We haven’t pinned down the masses of any individual neutrino, and we don’t even know which ones are heavier than the others. When it comes to our ability to collect raw data, neutrinos present a triple threat: they’re incredibly lightweight (even the electron weighs over 5 million times more than all the neutrinos combined), they shift their identity as they travel (and their rate of flavor oscillation changes as they travel through different substances, so there’s no one-size-fits-all solution), and they barely interact with anything in the first place...