Massless particle has substance after all

Neutrinos -- a supposedly massless kind of particle originally invented to help nuclear physicists balance their accounts -- now appears to be an important part of the mass of the universe.

If this prelimary discovery is confirmed, it could have significant, although not revolutionary, effects on theories in particle physics, astrophysics, and cosmology.

Among other things, it might resolve the question of whether the universe will expand forever or will ultimately halt its expansion and begin to collapse upon itself. Right now, astronomers cannot identify enough mass in the universe to provide the gravitational force needed to reverse the expansion. Neutrinos are believed to be one of the more abundant types of particle in the cosmos. The new findings suggest they could supply the "missing mass," as cosmologists call it, needed to "close" the universe.

Neutrinos first appeares on the particle scene as a "fudge factor" in physicists' calculations. In certain types of radioactive nuclear decay, small amounts of momentum and energy seemed to disappear.This would violate laws of conservation of energy and momemtum. So in 1931, physicist Wolfgang Pauli suggested that an unknown particle formed in the decay process carries away the missing momentum and energy. It would have no measureable mass when at rest, no electric charge, and negligible magnetic properties. Three years later, Enrino Fermi elaborated the concept and the neutrino (little neutral one) was born as a useful theoretical entity.

However, neutrinos would interact only weakly with matter. They could zip through the Earth as though it were not there. It was hard to prove they actually do exist. But in 1955, Frederick Reines and the late Clyde Cowan demonstrate the reverse of the process that produces neutrinos -- an interaction involving the absorption of neutrinos -- thus showing the particles to be real. Since then, physicists have found that neutrinos come in several different varieties depending on the processes in which they are involved.

Now Reines and his colleagues Henry Sobel and elaine Pasierb at the University of California's Irvine campus have evidence suggesting that the "little neutral one" has a small rest mass after all. This is an indirect conclusion. Their evidence suggests that neutrinos can change type, oscillating between one form of neutrino and another. Theoretically, they can only do this if they have some rest mass.

The experiment is not clear-cut and needs confirmation. A different type of experiment involving possible neutrino oscillation at the Institut Laue-Langevin in Grenoble, France, has so far found no sign of any identity changes. On the other hand, unconfirmed reports from the Soviet Union tell of experiments at the Institute of Theoretical and Experimental Physics in Moscow in which neutrino masses on the order of 12 to 40 electron volts have been measured -- roughly 43, 000 to 13,000 times less than the mass of an electron (511,000 eV).

With this much uncertainty, physicists will wait for more solid evidence before committing themselves to redoing particle theory or cosmology. But it may be that a particle once invented as a theoretical convenience will turn out be a major component of the universe.

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