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Asked by anon-258065 on 3 Jul 2020.
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Julian Onions answered on 3 Jul 2020:
They do, but no one knows what it is – but it is very small in any case. Someone with more expertise will be along shortly I’m sure!
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Scott Lawrie answered on 3 Jul 2020:
We see neutrinos oscillate, which means they change from one type to another. So if you start off with an electron-neutrino, after a few hundred km, it will have changed into a muon-neutrino or a tau-neutrino. Neutrinos are the only particles we know of that change like this. It’s very wierd. The only way they can do this change is because they have a very small mass: 500,000 times smaller than the (already very lightweight!) electron.
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Joel Goldstein answered on 8 Jul 2020:
Expanding on previous answers a bit:
Scientists have been trying to measure the masses of the different types of neutrinos for years, but all direct measurements so far have been consistent with zero. For a long time, therefore, we said that if neutrinos have masses they must be too small to measure (much, much smaller than the electron mass, and the electron is by far the lightest of particles currently known other than those which are massless).
Now, however, we have indirect evidence that neutrinos have mass because they change behaviour over time, a very strange effect known as neutrino oscillations. If neutrinos were truly massless they would travel at the speed of light and therefore (in accordance with relativity) could not change at all. The rates at which the oscillations happen depend on the differences in mass between the different types of neutrino, and these differences can therefore be deduced but are very tiny indeed.
So now it looks like neutrinos have masses, but these are probably roughly a hundred million times smaller than the mass of the electron.
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