Testing the Pauli exclusion principle.

Testing the Pauli exclusion principle 

The Pauli exclusion principle stands at the heart of modern molecular, atomic and nuclear physics. By insisting that no two electrons, protons or neutrons can occupy exactly the same quantum state, the principle explains why matter doesn't collapse on itself. Several research groups, prompted by theorists who have recently questioned the principle's validity under certain circumstances (SN: 2/27/88, p.132), are now subjecting it to sensitive experimental tests.

"The theorists have called for wide-scale testing in a variety of systems," says John D. Gillaspy of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. "It's rather surprising that such a fundamental principle has for so long not really been examined."

In the most precise experimental test to date, Erik J. Ramberg and George A. Snow of the University of Maryland in College Park sent a large electrical current through a copper wire, then searched for certain X-ray signals that would appear only if an electron were to fall into a quantum state already occupied by another electron. By finding no such X-ray, the researchers showed that any violation of the Pauli principle must be smaller than 2 parts in [10.sup.26].

NIST scientists are developing a simpler experiment to check some of the assumptions underlying the University of Maryland experiment. "There have been no hints of violations to this point," Gillaspy says. "Even though we believe there won't be a violation, we think it's important to put some very stringent and very rigorous limits on when such violations could occur."