Starting in the late 1960s, neutrino detectors began to see signs that neutrinos, now known to come in the flavors electron (νe), muon (νμ), and tau (ντ), could transform from one flavor to another. The findings implied that neutrinos must have mass, since massless particles travel at the speed of light and their clocks, so to speak, don’t tick, thus they cannot change…..
Read more: http://physics.aps.org/articles/v4/57
