Violating a not-so-exclusive exclusion principle.

Violating a not-so-exclusive exclusion principle 

The Pauli exclusion principle stands at the basis of the structure and stability of matter. It prevents, atoms, nuclei and larger structures, up to and including neutron stars, from collapsing on themselves. It does so by decreeing that no two particles of the class called fermions that have the same set of properties (the same quantum numbers) can be in the same place at the same time. The particles of physics are divided into fermions and bosons, and while any number of bosons can be in the same place, the exclusion principle maintains structures made of fermions (electrons, protons, neutrons) by restricing them.

Historically, physicists have believed the principle to be absolute. But now a few are asking, "Can it be violated?"

To explore that question, Oscar W. Greenberg and Rabindra N. Mohapatra of the University of Maryland at College Park have developed a theory that permits a slight violation of the exclusion principle. Experiments to test their theory are likely to follow.

According to Greenberg, the argument underpinning the absoluteness of the exclusion principle, using the statistical laws obeyed by fermions and bosons, makes a couple of false assumptions. As he said in a lecture last week at the National Bureau of Standards (NBS) in Gaithersburg, Md., those statistical principles are not general enough to support the argument. Greenberg and Mohapatra searched for a more generalized statistical law. When they found it, they discovered that it permits the existence of particles they call "parafermions" or "parons," which can violate the exclusion principle. The chance of violation is extremely small, about 1 in 100 million. However, this could produce unusual atomic states.

The experiment that may be closest to realization is one proposed by Daniel Kelleher of NBS, which would use helium atoms. Helium has two electrons, and according to the exclusion principle the spins of these two electrons must always be in opposite directions. However, a "paronic" helium atom, if one exists, could have its electron spins parallel. Each spinning electron is a little magnet producing a small magnetic field, or magnetic moment. With the spins antiparallel, the two magnetic moments cancel each other and the helium atom has no magnetic moment overall. Parallel spins mean parallel magnetic moments, which add together to give the atom a net magnetic moment. Kelleher's experiment would use magnetic fields to deflect and count paronic helium atoms. He told SCIENCE NEWS that he expected to hear in a few days whether NBS would fund the work.

In addition to producing unusual atomic states, the proposed violation of the exclusion principle relates to two important questions in modern particle physics, the many-dimensioned or Kaluza-Klein theories and the CPT theorem. Attempting to unify all of physics in one grand theory, theorists have postulated that space really has up to a dozen dimensions. We do not perceive the extra dimensions because they are tightly curved into microscopic balls around each point in ordinary space. The exclusion-principle violation can be related to the existence of these extra dimensions, and finding it could be evidence that they are really there.

The CPT theorem, one of the fundamental principles of physics, proposes that nature is symmetric with respect to matter and antimatter. If the exclusion principlke is violated, so is CPT.

Greenberg doesn't know what the ultimate effects of a violation of the exclusion principle might be, but as he said at NBS, "Small-scale phenomena can have large-scale effects."