Neutrino are exotic subatomic particles whose existence was deduced when physicists noticed that the subatomic event called beta decay didn't add up. Mother Nature always balances her books, the law of conservation of mass-energy forces that.

(Well, technically the Uncertainty principle allows one to embezzle mass-energy as long as you pay it back in the few nanoseconds before the cosmic accountant checks the ledger. But I digress.)

So with all nuclear reactions, the sum of all the starting particles mass-energy before the reaction must be exactly equal to the sum after. The problem with beta decay is that they weren't.

In 1930 physicist Wolfgang Pauli said "I've got it! What if beta decay produces some as-yet undiscovered weird invisible particle with properties that will exactly balance the books? We just thought things were not balancing because we couldn't see the blasted thing."

The other physicists rolled their eyes at Pauli. This sounded too much like your infant son telling you that he didn't break the lamp, it was an invisible green monster trying to frame him. Violates Occam's razor, that does.

Twenty-six years later Pauli was vindicated when Clyde Cowan and Frederick Reines finally managed to detect the weird invisible particle.

Why did it take them so long? Because neutrinos are real invisible. The slippery little devils can pass through about one entire light-year of solid lead before it hits a lead nucleus. They are beyond elusive, but can be detected by a sufficiently sensitive detector. These are typically 1,000 metric tons of ultra-pure water in a tank coated with photocells buried deep underground in an abandoned mine.

Since neutrinos are so darn penetrating, they can be used to observe astronomical objects. In 1987 a couple of neutrino detectors accidentally spotted a few from Supernova 1987A. The neutrinos arrived about two hours before the visible light of the supernova. This is because the neutrinos were created by the initial stellar core collapse, while the light was not created until two hours later when the shock-wave reached the star's surface. The important point is that neutrinos can be used to observe conditions inside the cores of stars.

Which leads to this Weird Astronomical.

In 1970 astrophysicists Raymond Davis, Jr. and John N. Bahcall figured they could use a neutrino detector to measure the rate of nuclear fusion in our primary star Sol. The Sun in our sky that gives us daylight. A single photon of light created by a fusion reaction in the core of Sol can take between 100,000 years and 50 million years to gradually work its way to the surface of Sol, then 8 minutes more to travel to Terra. But the slippery neutrino acts like the body of Sol ain't there. It takes a mere 2.3 seconds to reach the surface of Sol, and 8 minutes more to reach Terra.

This will allow a much more current report on the state of affairs at Sol's core. 2.3 seconds instead of 50 million years.

So Davis and Bahcall set up a 100,000 gallon tank of perchloroethylene 1,478 meters underground in the Homestake Gold Mine in Lead, South Dakota.

Bahcall had done the theoretical calculations on how many solar neutrinos would reach the detector. Davis used the detector to count the number of neutrinos that actually arrived. That's when all the fun started.

The trouble was that the actual number of neutrinos detected was consistently about one-third the number predicted by Bahcall's calculations. Now, keep in mind that such trouble in Science is actually a good thing. Isaac Asimov noted "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny ...'"

Properly following the dictates of the scientific method, the scientific community immediately declared that either Davis, Bahcall, or both had made a mistake. Both Davis and Bahcall double checked their work, but could find no errors.

The next step in the scientific method is for others to try and recreate the experiment and recreate the experimental results. Kamiokande in Japan, SAGE in the former Soviet Union, GALLEX in Italy, Super Kamiokande, also in Japan, and SNO (Sudbury Neutrino Observatory) in Ontario, Canada all tried it. Lo and behold, they got the same results. There were only one-third of the predicted amount of neutrinos coming out of Sol.

When you ask the question Why? is when things started getting edgy.

The two possibilities are: neutrinos are not understood as well as they thought or Sol is not understood as well as they thought. Or both.

The nuclear physicists shrugged, and promised to take a closer look and neutrino theory to see if there was anything they'd overlooked. If they found anything, particle physics would be updated with the new information. No big deal, science marches on and all that.

The solar astronomers started to sweat. The experimental results could mean that the rate of solar thermonuclear fusion had been drastically decreased by two-thirds. Which means the energy Terra receives from Sol could suddenly drop by two-thirds, at any moment from 50 million years in the future to eight minutes from now. The decrease would not be immediately apparent due to the 50 million year time lag of solar photons escaping the body of Sol. In that case the only question is how many millions of years in the past did the fusion rate stop?

Of course when the solar energy drops by two-thirds, Terra will die. Colder than a sno-cone in Niflheim.

Sir Arthur C. Clarke used this nifty situation as the background for his 1986 novel The Songs of Distant Earth. And there were a few popularization of science articles published with arresting titles along the lines of SCIENTISTS ARE UNSURE IF THE SUN WILL RISE TOMORROW.

The situation was resolved 2001 when the results came in from the Sudbury Neutrino Observatory (SNO) in Canada, to the relief of astronomers and the dismay of science fiction authors.

Since way back in the 1970's the Standard Model of particle physics predicted that there were three kinds of neutrinos, ordinary garden-variety electron neutrinos plus the exotic muon neutrinos and tau neutrinos. Solar fusion produced electron neutrinos but none of the other two.

Clever readers will have already noticed the coincidence between the number of different neutrinos being three, and the neutrino detectors finding only one third of the expected number of solar neutrinos.

Back in 1957 Bruno Pontecorvo proposed the theory of Neutrino oscillation. It predicted that a neutrino of one type could spontaneously transmute into another type, then another type as it shot through space. This was an interesting conjecture, but it was not for decades that physicists could figure out an experiment which could detect this.

Finally several experiments did. Ironically one of them was was the data from the previously mentioned Sudbury Neutrino Observatory looking for solar neutrinos. As it turns out, most of the other neutrino telescopes could only detect electron neutrinos but not the other two kinds.

The Super-Kamiokande collaboration in Japan produced evidence strongly suggesting that it was seeing cosmic-ray created muon neutrinos transmuting into tau neutrinos.

But the SNO used heavy water as the detection medium. This allowed it to detect all three types of neutrinos. It could not distinguish between muon neutrinos and tau neutrinos, but it could see both. But it could distinguish between electron neutrinos and the other two. In 2001 it showed that 35% of the arriving solar neutrinos are electron neutrinos, with the others being muon- or tau-neutrinos (invisible to most neutrino detectors). Notice that 35% is quite close to one-third. If the muon- and tau-neutrinos represented solar fusion electron neutrinos that had transmuted, then the rate of solar fusion was as predicted, and all's well with the world. Sol is not going to unexpectedly go ppssssst!, turn dark, and condemn the world to an arctic death.

Alas, yet another thrilling science-fictional idea first offered by Science but then snatched away.