John G. Learned, professor of physics in the department of physics and astronomy at the University of Hawaii, relays this answer:

"The nuclear reactions that power the sun produce neutrinos, uncharged subatomic particles that fly freely from the center of the sun to Earth and beyond. Four experiments have indicated that the flux of neutrinos coming from the sun is less than predicted by calculations based on the standard model of solar physics. A debate has raged now for more than 20 years as to the cause of this apparent neutrino deficit. Is it a flaw in the experiments? Is it a problem with the standard solar model? Are the neutrinos somehow disappearing en route to Earth, perhaps by decaying into another kind of unknown particle which is even harder to detect? Or, perhaps most strange of all, are the expected neutrinos mutating into different kinds of neutrinos during their eight-minute flight from the solar core (a hypothetical process called neutrino oscillation)? Current evidence somewhat favors this last explanation, but the case is still very much open.

"We at the University of Hawaii are participating in a new experiment in Japan, called SuperKamiokande. This project, which began operation on April 1, 1996, already has received neutrino signals from the sun and should be reporting significant results by the end of the year. By several means, Super Kamiokande may be able to find the 'smoking gun' indicating whether or not neutrino oscillations are hiding the neutrinos from the sun. Yet another new experiment, the Sudbury Neutrino Observatory (SNO), will begin operations next year in Sudbury, Canada. SNO will also help settle the question by collecting statistics on solar neutrinos using a somewhat different detection technique.

"Fortunately, the solar neutrino deficit does not seem to have any implications for Earth, at least not in the short term. Nothing indicates that the neutrino flux is decreasing nor that the rate of solar burning is slowing. Hence, there is no cause for concern that the sun has somehow 'switched off,' bringing on an ice age. There are indications of slight, long-term changes in solar output as occurred during the 17th-century Maunder Minimum, a period of depressed solar activity when temperatures in Europe were unusually cold; these appear to be transient phenomena, however.

"The sun burns, fear not, and it will not soon fail us. In fact, even if we learned that the nuclear fires at the center of the sun had completely extinguished because the flux of neutrinos ceased (a scenario impossible by any means we know), it would take a very long time very long time for anything visible to occur, because the photons we see take about a million years to diffuse to the surface of the sun. Humankind has many more imminent problems to face."

A. Baha Balantekin, an expert in nuclear theory and astrophysics at the University of Wisconsin, adds the following information, including a deeper exploration of the background physics:

"It probably is a good idea to talk about neutrinos first. More than half a century after their existence was first postulated, we finally seem to be getting close to understanding these elusive particles.

"The neutrino was originally proposed as a solution to a serious problem observed in the radioactive decays of certain atoms. In a process known as nuclear beta decay, one of the neutrons in the decaying nucleus is transformed into a proton, accompanied by the emission of an electron. The law of conservation of energy--one of the most basic principles of physics--indicates that all the electrons produced in this way should carry the same energy, determined solely by the mass difference between the neutron and the proton (that mass would be converted into its energy equivalent). In reality, however, the electrons emitted by beta decay carry a continuous range of energy; some of them carry all of the expected energy, but others emerge with hardly any energy at all. Somehow energy seems to have been destroyed.

"Wolfgang Pauli recognized that the observations could be explained by assuming that radioactive beta decays produce another, undetected particle--the neutrino--which shares the released energy with the electron. Later, the existence of neutrinos was directly confirmed by Frederick Reines and Clyde L. Cowan, Jr., who detected the inverse reaction, in which a neutrino is captured by a proton, producing a neutron. Subsequent experiments demonstrated that there are three different kinds of neutrinos, associated not only with electrons but also with muons and tau particles, more massive cousins of the electron.

"Neutrinos have at most a tiny mass and only feebly interact with ordinary matter. For a long time, therefore, little experimental information was available about the properties of these particles; nobody even knew for sure if neutrinos had any mass at all. That situation is starting to change. Strong indications of neutrino oscillations, which can occur only if neutrinos have a mass, are now seen in the observations of solar neutrinos, in studies of atmospheric neutrinos and in some recent particle accelerator experiments. Cosmologists are intrigued by these results; neutrinos are so common that they could actually make up a significant fraction of the universe.

"At about the time when Pauli postulated the existence of the neutrino, other researchers deduced the basic mechanism by which the sun shines. The sun's energy derives from nuclear fusion reactions taking place near its center. Most of this energy emerges as light and other forms of electromagnetic radiation from the solar surface, but about 3 percent of the energy is emitted as neutrinos. These particles interact so little with matter that they can escape directly from the solar core, where nuclear reactions are occurring. Electromagnetic radiation (such as light) takes as much as a million years to reach to the surface, whereas neutrinos arrive within seconds.

"The first experiment designed to look for these solar neutrinos is located in the Homestake mine in South Dakota. This detector has been collecting solar neutrinos for the past 25 years using a cleaning fluid target--every once in a while, a neutrino will interact with a chlorine atom in the fluid, transforming it into an argon atom. Much to everyone's surprise, the number of neutrino counts was significantly lower than expected. That pioneering experiment was joined by several others within the past decade. One of these is the Kamiokande detector located in the Japanese Alps. Kamiokande is a water Cerenkov detector: an incoming neutrino hits an electron in the water, making it move faster than light does through water. Just as a Concorde jet emits a shock wave when it starts moving faster than the sound in air, a high-energy electron traveling faster than light in water will radiate a kind of radiation shock wave, known as a Cerenkov cone of light. Unlike the Homestake experiment, Kamiokande could detect the incoming directions of the neutrino and prove that they are indeed coming from the sun. Kamiokande also found fewer neutrinos than predicted.

"Both Homestake and Kamiokande are sensitive to relatively high-energy neutrinos. Two other recent experiments using a gallium target (the GALLEX experiment in the Gran Sasso tunnel in Italy and the SAGE experiment in Southern Russia) have also detected the lower-energy solar neutrinos that make up the bulk of the solar neutrino flux. Physics calculations give more reliable predictions of the expected neutrino flux at low energies than at higher ones. But like the other experiments, GALLEX and SAGE are detecting fewer neutrinos than the models predict.

"These solar neutrino observations are not easily reconciled with the predictions of the standard solar model. All experiments observe a deficit, but the amount of the deficit appears somewhat different in each experiment (except the two gallium experiments, which agree with each other). Because the various experiments are sensitive to different neutrino energies, it seems that the amount of neutrino deficit is energy dependent.

"It does not seem to be possible to explain away the neutrino deficit by changing our assumptions about the physical makeup of the interior of the sun. Indeed, given the new observations, we can say that a solar neutrino problem exists independent of any detailed solar model--with 95 percent confidence, no combination of low-, medium- and high-energy neutrino fluxes fits the experimental data. A comparison of the fluxes detected at the various experiments supports a theory known as neutrino oscillations: that electron-neutrinos from the sun are transformed into other kinds of neutrinos (muon-neutrinos or tau-neutrinos) that are more difficult to detect.

"It does not seem likely that the neutrino shortfall is caused by a shutdown in the core of the sun. If the fusion were simply switching off--that is, if the sun were dying--the counts of all neutrinos (low-, medium- and high-energy ones) would be reduced by about the same amount, but that is not what is observed. Instead we seem to be discovering that neutrinos are more complex particles than we initially thought.