Another limit, calculated through similar methods by physicists Shouvik Roy Choudhury and Steen Hannestad, was also recently posted on and is now under peer review at the Journal of Cosmology and Astroparticle Physics. It’s quite a nice paper.” The results, initially posted to the preprint server, were published on August 22 in Physical Review Letters (PRL). André de Gouvêa, a theoretical physicist at Northwestern University, says, “It’s a slightly more detailed analysis of cosmological data than people had done before. “What they have done is really nice work,” says Olga Mena of the Institute of Corpuscular Physics in Spain, who has worked on similar calculations. The analysis also used laboratory data on neutrinos, such as measurements of the rates that they switch between flavors, to arrive at an estimate of the maximum weight of the smallest one: 0.086 electron volt, or 0.00000000000000000000000000000000000015 kilogram-making it at least six million times lighter than an electron. The new mass limit comes from a supercomputer calculation that combined data on the distribution of galaxies throughout the universe, the remnants of the first light released after the big bang and supernova measurements that reflect the expansion rate of the cosmos. “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. “Understanding why particles have mass is something very fundamental in how we understand physics,” says physicist Joseph Formaggio of the Massachusetts Institute of Technology. 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). 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. Perhaps the oddest aspect of these particles is their tendency to switch identities, cycling between the three possible “flavors,” or types. 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. Now a new calculation based on cosmological observations places an upper limit on how heavy the lightest kind of neutrino can be. They were predicted to be completely massless, but experiments roughly 20 years ago found they surprisingly do have some mass. Neutrinos, some of nature’s weirdest fundamental particles, are nearly massless-emphasis on nearly.
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