Quantum Mechanics Gets Real

By Lindley, David | Science News, March 1, 1997 | Go to article overview

Quantum Mechanics Gets Real


Lindley, David, Science News


Writing to Niels Bohr in 1935, physicist Erwin Schrodinger lamented his inability to understand a principle that Bohr deemed essential to the interpretation of quantum mechanics: "It must belong to your deepest conviction-and I cannot understand on what you base it," Schrodinger complained.

Bohr's principle concerns the way in which a measurement of a quantum mechanical system-the position of an electron, for example-produces a specific result. Quantum mechanics requires that a system exist in a range of possible states, a superposition, until a measurement is made, at which point one of those states takes on a definite reality. But how?

To illustrate his perplexity, Schrodinger imagined placing a cat in a closed box, along with an atom that could be in one of two states and a device to measure its state. If the measurement goes one way, the cat stays alive; if it goes the other way, the unfortunate cat dies. The quantum system starts as a superposition of two possible states, Schrodinger noted, but does that mean that the cat is simultaneously dead and alive? If not, what is it about a cat that requires it to be dead or alive rather than some unimaginable combination of the two?

Last year, scientists at the National Institute of Standards and Technology (NIST) in Boulder, Colo., created a small-scale scenario resembling the box with the fanciful cat. They trapped a single atom in such a way that it could occupy either of two distinct states. Then, using lasers, they nudged the two states in opposite directions, physically separating the two halves of the superposition.

Great precision was needed to maintain coherence between the separated states. Even the tiniest disturbance could have upset the system, forcing the atom to take up a definite position in one place or the other. Theoretical investigations in recent years suggest that the delicacy of such states holds an explanation for why atomic superpositions-let alone superpositions of cats-are not normally seen.

The atoms of a real purring, yowling, or napping cat constantly jiggle around, preventing a quantum mechanically coherent state from encompassing the entire animal, except perhaps for an instant. Moreover, the aliveness or deadness of a cat are qualities that have durable meaning, even though the cat's internal quantum disposition is in a perpetual state of flux.

These insights have been mathematically refined to form the basis of a physical process called decoherence. According to its proponents, decoherence confers long-term stability only on those properties of a macroscopic system that correspond to what an observer would recognize. A cat, in other words, remains dead or alive long enough for that state to be recorded; a superposed dead-and-alive cat, however, can never exist long enough to be noticed.

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