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SCIENCE FILE | Mind Over Matter

Schroedinger's Cat Unravels Paradoxical Ball of Thread

September 12, 1996|K.C. COLE | TIMES SCIENCE WRITER

One of the most notorious characters in physics is a cat named Schroedinger, who is both dead and alive at the same time.

Schroedinger isn't a real cat, but a figure in a thought experiment concocted by physicist Erwin Schroedinger in 1935 to expose what he felt were the logical absurdities of quantum mechanics--the rules that govern the behavior of subatomic particles.

At these submicroscopic scales, the theory says, particles are neither here nor there until they are measured--at which point they choose where to be. A quantum particle, in other words, can be on your desk and in the next room, simultaneously. Until you decide to look at it.

Mathematician Ian Stewart has compared the situation to a spinning coin. As long as it's spinning, it's both "heads" and "tails." Only when you slam it down with your hand does it actually choose whether to be heads or tails.

Schroedinger found this situation ridiculous. Say you put a cat in a box, he said, with a vial of poison, and a single radioactive atom that shoots out a particle at some random time. The particle breaks the vial and kills the cat. But until you look inside the box, he said, the atom doesn't choose whether to shoot out the particle; therefore the vial of poison remains both broken and unbroken at the same time, and the cat is both alive and dead.

When you open the box to look, the very act of looking either frees the creature or does it in. But since a cat obviously can't be both dead and alive at the same time, the theory must be wrong.

Until recently, the cat remained primarily a figment of the physicist's imagination. Then earlier this year, much ado was made of the fact that real atoms, in a real laboratory in Boulder, Colo., seemed to exhibit the properties of this mythological cat: A beryllium atom was seen in two places at once.

Schroedinger's cat, it seemed, was real.

I never understood what the big deal was about Schroedinger's cat--since any cat owner knows that half dead is a normal state for the species.

However, most everyday objects are not both here and there at the same time. And that is a profound paradox. If the elementary particles exist in a state of eternal fuzziness, how can objects composed of them be so sure of themselves? Where does quantum uncertainty end and everyday definiteness begin?

The answer lies partly in quantity. As physicist Phil Anderson has said so succinctly: More is different. A typical cat, Stewart points out, contains many atoms--about 10 to the 26th power. You wouldn't expect a conglomeration of atoms that big to behave like a single atom any more than a piece of music would have the same impact as a single note.

The cat, in other words, is what physicists call an "emergent property." Like music, or time, or even cats and people, it doesn't exist on the scale of its most fundamental parts. There is no hint of humanity in a single atom.

Schroedinger himself developed equations that describe any atom to perfection, but they have nothing to say about whether a cat made up of those atoms will scratch the furniture or sleep on your head.

But in another sense, the dual nature of Schroedinger's cat is present in everything. A blank piece of paper is both a potential poem and a potential drawing, just as a subatomic particle is both a potential wave and a potential particle, depending on how you measure it. Whichever it becomes, it can't be the other at the same time. In a sense, potential is all there is.

Or as Stewart and biologist Jack Cohen put it in "The Collapse of Chaos": A tree is a potential boat, or a desk.

The tree, like the cat, exists in a state of endless potential until it (or something else) decides what it will be when it grows up. Not so paradoxical at all, when you think about it.

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