Zurek is the leading advocate of a theory called decoherence, which is based on the idea that the environment destroys quantum coherence. He formulated it in the 1980s (although some of it harkens back to Bohr and other quantum founders) and with various collaborators has been investigating its consequences ever since.
The destabilizing environment essentially refers to anything that could be affected by—and hence inadvertently “measure”—the state of the quantum system: a single photon, a vibration of a molecule, particles of air. The environment is not simply “noise” in this theory; it acts as an apparatus that constantly monitors the system.
The ENS experiment makes that effect clear. “The system decoheres because the system leaks information,” Zurek notes. Some photons can escape the cavity and hence betray the state of the remaining ones to the rest of the universe. “So in a sense, Schrödinger’s cat is having kittens crawling out,” Zurek says.
Having the environment define the quantum-classical boundary has the advantage of removing some of the mystical aspects of quantum theory that certain authors have promulgated. It does away with any special need for a consciousness or new physical forces to effect a classical outcome. It also explains why size per se is not the cause of decoherence: large systems, like real-life cats, would never enter a superposition, because all the particles that make up a feline influence a vast number of environmental parameters that make coherence impossible. Given a one-gram bob on a pendulum and a few reasonable assumptions, the interference terms in the system’s wave function drop to about 2.7–1,000 of their original value in a nanosecond—a virtually instantaneous disappearance of quantum weirdness. “The old intuition going back to Bohr is on the money,” although now there is a physical mechanism to substantiate his mandate, Zurek concludes.
Still, Zurek’s decoherence model is flawed in some eyes. “In my view, decoherence doesn’t select a particular outcome,” opines Anthony J. Leggett of the University of Illinois. “In real life, you get definite macroscopic outcomes.”
Zurek argues that the environment does indeed dictate the quantum possibilities that end up in the real world. The process, which he refers to as environment- induced superselection, or einselection, tosses out the unrealistic, quantum states and retains only those states that can withstand the scrutiny of the environment and thus might become classical. “The selection is done by the environment, so you will not be able to predict which of the allowed possibili- ties will become real,” Zurek observes.
The explanation feels less than satisfying. Zurek’s approach is “very appealing. It allows you to calculate things, to see how the interference fringes wash out as the superposition gets bigger,” NIST’s Monroe says. “But there’s still something funny about it. He’s sweeping things under the rug, but it’s hard to say what rug.” The problem is that decoherence—and in fact any theory about the quantumclassical transition—is necessarily ad hoc. Quantum superpositions must somehow yield outcomes that conform to our everyday sense of reality. That leads to circuitous logic: the results seen in the macroscopic world arise out of the quantum world because those results are the ones we see. A solution of sorts, advocated by a few prominent cosmologists, is the unwieldy “many worlds” interpretation, which holds that all possibilities stipulated by the wave function do in fact happen. They go on to exist in parallel universes. The idea, however, is untestable, for the parallel universes remain forever inaccessible to one another.