Solar neutrino problem

The solar neutrino problem was a major discrepancy between measurements of the neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002. The discrepancy has since been resolved by new understanding of neutrino physics, requiring a modification of the Standard Model of particle physics. Essentially, if neutrinos do have mass, then they can change from the type that had been expected to be produced in the sun's interior into a type that would not be caught by the detectors in use at the time.

History of the problem

As neutrino detectors became accurate enough to measure the flow of neutrinos from the sun, it became clear that researchers weren't getting as many of them as the models of solar combustion predicted. In various experiments, the number of detected neutrinos was between 1/3 and 1/2 of the predicted number. Therefore either the current models of the sun were wrong or the models of neutrino behavior were wrong. This came to be known as the solar neutrino problem. As one researcher put it, "we must trust the measurements, because even if they are wrong by three standard deviations, theory is incorrect. To give you an idea of how large three standard deviations of error is, if a graduate student made such a bad measurement, it would cause him to be dismissed. If a professor made such a bad measurement, it would cause him to be made head of the department."

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The problem was troubling because it meant that either General Relativity was incorrect, models of stellar evolution were incorrect, or the "standard model of physics" was incorrect. Since each of these models had proven remarkably accurate, the choice was an unpalatable one.

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Early attempts to explain the discrepancy proposed that the models of the sun were wrong, i.e. the temperature and pressure in the interior of the sun were substantially different from what was believed. For example, since neutrinos measure the amount of current nuclear fusion, it was suggested that the nuclear processes in the core of the sun might have temporarily shut down. Since it takes thousands of years for heat energy to move from the core to the surface of the sun, this would not immediately be apparent. However, these solutions were rendered untenable by advances in helioseismology, the study of how waves propagate through the sun. Based on such observations it became possible to measure the interior temperatures of the sun and these agreed with the standard solar models. There are unresolved problems of the structure of what was found with helioseismology. Instead of the old pot on the stove vertical convection, horizontal jet streams were found in the top layer of the convective zone. Small ones were found around each pole and larger ones extended to the equator. As you might expect, they had different velocities.

Related Topics:
Temperature - Pressure - Helioseismology

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Currently, the solar neutrino problem is believed to have resulted from an inadequate understanding of the properties of neutrinos. According to the Standard Model of particle physics, there are three different kinds of neutrinos: electron-neutrinos (which are the ones produced in the sun and the ones detected by the above-mentioned experiments), muon-neutrinos, and tau-neutrinos. In the 1970s, it was widely believed that neutrinos were massless and their types were invariant. However, theoreticians in the 1980s realized that if neutrinos had mass then they could change from one type to another. Thus the "missing" solar neutrinos could be electron-neutrinos which changed into other types along the way to Earth and therefore escaped detection.

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The television show Law & Order had an episode that used the solar detection problem and the physics involved as a plot device.

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