Date: Prior to 1993
What exactly are neutrinos and why are they important?
The existence of neutrinos was postulated to explain what was
observed when a nucleus undergoes beta decay via electron emission (a neutron
in the nucleus changes into a proton, and the nucleus emits an electron).
Observation seemed to indicate that, in general, neither momentum nor total
energy was conserved in such a reaction. This was, understandably,
disturbing. In 1931 the physicist Pauli proposed that another, unseen
particle was also being emitted in the reaction, accounting for the "missing"
momentum and mass-energy. Fermi named this particle neutrino" meaning "little
neutral one". The name reflected the assumption that it had no electric
charge (so that the law of charge conservation would remain intact);
additionally, it was postulated to have zero spin (keeping the law of
conservation of angular momentum) and to interact only weakly with matter (to
explain why it was virtually undetectable). Eventually the neutrino was
detected, in the early 1950's, through nuclear reactors (which produce huge
numbers of neutrinos). Currently it is believed that there are three kinds of
neutrinos (plus their antiparticles, making a total of six) -- the electron-
neutrino, the muon-neutrino, and the tau-neutrino. Neutrinos are considered
to be the force-carriers associated with the fundamental force called the weak
force. The weak force is very important -- it contributes to the production of
elements of higher atomic number, without which life as we know it would be
And they are important in astronomy for several reasons:
supernovas emit huge quantities of neutrinos (according to theory). Supernova
1989A, which was nearby in one of the Magellanic clouds, was detected by
several neutrino detectors that had been designed and built to detect
neutrinos from the sun: the best theories of how the sun works predict three
times the number of neutrinos than are actually detected! Neutrinos may play
a major role in the dark or missing matter problem of cosmology.
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Update: June 2012