Quantum Physics


Debating the Meaning of Quantum Mechanics

Why is quantum mechanics like cricket?

Because for me, no matter how many times the rules are explained, I can’t seem to get my head around what the game is actually about.

Is quantum theory a system of equations? A description of the behavior of invisible particles? A philosophy for the post-post-modern age?

And how strange is it that we even have to ask? Unlike other scientific theories, quantum physics is so slippery that its formalism—the equations that add up to a mathematical representation of what we humans call reality—is divorced from its physical interpretation. Sure, we can solve the Schrödinger equation for the case of a particle stuck in a box, but what is that telling us about how the natural world really works?

This isn’t a question you’d even think to ask about classical mechanics. Remember Newton’s Second Law, the one relating force to mass and acceleration? Its formalism is F=ma, and its interpretation is pretty simple: If you want to know the force an object is exerting, just multiply its mass by its acceleration.

That’s F=ma. But what about:


“Quantum mechanics needs an explanation worse than other theories do because others always had a physical picture that guided the formulation of the mathematics,” explains John Cramer, a physicist at the University of Washington who also happens to be the author of his own interpretation of quantum mechanics—more on that later. Newton had his (possibly apocryphal) apples, his inclined planes, his cannonballs. Werner Heisenberg, one of the “fathers” of quantum mechanics, by contrast, had some elegant mathematics, a vision more akin to numerology than to a picture of the physical world, in Cramer’s view.

“The Copenhagen interpretation is like a religious text,” says MIT physicist Max Tegmark. “It leaves a lot open to interpretation.”

Yet Heisenberg, like his colleague Niels Bohr, felt that quantum mechanics needed no further interpretation. This view, which is now known as the Copenhagen interpretation, holds that there is no “objective reality” lurking beneath the formalism. If the equations say that I have a 50% chance of measuring a particle in a certain state—say, spin up—and then I go ahead and measure it in that state, what more is there to say? To guess at what the particle was doing before I made the measurement would be worse than speculation; nothing can be said about the particle except in the context of a measurement. “Reality” is no more and no less than what our instruments and senses reveal it to be. The Copenhagen interpretation may give you a headache, but according to Anton Zeilinger, the University of Vienna physicist most famous for his teleportation experiments, “It works, is useful to understand our experiments, and makes no unnecessary assumptions.”

Still, many physicists find this notion unsatisfying. “Quantum mechanics is full of strange things that cry out for an interpretation,” says Cramer. There’s the problem of “spooky action at a distance,” the apparent connection between “entangled” particles that seems to violate the finite speed of light; and there’s Einstein’s famous discomfort with the idea that no reality exists outside of our own perceptions. As Einstein put it: “Do you really think the moon isn’t there if you aren’t looking at it?”

There’s also a niggling problem with exactly what defines “looking at it”—or, in quantum-speak, what defines a “measurement.” If we truly cannot say anything definite about a particle until after we’ve measured its state, then the act of measuring it must be pretty special. But why? What happens in that moment? Physicists often talk about it as the “collapse of the wavefunction”—that is, the moment when all of the possible particle states represented in the probability equation called the wavefunction collapse into a single, measured state. The instantaneous collapse of an entity that wasn’t physically real to start with is weird in itself. But physicist Steven Weinberg pointed to another weak link in this interpretation in a 2005 article in Physics Today: “The Copenhagen interpretation describes what happens when an observer makes a measurement, but the observer and the act of measurement are themselves treated classically. This is surely wrong: Physicists and their apparatus must be governed by the same quantum mechanical rules that govern everything else in the universe.”

If not Copenhagen, then what? Let’s take a quick tour of a handful of the (many!) competing interpretations of quantum mechanics.

  • Copenhagen interpretation: This is the interpretation we’ve just met, and the one you’ll see in most physics books—though even Heisenberg and Bohr didn’t always agree on the particulars. To put it in terms of our cricket analogy, let’s say that you’re following a cricket match on your cell phone. Actually let’s make it a baseball game because, as I’ve already confessed, I don’t understand cricket. So you’re using one of those apps that updates the box score every time you press “refresh,” but you can’t actually see the game in progress. According to the Copenhagen interpretation, there is no game—just the results you get when you ping the server. So it’s no use talking about whether the batter is getting into the pitcher’s head, or the appearance of the rally squirrel, or even the trajectory the ball takes on its way into the first baseman’s glove. The box score is real; the game isn’t.
  • Consistent histories: The Copenhagen interpretation applies to a situation in which an observer (the baseball fan) makes a measurement (checks the score) on some external system. But what happens when the observer is himself part of the system—say, the shortstop? That’s the problem that a special breed of physicists called quantum cosmologists encounter when they attempt to study the entire universe as a single quantum system. The Copenhagen interpretation falls short in this case, but the consistent histories interpretation, developed in the 1980s and early 1990s, does away with external “observers” and “measurements”—they are treated as part of one big system.
  • Many worlds: We talked earlier about the problem of the collapsing wavefunction. But what if the wavefunction never actually collapses? What if every possibility it represents really does happen in its own universe? With every measurement, each universe branches off into countless others, each of which in turn branches into ever more universes. The many worlds interpretation was first proposed in the 1950s by the young physicist Hugh Everett, and though it never gained much traction at the time, its star is now ascending: In the film Parallel Worlds, Parallel Lives, Tegmark called the many worlds interpretation “one of the most important discoveries of all time in science,” and he and his colleagues recently posited that Everett’s parallel universes might be congruent with the parallel universes proposed by cosmologists. Of course, plenty of physicists can’t stomach the idea of a multiplicity of fundamentally unobservable universes. Yet—back to baseball for a moment—there is something appealing about an interpretation that insists upon the existence of a universe in which the baseball rolls squarely into Buckner’s glove; an interpretation that guarantees that every heartbreaker in our universe is shadowed by a heroic comeback in another; an interpretation in which the Red Sox and the Yankees win, year after year after year.
  • Transactional interpretation: The transactional interpretation might solve some of quantum theory’s biggest quandaries, if you can get your head around the idea of a wave with negative energy that travels back in time. The transactional interpretation was first proposed in the 1980s by John Cramer, and suggests that the wavefunction includes not just one but two probability waves—the familiar one that travels forward in time, plus an exotic twin that travels backward. When they meet, they exchange a “handshake” across space-time, says Cramer; at other points, they cancel each other out completely, removing any telltale traces of the journey backward in time.

So, is there any way to know which interpretation is right or wrong? “Unless you can catch an interpretation deviating from the mathematics, you can’t rule it out,” says Cramer. And though some experiments could maybe, possibly tip the scales in favor of one interpretation or another, there is no consensus that any of the contenders above have been favored or nixed by experiment. Perhaps, some physicists argue, the pursuit of an interpretation is a flawed endeavor. “There is no logical necessity of a realistic worldview to always be obtainable,” wrote Christopher Fuchs and Asher Peres in a Physics Today opinion piece titled, transparently, “Quantum Theory Needs No ‘Interpretation’.” “If the world is such that we can never identify a reality independent of our experimental activity, then we must be prepared for that, too.” Perhaps the interpretation problem isn’t a problem of quantum physics at all, but a problem of human beings.

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Kate Becker

    Kate Becker is the editor of The Nature of Reality, where it is her mission to blow your mind with physics. Kate studied physics at Oberlin College and astronomy at Cornell University, and spent seven years as senior researcher for NOVA and NOVA scienceNOW. Follow her on Twitter and Facebook.