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Physics + MathPhysics & Math

Is Quantum Intuition Possible?

Quantum physics defies our intuitions about the physical world. Does it have to be that way?

ByKate BeckerThe Nature of RealityThe Nature of Reality

Quantum physics defies our physical intuition about how the world is supposed to work. In the quantum world, objects resist such classical banalities as “position” and “speed,” particles are waves and waves are particles, and the act of observing seems to change the system being observed

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But what if we could develop a “quantum intuition” that would make this all seem as natural as an apple falling from a tree?

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Physical intuition starts developing early, long before we ever encounter Newton’s laws on a blackboard. “Babies have a few skeletal principles that are built in to the brain and help them reason about and predict how objects should act and interact in the world,” says Kristy vanMarle , an infant cognition researcher at the University of Missouri. They understand, for instance, that objects can’t pass through each other, a notion that’s at odds with a quantum effect called tunneling, which allows objects to slip through barriers that, in the classic world, would be impenetrable. Presented with demon strations in which objects appear to materialize inside closed boxes and pass through solid walls , babies consistently stare longer at these “magic” shows than they do at demos in which boxes act like boxes. Psychologists Susan Hespos (now at Northwestern University) and Renee Baillargeon (University of Illinois) found that this physical intuition kicks in as early as two and a half months, and vanMarle and her colleagues think that it is probably present from birth.

Babies also intuitively grasp that objects exist even when you’re not looking at them, a concept called “object permanence” that goes against the classic Copenhagen interpretation of quantum mechanics, in which an object can’t be said to have any definite properties until the moment at which it is observed. Since Jean Piaget first pegged object permanence as a milestone in infant development, psychology researchers have found evidence that ever-younger babies have some sense of it; affirming object permanence seems to be the main theme of peek-a-boo. (To someone who has truly taken quantum physics to heart, perhaps peek-a-boo never gets old.)

These innate notions, plus “elaborations” born from watching and interacting with the world, add up to a sort of “naïve physics” that we all grasp without any formal physics training, says vanMarle.

But what about building quantum intuition after that early mental groundwork has already been laid? Most students don’t begin studying quantum physics until college , when they already have both an intuitive and a formal, or mathematical, toolkit for classical physics.

Some college educators maintain that students should just stick to the math and forget trying to establish a “gut feeling” for quantum mechanics; I’ve argued the same about similarly difficult concepts in cosmology. No less an authority than Max Born, who received the 1954 Nobel Prize for his contributions to the foundation of quantum mechanics, felt that our minds just weren’t up to the task of “intuiting” quantum physics. As he wrote in “ Atomic Physics ,” first published in English in 1935, “The ultimate origin of the difficulty lies in the fact (or philosophical principle) that we are compelled to use the words of common language when we wish to describe a phenomenon, not by logical or mathematical analysis, but by a picture appealing to the imagination. Common language has grown by everyday experience and can never surpass these limits.”

Lord Kelvin took a similar tack, points out Daniel Styer , a professor of physics at Oberlin College and the author of “ The Strange World of Quantum Mechanics “. In his “ Baltimore lectures ,” a series of talks delivered in 1884 at Johns Hopkins University, Kelvin said, “It seems to me that the test of ‘Do we or not understand a particular subject in physics?’ is, ‘Can we make a mechanical model of it?’” By that yardstick, says Styer, all efforts to understand quantum mechanics are doomed to fail. “The experimental tests of Bell’s inequality prove that no mechanical model, regardless of how intricate or fanciful or baroque, will ever be able to reproduce all the results of quantum mechanics.”

But Kelvin wasn’t talking about quantum mechanics; he was struggling to grasp the theory of electromagnetism. Quantum mechanics doesn’t have a monopoly on mind-blowing, after all; physicists have been upending intuition for thousands of years. “When I teach freshman physics, the thing that’s hard is not that the students are ignorant. It’s that they already know the answer—and it’s wrong,” says Steve Girvin , a physicist at Yale University. Newton’s first law claims (roughly) that objects in motion tend to stay in motion, but tell that to the guy trying to push a moving box full of books across the floor. Our “naïve physics” is actually closest to Aristotle’s 2,300-year-old theories, in which heavy objects fall faster than light ones and objects in motion ease to a stop unless you keep pushing them. Quantum mechanics may seem weird, but to Aristotle, Newton’s laws would have been just as head-spinning.

To get from Aristotle to Newton, you have to be able to imagine a world without friction. Luckily, that isn’t so hard; if you’ve ever played air hockey or laced up ice skates, you can vouch for Newton’s first law.

But what is the quantum equivalent of an air hockey table–an everyday object that provides us hands-on access to quantum physics? If there is one, I haven’t thought of it. Computer simulations may provide the next best thing, and physics educators like Kansas State University’s Dean Zollman are actively developing and testing new software that puts students into a (virtual) quantum world where they can actively manipulate the parameters of quantum systems and see how their tweaks play out. “It’s certainly easier to be a student of quantum mechanics now that it was when I went through school,” says Zollman. “We had drawings in books, but visualizing things, even twenty-five years ago, was not the way most people went about teaching. And still images are still images—they don’t give you the same feeling, the same kind of understanding, that we really can do these days.”

Perhaps all of this should give us fresh respect for the scientists who discovered and codified the rules of quantum physics. “It was just incredibly difficult for classical physicists to make the leap from that worldview, which was confirmed by the things they saw in the everyday world around them, to understanding the strange implications of quantum mechanics,” says Girvin. “Every student today—90-100 years later—still has to make that same leap.” Each individual who aims to learn modern physics must personally recapitulate thousands of years of discovery.

And at the end of that road? “Practicing, professional people who have been doing this for decades still have arguments about what the results of the experiments will be,” says Girvin. There are no “native speakers” of quantum mechanics. “What is to be done about this?” asks Styer. “There are only two choices. We can either give up on understanding, or we can develop a new and more appropriate meaning for ‘understanding.’ I advocate the second choice.”

“Our minds evolved to find food and to avoid being eaten,” says Styer. “The fact that our minds ‘overevolved’ and allow us also to find beauty in sunsets and mountains, waterfalls and people; allow us to laugh and to love and to learn; allow us to explore unknown continents, and outer space, and (most bizarre of all) the atomic world, is a gift that we neither deserve nor (in many cases) appreciate. That we can make any progress at all in understanding quantum mechanics is surprising. We must not berate ourselves because our progress is imperfect. Instead, we must continue poking around, in joy and in wonder and sometimes in pain, exploring and building intuition concerning this strange and beautiful atomic world.”

This project/research was supported by grant number FQXi-RFP-1822 from the Foundational Questions Institute and Fetzer Franklin Fund, a donor-advised fund of Silicon Valley Community Foundation.