At what point does a mechanical system—governed by classical dynamics—become a quantum system, or vice-versa? In 1920, Neils Bohr argued that quantum systems became describable with classical mechanics when the correspondence (or classical) limit had been reached. He described this as occurring "when the quantum numbers describing the system are large." The exact interpretation of "large" has been left to the reader as a 90-year-old take-home exercise.
The broad implications of Bohr's correspondence principle is that (as Bohr himself argued) one cannot derive classical mechanics from quantum mechanics; classical systems in the every-day world will be governed by classical laws, and the very small quantum systems by quantum laws. There are examples of macroscopic systems that obey quantum mechanics—Bose-Einstein condensates for example—but they are inherently quantum systems. However, in March of this year, a research group at the University of California, Santa Barbara reported on the first ever classical system—a vibrating mechanical resonator—where the behavior could be described and manipulated through quantum mechanical means.
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Jennifer Scholle Tatiana Zavialova Tila Tequila Tamie Sheffield Kelly Monaco
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