Neutrino

Neutrinos are elusive subatomic particles that result from certain nuclear reactions. Neutrinos have no electrical charge and only a tiny mass, travel at nearly the speed of light, come in three types—electron neutrinos, muon neutrinos, and tau neutrinos—and barely interact with normal matter. Because their interaction rate is so low, neutrinos produced in the core of the Sun fly directly out through the outer layers of the Sun and flood surrounding space, providing direct (though hard-to-intercept) information about nuclear reactions in the Sun's core. In 1968, however, when scientists first tried to detect electron-type neutrinos emitted by the Sun, they found less than half those expected from the then-current theory of nuclear reactions in the Sun. This shortage was known as the solar neutrino problem, and was only resolved in 2001 by elaborate experiments that proved that the Sun is in fact, producing the number of neutrinos predicted by theory, but that some of these neutrinos change type (electron to muon or tau) en route from the core of the Sun to detectors on Earth. The total number of neutrinos detected on Earth is in accord, as it turns out, with the standard model of the solar core.