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Why so little neutrinos?

  1. Apr 24, 2008 #1
    If the sun produces so many neutrinos from fusion, then I would assume that so do all other fusion stars, yet why is it that our detectors detect only the number of neutrinos that would be expected to be produced from our sun? If neutrinos travel at nearly the speed of light and barely ever interact with anything, then shouldn't we be constantly bombarded by an endless number of them coming from the trillions of stars in the universe?
  2. jcsd
  3. Apr 24, 2008 #2
    I think this is basically the same question as in Olbers paradox - if there are so many stars out there, why is the sky not bright in the night ? you just ask the question for neutrinos instead of photons.

    maybe we should call this one "moe darklights paradox" ?
    Last edited: Apr 24, 2008
  4. Apr 24, 2008 #3


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    yes, it is basically the same question.

    It is also related to intensity and flux. A star 5Ly away will radiate over the whole solid angle and only a tiny tiny tiny tiny fraction will reach earth, and only a tiny tiny tiny fraction will interact here. So the main component of neutrino flux here on earth is our star the sun.
  5. Apr 24, 2008 #4
    Neutrino telescope?

    I can understand why our sun would be the largest source of neutrinos that we can "see", but all other stars should also produce neutrinos, but to us not in the quantity as from our sun.

    This brings up a question I have. Since neutrinos are "particles", is it possible to build something like a "neutrino telescope"? With enough resolution, it might be possible to identify those sources with greater or lesser amounts of neutrino emission. Would this be another way to look at our universe? Does anyone know of anything being done along these lines? I'm aware of the various mines that have been converted into neutrino detectors, but to my knowledge, they're not particularly directional in nature, but I could be wrong.

    Last edited: Apr 24, 2008
  6. Apr 24, 2008 #5
    The problem is, that neutrinos only interact VERY weakly with ordinary matter. Even from the sun we do not catch very much neutrinos in this detectors (though enough to have fun with them and verify our theories about how the sun is working). But as far as I know from normal stars there is currently no chance to catch any neutrinos. But in case of a supernova there are so much neutrionos emitted (I think the "neutrino luminosity" of a supernova is even bigger than the luminosity in the visible spectrum) that we can detect them. One of the early detectors just went operational just before SN1987 went KAWOOM, (or, more correctly, just before the light and the neutrinos from this explosion reached us), so the guys who built it were very happy about the many neutrinos they caught from this supernova.
  7. Apr 24, 2008 #6


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    Correct - but like Olbers paradox they should produce them in the same proportion as the light from the other stars compared to the sun. (that was really badly phrased!)

    The kamiokande detector is somewhat directional - using cerenkov radiation from the particles. The cl->ar detectos aren't
  8. Apr 24, 2008 #7
    I thought the neutrinos change type at random
    and that limited the detection rate
  9. Apr 24, 2008 #8


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    In reference to the Obler's Paradox type situation, I think the solution would be much the same as it is in reference to light. The universe is not infinite and therefore is not infinitely old, therefore light (or neutrinos) from all the stars has not had a chance to reach us.
  10. Apr 25, 2008 #9


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    Last edited: Apr 25, 2008
  11. Apr 25, 2008 #10
    but for the light from stars which is reaching us? together with light,we should also find neutrinos?isn't?
  12. Apr 25, 2008 #11


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    Yes but the neutrinos interfear so incredible little, their cross section is of the order 10^-20 Barns! Photon has high cross section.
  13. Apr 25, 2008 #12
  14. Apr 29, 2008 #13

    Yes, very interesting. Thank you.
  15. Apr 29, 2008 #14
  16. May 1, 2008 #15
    neutrinos are extremely difficult to detect - they were thought of as massless particles but recent work suggests that neutrinos possess a very small mass.

    I dont understand the comparison made to the night sky being filled with light at night because of the number of stars.

    Remember that's the point - if the universe was infinite then the night sky would be filled with light from the INFINITE number of stars out there. This is strong evidence that there are a finite number of stars out there and that the universe is almost entirely composed of non-baryionic material (about 70% dark energy and 25% dark matter.

    As EInstein said "the universe is FINITE but UNBOUNDED"
    Last edited: May 1, 2008
  17. May 6, 2008 #16
    Is a "Barn" a standard unit of measurement?

    Chuckle, thanks for a good laugh!
  18. May 6, 2008 #17


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  19. May 7, 2008 #18
    Live and Learn, I always say. Thanks for letting me know that there really is a BARN in quantum mechanics. Now I'm getting an even better chuckle out of my ignorance!
  20. May 28, 2008 #19
    i thought that we were being constantly being bombarded by neutrinos,you should read the book SPACESHIP NEUTRINO (no it's not a picture book)
  21. Jun 5, 2008 #20
    I thought we were, also. Can you shed a few sentences from your reference?

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