NEUTRINOS. “The standard model requires them to have non-zero mass in order to explain their experimentally observed properties. But no experiment has been able to measure this mass directly, and it must be very small—no more than about one-millionth of the mass of the electron, which is the next lightest particle. Therefore this (hypothetical) non-zero mass is a prediction of the standard model that has not yet been confirmed by experiment. While most physicists regard this prediction as a solid fact, I do not. There is simply not enough physical evidence to support this hypothesis, in my opinion.”
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