B Neutrinos: Fast or Slow? Can We Observe It?

[planet-75]

TL;DR Summary

Can neutrinos also be in a state of slowness or rest?

As far as I know neutrinos are normally fast/ connected with a velocity close to the speed of light. This seems to be characteristic.
Can neutrinos also be in a state of slowness or rest? If yes, has there ever been an indirect observation of such cases?

 

[Orodruin]

Since neutrinos are massive, they travel at speeds lower than the speed of light. As such, you can always find a local inertial frame where a particular neutrino is at rest. However, the neutrino masses are very small and this leads to them traveling very close to light speed unless you somehow manage to give them very very little kinetic energy.

Note that there is no such thing as ”being at rest”. You can only be at rest relative to something else.

 

[mfb]

About 1 in 5 trillion neutrinos from tritium beta decay have an energy below 1 eV assuming small neutrino masses. Maybe 1% of these (so 1 in 500 trillion) would have a speed below 30% the speed of light. Tritium has a very low decay energy - for most beta decays the fraction is much smaller. If the constraints from cosmology are wrong for some reason then the fraction could be larger, but still very small.

Some neutrinos from the cosmic neutrino background can be non-relativistic today. PTOLEMY is a proposed detector to find them.