Pauli postulates hypothetical “neutron” particle

Before 1930, there was a baffling issue in nuclear physics: a certain nuclear decay called beta decay appeared to lose energy, which contradicted the law of the conservation of energy. So Wolfgang Pauli, age 30, had the idea that maybe there was a lightweight particle produced in the decay that was responsible for the missing energy. At the time, Pauli dubbed the particle a “neutron,” but the particle he hypothesized would eventually be named the neutrino.

On December 4, 1930, Pauli wrote a letter to a group of nuclear physicists outlining his idea for what is now known as the neutrino. It opened, “Dear radioactive ladies and gentlemen…” Read more about the letter here.

Scan of Pauli's famous letter from 1930 in which he postulates a hypothetical "neutron" particle that would be found to be the neutrino.
Credit: Pauli Letter Collection, CERN