The Classical Limit of an Atom; June 1994; Scientific American Magazine; by Nauenberg, Stroud, Yeazell; 6 Page(s)

Throughout this century, physics has made use of two quite different descriptions of nature. The first is classical physics, which accounts for the motion of macroscopic objects, such as wheels and pulleys, planets and galaxies. It describes the continuous, usually predictable cause-and-effect relationships among colliding billiard balls or between the earth and orbiting satellites. The second description is quantum physics, which encompasses the microscopic world of atoms, molecules, nuclei and the fundamental particles. Here the behavior of particles is described by probabilistic laws that determine transitions between energy levels and govern tunneling through energy barriers. Because quantum mechanics is the fundamental theory of nature, it should also encompass classical physics. That is, applied to macroscopic phenomena, quantum mechanics should reach a limit at which it becomes equivalent to classical mechanics.

Yet until recently, the exact nature of this transition had not been fully elucidated. Now that goal is within reach. Atomic systems have been created that behave--for a short period--according to the laws of classical mechanics. Researchers fabricate such systems by exciting atoms so that they swell to about 10,000 times their original size. On such a scale the position of an electron can be localized fairly closely; at least its orbit no longer remains a hazy cloud that represents only a probable location. In fact, as the electron circles the nucleus, it traces an elliptical path, just as the planets orbit the sun.