Neutrinos

Episode #8 of the course “Strangest Things in Space”

Neutrinos are tiny subatomic particles that carry no electrical charge and thus weakly interact with other particles. The idea of neutrinos was first proposed by Wolfgang Pauli in 1930. For a long time, neutrinos were speculated to be massless (as impossible as that sounds), but scientists have been able to confirm their infinitesimal mass. Another early confusion surrounding neutrinos was that there was only believed to be one kind, but today we now classify them in three categories: electron neutrinos, muon neutrinos, and tau neutrinos.

The way in which scientists got from one neutrino to three is part of the great mystery surrounding them: neutrinos seem to remember their origins. In an experiment in 1962, a neutrino was created at the same time as a muon. The neutrino was then shot through an atomic nucleus where it changed into a muon. This led scientists to conclude that the neutrino remembered how it was made. Not once during their experiment did a neutrino created with a muon change into an electron or tau particle.

Because neutrinos do not carry an electrical charge and are therefore not affected by electromagnetic forces that act on charged particles, a neutrino typically passes through normal matter unimpeded. In fact, billions of neutrinos pass through the earth every second, most emanating from the sun.

Neutrinos can be created in several ways—in nuclear reactions such as those that take place in the sun, during certain types of radioactive decay, in man-made nuclear reactors, or when cosmic rays hit atoms. In spite of their miniscule mass, neutrinos are so plentiful in the universe that they help to account for the mysterious 27% of the universe that is dark matter.

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