The conventional way physicists describe neutrinos is that they have a
very small amount of mass which entails they are traveling close to the
speed of light. If you don't believe me, here's a Wikipedia quote which
is also reflected in many textbooks:
"It was assumed for a long time in the framework of the standard model
of particle physics, that neutrinos are massless. Thus they should
travel at exactly the speed of light according to special relativity.
However, since the discovery of neutrino oscillations it is assumed that
they possess some small amount of mass.1 Thus they should travel
slightly slower than the speed of light..." -- Wikipedia (Measurements
of Neutrino Speed)
Taken at face value, this is surely total nonsense. If a particle has
mass (no matter how small), its speed is completely relative, and to say
that neutrinos travel close to the speed of light, without
qualification, is just as incorrect as saying electrons or billiard
balls travel close to the speed of light.
So what is the reason everyone repeats this nonsense? Is it because all
the neutrinos we detect in practice travel close to the speed of light?
If so, then I have this question:
Neutrinos come at us from all directions and from all sorts of sources
(stars, nuclear reactors, particle accelerators, etc.), and since they
have mass, just like electrons, I would have thought we should see them
traveling at all sorts of speeds. (Surely some cosmic neutrino sources
are traveling away from the earth at very high speeds, for example. Or
what about neutrinos emitted from particles in accelerators?)
So like I said at the start: Where are all the slow neutrinos? And why
do we perpetuate this misconception of there being such a thing as
'close to the speed of light'?