Thanks to an elusive little particle called the neutrino, physicists expect they may soon make a breakthrough in their understanding of matter. What's more, they don't need a new multibillion-dollar accelerator to do it.
The sun provides their experiments in the form of emissions of neutrinos. All physicists need do is refine their ability to read the neutrinos' message. That's what equipment being readied in several neutrino observatories around the world is designed to do. It is the latest -- and possibly most fruitful -- phase of research, begun three decades ago, into the power source of the sun.
In what physicists call the standard theory of matter's basic structure, neutrinos are electrically uncharged particles that have no mass and that carry energy and momentum away from particle interactions at the speed of light. There are three types of neutrinos. They are paired, respectively, with the electron and its electron-like siblings, the muon and tau particles.
The ghostly neutrinos interact so weakly with other material particles that millions of them pass through our bodies every second and we never know it. That's what makes them intriguing to solar astronomers. The sun and sun-like stars draw power from the fusion of hydrogen atoms deep within their cores. It takes millions of years for energy produced in that fusion to reach the sun's surface and emerge as sunlight. But fusion-produced neutrinos zip out of the sun at light speed.
Astronomers can't see into the sun's 15 million-degree hot core. But solar neutrinos carry detailed up-to-date information on the hydrogen-fusion furnace. Physicists have been tapping that information by capturing a few of the billions of solar neutrinos in tanks of cleaning fluid or water or in masses of gallium.
This has helped those scientists develop a successful, well-tested theoretical model of the sun. It also has produced a conundrum. Their observatories detect a smaller neutrino flow than the solar theory predicts. This is where the possibility of discovering ''new physics'' lies.
Skeptics have questioned the accuracy of the neutrino measurements. However, the neutrino deficit has been found repeatedly by different observatories using different techniques over three decades. Also, the international observatory at Gran Sasso in Italy recently reported a test of its equipment using a radioactive source with a known neutrino flux. It measured that flux accurately.
John Bahcall at the Princeton Institute for Advanced Studies in New Jersey -- an author of the solar theory -- says he used to be skeptical of the neutrino deficit but has changed his mind. He now believes that the deficit is real and points to ''new physics.''
The favored possibility is that neutrinos can change their spots. They can oscillate between the electron, muon, and tau types. Up to now, the observatories could detect mainly electron-type neutrinos. If some of the expected electron neutrinos had become another type, that would account for the observed deficit. Over the next two years, new equipment at several observatories should be able to detect all three neutrino types and settle this question.
If neutrinos can change type, physicists say that would mean that at least some neutrino types have a small amount of mass. This, again, is not allowed by the present standard theory of matter. However, experiments by D. Hywel White and colleagues using a small particle accelerator at the Los Alamos National Laboratory in New Mexico suggest -- but do not yet prove -- that neutrinos do have a small amount of mass.
If neutrinos do have mass and therefore can oscillate between their three different types, this will give physicists new facts with which to extend their theory of matter. It also could account for at least some of the so-called invisible matter that astronomers say exists throughout the universe.
As Dr. Bahcall has observed, it may soon turn out that the long effort to use solar neutrinos to test theories about the sun has unexpectedly opened a new window on particle physics.