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Re: [Phys-l] The speed of neutrinos

On 10/01/2010 11:45 PM, Derek McKenzie wrote:
sed, it is usually asserted that they travel very close to the speed
of light, which was responsible for people thinking they were
probably massless. But the statement 'close to the speed of light' is
meaningless. A particle is either traveling at the speed of light or
it is not, and there are no degrees of closeness to it. So this
choice of phrase obviously means that physicists always observe
neutrinos to be traveling close to the speed of light in their
lab-frame. But how can *all* neutrinos, no matter how and where they
were produced, appear in the lab-frame to be traveling close to the
speed of light? Surely there should be plenty of neutrinos floating
around at nice slow speeds relative to the E

That depends on what you mean by "slow".

Try this:"cold+neutrinos";

which leads to:

... which I can summarize: In analogy to the cosmic background
microwave radiation, there must be cosmic background neutrinos,
with an energy on the order of a few kelvin. Given plausible
estimates of neutrino mass, they will have speeds on the order
of 2e7 m/s. That is non-relativistic, but not exactly slow on
the human scale, and not even slow compared to planetary,
stellar, or even galactic escape velocities.

It hardly matters how many of them there are, because you will
never see them. Neutrino cross sections scale like energy.
It takes heroic measures to see the high-energy ones, and the
low-energy ones are out of the question for now, and for a
long time to come.

Any neutrinos produced recently (on earth, or in any known
star, or in any known galaxy) will be moving at very nearly
the speed of light relative to the lab frame.