Decoherence: Destroyer of Weirdness

Last night I watched an episode of one of my favorite TV shows, PBS's Closer to Truth, which deals with scientific perspectives on questions of philosophy and theology. The episode was called “Why Is the Quantum So Weird?” From Scientific American magazine to popular pseudoscience books like The Secret, we are told that the world of the very small is extraordinarily strange, counterintuitive, unlike anything we can relate to in everyday life — where particles can seem to be in two places at once, go backward in time, “tunnel” through impermeable barriers, etc. We know that on very small scales, these quantum phenomena do occur, and in fact quantum mechanics establishes the theoretical basis behind everything from transistors to quantum computing. So, why is the quantum world so strange?

The episode provided an excellent run-down of quantum theory, but it didn’t provide a satisfying answer to the question. That’s because it’s not the right question. We should be asking, Why is the ordinary world not weird? Because this is a question we actually have an answer for.

Assigning a value-judgment word such as “weird” to quantum phenomena betrays how biased we humans can be. We expect things to behave the same on all scales, large and small, because that’s how a physically consistent universe should be. If a tennis ball can be in only one place at once, we assume that the same must be true of an electron. In fact, today many physicists agree that the world does behave the same on all scales; however, this behavior is most accurately described by the laws of quantum mechanics — even the behavior of the entire universe as a whole. (This is the scientific basis for the parallel universes of the famous “many worlds interpretation.”) In other words, the whole entire universe on all scales is “weird.” So why does it make sense to us? Why do we never see evidence of a tennis ball being in two places at once, or passing through a brick wall, as we do for subatomic particles?

The answer relates to something called quantum decoherence. Discovered in the late 1980s, decoherence refers to the loss of coherence, which is the property of a quantum system (such as an electron) that can give it an uncertain, blurry or smeared out physical description. An electron in a coherent state can be in superposition, meaning that its precise location, momentum, spin, etc., is undefined or blurred out: It appears to possess many values for these things at once. (Most people learn about this bit of quantum weirdness by way of the electron cloud that surrounds an atom's nucleus, but free electrons and other particles have this property as well.) However, if that electron encounters an electron detector, the system of the electron and the detector will undergo decoherence, and the electron will appear to suddenly “snap” into one definite state. You often hear this process described as the collapse of the wavefunction, although that phrase is becoming increasingly archaic among the physics crowd.

Decoherence causes ordinary macroscopic objects to behave differently than subatomic particles; unlike electrons or photons, they always exist in definite places and follow well-understood and predictable or classical laws of motion. To experimentally prove that decoherence is responsible, just take an object and put it into a coherent state of superposition, and keep it that way — prevent decoherence from happening. To achieve this feat, though, there’s one thing you need to do: The object must be completely removed, or decoupled, from interaction with its environment. For example, it needs to be kept incredibly cold, at a fraction of a degree above absolute zero. This is because the moment the object starts getting hit with photons of heat or light, those photons begin to “observe” the object. In doing so, they carry away enough information about the superposition that the object appears to collapse into one definite state, with astonishing speed. Decoherence ensures that anything that’s directly observed in any way at all cannot remain in a state of superposition. Even though everything in the universe obeys those “weird” laws of quantum behavior — all the time — whenever there’s any kind of observation going on, decoherence destroys that quantum weirdness. In the process, it creates a world that makes sense.

Actually, decoherence only destroys the weird aspect of nature; it doesn’t destroy the alternate states represented by a superposition, or change anything about the way quantum mechanics works. If a tennis ball in a quantum superposition undergoes decoherence, information describing those potential alternate states still technically exists in the world. It’s just that it has been irreversibly lost to the chaos of the environment, and like Humpty Dumpty, no amount of effort will be able to restore it. The “blurry” aspect of a tennis ball that that has undergone decoherence is a little like the kinetic energy of a car with the brakes applied: It isn’t destroyed altogether, but only gets dissipated into the environment. Once this happens, the probability of any alternate state reappearing — for the alternate positions or momenta of all of the ball’s particles to randomly reconstitute themselves, allowing us to see a second ball — becomes vanishingly tiny.

So the next time you’re playing tennis, and you hit one definite ball back to a definite spot on the court, you can thank quantum decoherence for making it possible.