In December 1933, Fermi formulated his revolutionary theory of beta decay. The nucleus, consisting of only protons and neutrons, undergoes beta decay when a neutron turns into a proton, simultaneously emitting an electron, called a “beta ray”, and a neutrino, both created during the decay process and previously not existing in the nucleus . Fermi thus introduces a new type of fundamental interaction: the weak interaction. Fermi’s explanation of beta decay represents his major contribution to theoretical physics: both the possibility that a particle changes its identity and the assertion that in addition to gravity and electromagnetism there is a third force, will be central to the development of nuclear physics.
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