Of all the known particles in the universe, only photons outnumber neutrinos. Despite their abundance, however, neutrinos are hard to catch and inspect, as they interact with matter only very weakly. About a thousand trillion of the ghostly particles pass through your body every second — with nary a flinch from even a single atom.

“The fact that they’re ubiquitous, yet we don’t even know what they weigh, is kind of crazy,” said Deborah Harris, a physicist at the Fermi National Accelerator Laboratory near Chicago and York University in Toronto.

Physicists have long tried to weigh the ghost. And in September, after 18 years of planning, building and calibrating, the Karlsruhe Tritium Neutrino (KATRIN) experiment in southwestern Germany announced its first results: It found that the neutrino can’t weigh more than 1.1 electron-volts (eV), or about one-five-hundred-thousandth the mass of the electron.

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