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Re: [Phys-L] Where are all the slow neutrinos?



Thanks very much John. Your answer looks like it will satisfy my
curiosity about why we don't 'see' slow neutrinos, and I'll follow up
the reference pronto!

Meanwhile though, I still have a serious pedagogical issues with the
language, and I don't believe I'm just being pedantic.

Whether you can detect the slow neutrinos or not, they still exist (in
the terrestrial frame), and so talking collectively about neutrinos as
traveling close to the speed of light, even assuming the terrestrial
frame, is just empirically wrong.

But worse still is the implication that traveling close to the speed of
light (without contextual qualification) means anything. Physicists (I
like to think) know what they mean by this statement, but wouldn't you
agree it's bad phraseology in classrooms and textbooks?

Students already think that strange things happen to you as you approach
the speed of light, as if that were any different to any other speed.

Also, the Wikipedia quote I mentioned is even worse (and I've seen this
before), implying that the high speed follows logically from its low
mass. We could make good sense of that if you were given the energy of
these things as a constraint, but as it stands?

Derek McKenzie
http://PhysicsFootnotes.com



-------- Original Message --------
Subject: Re: [Phys-L] Where are all the slow neutrinos?
From: John Denker <jsd@av8n.com>
Date: Fri, July 08, 2016 11:20 pm
To: Phys-L@Phys-L.org

On 07/08/2016 02:48 PM, Derek McKenzie asked:

why do we perpetuate this misconception of there being such a thing
as 'close to the speed of light'?

It's like a 55 mph speed limit. The speed is, by convention, relative
to the local terrestrial frame.

Where are all the slow neutrinos?

Two answers:
-- If you had a fast neutrino, it would be hard to slow it down,
because
of the small cross-section.
-- If you had a slow neutrino, you would never be able to see it.
Seeing
the fast ones takes heroic efforts, and the cross section goes down
even more as the energy goes down.

Searching for
https://www.google.com/search?q=neutrino+%22inverse+beta%22+%22cross+section%22+energy

immediately turns up
J.A. Formaggio, G.P. Zeller
"From eV to EeV: Neutrino Cross Sections Across Energy Scales"
https://arxiv.org/pdf/1305.7513v1.pdf

which will tell you what you wanted to know.
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