Neutrinos are the lightest of the massive fundamental particles in the Standard Model. Why are neutrinos so small, how do they get their mass, why do they have mass at all, and how much do they actually weigh? These are some of the questions that keep neutrino physicists up at night. Because neutrinos are so small and interact so rarely, it’s a difficult puzzle to crack. It’s not easy to build an experiment that can answer this mystery. This is already shocking physicists’ best model of the universe (called the Standard Model) predicts that neutrinos should be massless. Put another way, a neutrino is 10 billion, billion, billion times smaller than a grain of sand. For comparison, one electron has a mass of 511,000 electronvolts. The current best estimate says that the sum of the masses of the three neutrinos should be below about one electronvolt. Particles are often measured (and weighed) in units called electronvolts. What we do know is that the three known types of neutrinos have different masses and that the sum of all three of those types is still less than one millionth the mass of an electron. Experiments showing that neutrinos change type proved that wasn’t the case, but we still don’t know the absolute mass of the neutrino. For many years, scientists thought they were massless. It’s a very basic question, and something we can answer about every other particle: How much does it weigh?
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