BY DAVID F. SALISBURY
Ever wonder what keeps giant boulders from collapsing, steel girders from crumbling away, or oceans from vanishing overnight?
After all, modern physics tells us that atoms are mostly space. All but a few hundredths of a percent of an atom's mass is concentrated in the tiny nucleus at its core, and the nucleus itself is constructed out of even smaller, point-like particles called quarks, so it is mostly space as well. So why don't atoms collapse?
Since the 1950's physicists have had a rule that explains this, but they haven't had direct evidence to support it. According to quantum mechanics, the theory that describes how the world works at the subatomic level, atoms are constructed from particles that take an arm's length attitude toward others of their kind. These particles are called fermions, and include the proton, neutron and electron. One of the most important ramifications of this mutual repulsion is that only two electrons can occupy a single orbit within an atom at the same time. This restriction, called the Pauli exclusion principle, helps to keep all of an atom's electrons from jumping into the lowest energy orbit, the one closest to the nucleus.
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Despite the obvious importance of this phenomenon, physicists have had only indirect evidence of its existence. Atoms, after all, don't collapse under normal conditions. Now, however, a research group headed by Yoshihisa Yamamoto, professor of applied physics and electrical engineering at Stanford, reports the first direct evidence for this effect. Writing in the April 9 issue of the journal Science, the researchers have shown that individual electrons in a beam produced by a thermal source arrive at a target one by one, rather than in bunches. That fermion beams should exhibit such "anti-bunching" was predicted in 1956 by Nobel Laureate Edward Purcell, but only recently has technology advanced to the point that such a subtle effect can be measured.
"They may not realize it, but the entire microelectronics industry relies on the anti-bunching behavior of electrons," says Yamamoto. "It keeps electrical current flowing evenly at even microscopic levels."