In a revolutionary finding that may play a crucial role in understanding how stars formed and the universe will evolve, physicists announced they have evidence that the neutrino - an elusive, subatomic particle - has mass, or weight.
If substantiated, the discovery by an international team of scientists at an underground lab in Japan could force rethinking of a central theory of modern science: That the universe began expanding at the "Big Bang" and will keep expanding forever.
It also may help solve the mystery of why the universe isn't as heavy as science says it should be.
Neutrinos are one of the basic building blocks of atoms and, in turn, everything else. They are also particularly elusive. Neutrinos have no charge and are so small that they pass right through matter, making them very tricky to track.
Physics' "standard model" theory of how fundamental particles work decrees that neutrinos should weigh nothing at all.
"We thought we had an inviolable law, but here we've got a counter-example," said Louisiana State University physicist Robert Svobodoa, who is on the team of 120 scientists in Japan.
The scientists, working on a project known as the Super-Kamiokande Experiment, believe they have found evidence that proves that part of the theory needs to be reconsidered. They can't measure a neutrino's mass, but have evidence that strongly suggests it has some.
"This is something that physicists have hoped for and eagerly sought for decades," said John Bahcall, an astrophysicist at the Institute for Advanced Study and a visiting professor at Princeton University.
"These new results could prove to be the key to finding the Holy Grail of physics, the unified theory," University of Hawaii physicist John Learned, a leader in the research effort, said in Thursday's Honolulu Star-Bulletin.
The unified theory is an effort to combine in one set of mathematical equations all of the fundamental forces in nature: gravity; electromagnetic energy, such as light; and the interactions between subatomic particles. The concept was first proposed by Albert Einstein, but, so far, the search for a unified theory has failed because of uncertainties - such as the mass of neutrinos.
Neutrinos come in three types (physicists call them flavors), named according to what other subatomic particles they hang around with. There are electron-neutrinos, muon-neutrinos, and tau-neutrinos.
The scientists in Japan say they have detected that some neutrinos oscillate, or change type, as they pass through space or matter. And another widely accepted theory of physics says anything that oscillates must have mass.
"We are very confident that this proves it," said Chang Kee Jung, member of the project team and a professor of Physics and Astronomy from the State University of New York at Stony Brook.
Neutrino mass could help explain a thorny problem: why the eight of the planets, stars and all other known stuff in the cosmos, when added together, is so much less than the model of the universe predicts.
The visible universe makes up barely 10 percent of that theoretical matter, leading astronomers to conclude that much more of it is hidden in mysterious dark spots or that the theory is seriously flawed.
The mass of an individual neutrino is likely infinitesimally small, perhaps billions of times less than an electron, the particle with the smallest known mass. But there are so many neutrinos that, together, they could account for a lot of the universe's mysteriously missing mass.
A neutrino with mass also could have played a crucial role in the formation of celestial objects, in their movement during the billions of years since the universe was born and in how the universe will evolve.
In calculating the expansion believed under way since the "Big Bang," scientists now will have to factor in a neutrino with mass and decide whether that changes their understanding of the fate of the universe.
The group in Japan, which includes researchers from Japan, the United States, Germany, and Poland, detected neutrinos using a 50,000-ton tank filled with highly purified water down a half-mile-deep mine shaft in the central Japan city of Takayama.
Surrounding the tank are 13,000 photomultiplier tubes that detect the faintest flash of light. The apparatus was buried in the mine to block cosmic rays and radiation.
Nuclear fusion at the sun's core produces neutrinos which are then hurled this way at the speed of light. The scientists hoped that at least a few of the trillions of particles that hit Earth every second would pass through their tank, collide with an atom there, and set off a little flash of light they could detect and measure.
Their conclusion that neutrinos have mass came after a year of data collection. Statistical analysis of the data, Jung said, strongly suggested that some of the neutrinos had changed type.
The researchers likely will have to wait for another laboratory to repeat their experiment and findings before it is accepted as scientific fact. The Sudbury Neutrino Observatory, 200 miles north of Toronto, is conducting similar experiments; only a handful of such facilities exist.
Written by Todd Zaun
©1998 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed
© 1998 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed.