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the erstwhile solar neutrino problem [Q&A]


Yoshita of Poornaprajna College asks, what is the solar neutrino problem?

A byproduct of the nuclear fusion reactions in the core of the Sun are neutrinos. Because neutrinos don’t react with much, they escape from the core as soon as they are formed. The number of neutrinos produced is a very sensitive indicator of the conditions at the core, which in turn has a very strong influence on observable properties like the size of the Sun and the temperature at the surface. When these neutrinos were first being detected on Earth, their numbers fell short of those predicted, with only a third of the number being detected. The experimenters (Ray Davis, et al.) spent decades trying to resolve the discrepancy, but could find nothing wrong with their instrumentation. Eventually John Bahcall, following in the footsteps of Sherlock Holmes (“When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” — The Sign of Four), suggested that the problem lay in the physics of neutrinos.

Usually astrophysics takes ideas from physics, and it is not often that we have the opportunity to return the favor. In this case, it turned out that Bahcall was right. Neutrinos were eventually discovered to oscillate between different flavors. The other flavors were not detectable in Ray Davis’s experiment, which explains the observed neutrino deficit.

A new detector, the Sudbury Neutrino Observatory, has been built since then, and it is capable of detecting all the different flavors of neutrinos, and the observations are now consistent with predictions.