One of the most vexing things about studying quantum mechanics is how maddeningly classical the world is. Quantum physics features all sorts of marvelous things—particles behaving like waves, objects in two places at the same time, cats that are both alive and dead—but we don’t see those things in the world around us. When we look at an everyday object, we see it in a definite classical state and not in any of the strange combinations of states allowed by quantum mechanics. Particles and waves look completely different, dogs can only pass on one side or the other of an obstacle, and cats are stubbornly alive or dead, not both at once.

Over the last 80 or so years, physicists have struggled to discover the origin of this apparent division between the quantum and classical worlds. Niels Bohr treated it as axiomatic in the “Copenhagen interpretation” of quantum theory, but this was an ad hoc addition to the theory. Nothing in the core equations of the theory says that a cat can’t be in two states at once, and physicists have had to work very hard to find possible explanations for why this doesn’t happen, proposing additions to the Schrödinger equation or invoking quantum gravity.

In recent years, new advances in materials and experimental techniques have made it possible to see quantum behavior in larger and larger objects, and a sort of cottage industry has sprung up in looking for quantum behavior of macroscopic objects. The DAMOP meeting in May featured an entire invited session on the subject with speakers from Yale, Vienna, and Munich, and another talk on experiments at LIGO that have pushed gram- and kilogram-scale mirrors toward the quantum limit (article here).

In “Quantum Mechanics in Ordinary Objects,” Veronique Greenwood reports on the latest development in this fast-growing field, a new experiment from the group of Michael Roukes at Caltech. The Roukes group has manufactured a “bridge” two micrometers in length next to an “artificial atom” consisting of a small loop of superconductor. The two are close enough together that their motion is coupled—when the “atom” is in a higher energy state, the “bridge” vibrates at a higher frequency, and vice versa. They have used the vibration of the “bridge” to detect the state of the “atom” and observed the discrete energy steps that give quantum mechanics its name. In the future, they hope to reverse the experiment, and use the state of the “atom” to detect quantized vibrations in the “bridge,” when the whole system is cooled down to low enough energy. Then they can try to prepare the “bridge” in a superposition of two states at once, and see what happens.

The Caltech group is still a long way from observing a cat in two places at once—only a physicist would consider a mass of 40 trillionths of a gram “macroscopic”—but this would be the largest object by far ever to show unambiguous quantum behavior. If they succeed, it could provide new insight into why the world we see is so depressingly ordinary compared to the world of quantum theory.

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Originally published July 29, 2009