© Daniel Eisenstein and the SDSS-III collaborationPhysicists used this three-dimensional map of galaxies in the universe, created by the Baryon Oscillation Spectroscopic Survey, to estimate a limit on how massive the lightest neutrino particle can be. Each dot in the image represents a single galaxy.
There are many strange things about neutrinos: their unexpected heft, for one thing, and that they rarely interact with other matter and are passing through our bodies by the billions each moment. Perhaps the oddest aspect of these particles is their tendency to switch identities, cycling between the three possible "flavors," or types. In fact, it was the observation of this shape-shifting ability in the first place that told scientists the three neutrino flavors must have different masses — which means, of course, that all of their masses cannot be zero.
Scientists would desperately like to know what they actually weigh, which would be a vital clue about why they have mass, given that they do not seem to acquire it the way other particles do: through the Higgs field (associated with the Higgs boson, which was discovered in 2012). "Understanding why particles have mass is something very fundamental in how we understand physics," says physicist Joseph Formaggio of the Massachusetts Institute of Technology. "What neutrinos pose is the possibility that the mechanism we think gives rise to masses for all the particles may not apply, for some strange reason, to neutrinos. I find that exciting."
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