really no escape from the mysterious — some might say, mystical — nature of the quantum world.

But what, if anything, does all this mean for us in our own lives? We should be careful to recognize that the weirdness of the quantum world does not directly imply the same kind of weirdness in the world of everyday experience. That’s because the nebulous quantum essence of individual elementary particles is known to quickly dissipate in large ensembles of particles (a phenomenon often referred to as “decoherence”). This is why, in fact, we are able to describe the objects around us in the language of classical physics.

Rather, I suggest that we regard the paradoxes of quantum physics as a metaphor for the unknown infinite possibilities of our own existence. This is poignantly and elegantly expressed in the Vedas: “As is the atom, so is the universe; as is the microcosm, so is the macrocosm; as is the human body, so is the cosmic body; as is the human mind, so is the cosmic mind.”