Neutrino Mass

Since their proposed existence in the 1930s, neutrinos and antineutrinos of all three lepton families have been thought to have zero mass and travel at light speed to conserve energy and angular momentum in nuclear decays. In 1969 came the first hints that at least one type of neutrino can become another type of neutrino, and a neutrino oscillation scheme was proposed. We now know that muon neutrinos created in Earth’s atmosphere can oscillate into electron neutrinos and tau netrinos before reaching an underground detector. Why cannot all three neutrino types still have zero mass?

Answer

For a change in a system to occur such as the change of a muon neutrino to an electron neutrino, for example time must elapse. That is, the reference clock must tick in the rest frame of the muon neutrino. We know that the greater the velocity of a real clock in our laboratory reference system, the slower is its ticking rate. In the speed limit of a massless particle such as a photon traveling at light speed, the clock would not tick. As a photon traverses the universe, no time elapses in its reference system. The photon can be absorbed by an atom and disappear, but the photon cannot change directly into another photon. Likewise, if all three neutrino types did not have any mass, none could oscillate into another neutrino type because they do not experience the passage of time. Therefore, for neutrino oscillations to occur, at least two neutrino types must have mass. The data indicate that the sum of the three neutrino masses cannot exceed about 1 eV/c², very much smaller than the 0.511 MeV/c² mass of an electron.