May 29, 2010

If an Electron Can Be in Two Places at Once, Why Can't You?

Sir Roger Penrose—Knight of the Realm, Emeritus Rouse Ball Professor of Mathematics at Oxford University, controversial author, and polymath extraordinaire—is worried that his car might be towed. It is parked in a temporary space beside Oxford’s Mathematical Institute, where we’ve arranged to have the first of our meetings. So before settling down to discuss his solution to one of the greatest mysteries in physics, he hustles out a couple of times to make sure the car is still there, displaying impressive bursts of speed for a 73-year-old.

I am sure that he would like to be in two places at once: here in an otherwise empty conference room with me and outside in the chill autumn rain, keeping an eye out for the bobbies. That’s impossible, of course, and therein lies the mystery that consumes Penrose.

About 80 years ago, scientists discovered that it is possible to be in two locations at the same time—at least for an atom or a subatomic particle, such as an electron. For such tiny objects, the world is governed by a madhouse set of physical laws known as quantum mechanics. At that size range, every bit of matter and energy exists in a state of blurry flux, allowing it to occupy not just two locations but an infinite number of them simultaneously. The world we see follows a totally different set of rules, of course: There’s just one Oxford University, just one car, just one Penrose. What nobody can explain is why the universe seems split into these two separate and irreconcilable realities. If everything in the universe is made of quantum things, why don’t we see quantum effects in everyday life? Why can’t Penrose, made of quantum particles, materialize here, there, and everywhere he chooses?

Many physicists find this issue so vexing that they ignore it entirely. Instead, they focus on what does work about their theories. The equations of quantum mechanics do a fantastic job describing the behavior of particles in an atom smasher, the nuclear reactions that make the sun shine, and the chemical processes that underlie biology. For Penrose, that is not nearly enough. “Quantum mechanics gives us wonderful predictions and experimental confirmations for small-scale scenarios, but it gives us nonsense at ordinary scales,” he says, relaxed now that a receptionist has assured him of his car’s safety. “If you just follow the equations, you get a mess. So you have to ask: What leads to this world?”

He has an answer, which, if correct, will lead to the first quantum theory that makes as much sense for people as for particles. Penrose believes he has identified the secret that keeps the quantum genie tightly bottled up in the atomic world, a secret that was right in front of us all along: gravity. In his novel view, the same force that keeps us pinned to the ground also keeps us locked in a reality in which everything is tidy, unitary, and—for better and for worse—rooted in one place only.

Aside from a frustrating inability to manifest in any number of places simultaneously, Penrose qualifies as something of a quantum phenomenon himself. There do indeed seem to be many Penroses; they just all happen to occupy the same body.

There is Sir Roger the physicist, knighted in 1994 for his contributions to science, among them pioneering efforts to reconcile Albert Einstein’s general theory of relativity with quantum mechanics. There is Penrose the puzzle master, creator of geometric illusions that M. C. Escher incorporated into some of his most famous works. There is Penrose the neuroscientist, who developed a controversial theory linking consciousness to quantum processes in the brain. And there is Penrose the author, most recently of a 1,049-page tome called The Road to Reality, which is modestly subtitled A Complete Guide to the Laws of the Universe. It’s an impressive résumé for someone who was demoted a grade in elementary school because he couldn’t master arithmetic.

On our second meeting, all of those Penroses are slumped on a sofa in the living room of his spacious home a few miles outside Oxford. A coffee cup and a plate of cookies rest on his chest, which, since he is sunk so deeply into the sofa, is almost perfectly horizontal. Tall windows look out on a lush green yard, damp from the rain. In this pensive setting, he looks back on the events that convinced him that quantum theory has serious problems, a view that would be heresy for a young physicist entering academia today.

Penrose’s faith began to waver while he was a graduate student at Cambridge. The crucial moment came during a lecture by Paul Dirac, one of the legendary early thinkers in quantum mechanics. “He was talking about the superposition principle, whereby objects could be in two places at the same time. To illustrate, he broke a piece of chalk in two and then tried to explain why you never saw superpositions in real life. My mind may have wandered briefly, because I never heard his explanation!” Penrose says, laughing. “But when I think about it, I’m not sure it did wander, because it’s not possible to explain why you don’t see objects in two places at once on the basis of present-day quantum mechanics. It’s a big problem. It’s what I’ve worried about ever since.”

The maddening part of that problem is that the ability of particles to exist in two places at once is not a mere theoretical abstraction. It is a very real aspect of how the subatomic world works, and it has been experimentally confirmed many times over. One of the clearest demonstrations comes from a classic physics setup called the double-slit experiment.

Copyright: Tim Folger