Solar Neutrinos are a very complex subject matter that defies anything like simple conclusions and assessments. Since attempting to deal with all the complex arguments, assessments, conclusions, data, evidence and so forth would be almost impossible in this setting, let me offer some reasons we should not rush to judgment. Here are snippets that reinforce what I am trying to say:
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But when detailed calculations of the expected neutrino flux were confronted with measurements, about 30 years ago, a significant discrepancy was found. Only about half of the expected neutrinos could be found. This anomaly persisted until quite recently, and is known as the "solar neutrino problem". For all the nitty-gritty details of the solar neutrino problem as it appeared before being solved, see John Bahcall's authoritative book Neutrino Astrophysics (Bahcall 1989), complemented by his more recent updates (Bahcall 1997a; Bahcall & Krastev & Smirnov 1998), and with a more accessible introduction in Bahcall (1990).
It is argued, in brief, that since the neutrino flux is wrong, there can't be enough fusion in the sun, in which case the sun can't keep shining for billions of years, so it must be recently created.
Speculations about the nature of the sun are as old as recorded history, but I shall not dwell on the more fanciful versions of old, as they are not pertinent to the solar neutrino issue.
The issue of where the sun's energy came from turned out to be a thorny one.
One of the leading theories of the formation of the sun was (and is) the 18th-century nebular theory of Kant and Laplace, in which the sun formed through the gravitational contraction of a large gas cloud. The potential gravitational energy of the cloud would be released as heat, as it contracted, and Hermann Helmholtz realized that this was a possible energy source for the sun, provided that it was still in the contracting phase. William Thomson (better known as Lord Kelvin) elaborated and promulgated this theory during the last decades of the 19th century. It was clear, however, that this energy source, while ample by human standards, couldn't last forever. Various calculations gave limits on the order of a few tens of millions of years of steady sunshine: "...it would, I think, be exceedingly rash to assume as probable anything more than twenty million years of the sun's light in the past history of the earth, or to reckon on more than five or six million years of sunlight for time to come" (Thomson 1889, p 369).
Alternatively, one might assume that star birth is an ongoing process, so that the sky is filled with stars of all different ages.
In the quest for a new energy source that took place in the early 20th century, radioactivity (discovered serendipitously by Henri Becquerel in 1896) played a prominent role
What are neutrinos?
Interestingly enough, the neutrino was first invented as an ad hoc hypothesis, in order to save the laws of conservation of energy and momentum from falsification. Around 1930, in the first detailed studies of radioactive beta-decays, it was found that some energy and momentum went missing in each decay. Beta decay involves the conversion of a neutron into a proton, accompanied by the emission of an electron, and nothing else visible. The energy carried away by the electron ought to match the energy released by the atom in the process – but it didn't! Wolfgang Pauli proposed to explain this discrepancy by postulating that an additional, invisible particle was emitted along with the electron, carrying away the missing energy and momentum. This "ghost particle" was named neutrino. (For some of Pauli's original musings about the neutrino, see Mössbauer (1998).)
Now, ad hoc hypotheses, invented purely to save our favorite theories, are generally frowned upon in science, and for good reason. But the neutrino hypothesis was ultimately vindicated, when the ghost particle was finally demonstrated to have a real existence, more than twenty years later. Today, the neutrino is well established as partner to the electron in our standard theory of elementary particles. It has the same basic properties as the electron, and participates in the same interactions, except that it lacks an electric charge, and has a nearly zero mass.
Solar neutrino experiments
The problem with solar neutrino experiments is that neutrinos are notoriously difficult to detect and measure. (After all, they were invented for the purpose of sneaking away unnoticed... ) The only way to detect them is through their occasional interactions with matter as they pass through. But the probability of such an interaction is extremely low; the vast majority of the neutrinos will pass straight through the earth without interacting at all.
Neutrino astronomers have invented a special unit, the SNU (solar neutrino unit), equal to one interaction per second per 1036 atoms (or equivalently, about one interaction per ton per year), that is convenient for solar neutrino studies. Expected interaction rates in realistic neutrino experiments amount to a few SNUs, up to 130 SNUs at best.
The flux of neutrinos observed in Super-Kamiokande is less than 40% of that predicted from standard solar models (Fukuda et al 1998c).
These four solar neutrino experiments (one chlorine, two gallium, and Super-K) were until recently the only ones. All show a significant deficit of neutrinos, measuring on the order of one third to one half the expected flux. Their measurements thus implied that either the standard solar model was wrong, or the standard model of particle physics (and thus neutrino behavior) was wrong.
Two important new solar neutrino experiments have reported results during the past two years. GNO (Altmann et al., 2000) is a new, larger gallium experiment, intended to add weight to the existing data.
On one hand, the number of electron neutrinos remains too low, same as the other experiments see
An even stronger argument against a solar solution, is the analysis of Hata & Langacker (1997). They show that even if one totally disregards the solar model, and allows the relative fluxes of neutrinos from different sources to vary freely, the neutrino discrepancies are not eliminated.
Neutrino oscillations
To have neutrinos disappear into thin air would be highly problematical, violating among other things the conservation of energy and momentum (the preservation of which was the main motive for inventing the neutrino in the first place). Converting the neutrinos into something else is a much more palatable solution. Luckily, there is ample precedent for such conversions among other elementary particles, and speculation about the possibility of similar behavior among neutrinos long predates the solar neutrino problem (Pontecorvo 1957). This conversion process is known as neutrino oscillations.
In particle physics, the equivalent of the suits are the three families, discussed above in the section 'What are neutrinos?'. A neutrino may belong to any one of the three families, making it an electron-neutrino, or a mu-neutrino, or a tau-neutrino. Or, it may be a superposition of the three family flavors, mixed in some proportions. Now, the standard model assumes that the neutrinos emitted from the sun are in a pure electron-neutrino state, with no mixing. If this assumption is wrong, however, interesting things may happen en route.
Neutrino oscillations is today the most promising of the proposed solutions to the solar neutrino problem. But until recently, the sun had provided no direct evidence that oscillations were indeed taking place.
