Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why are there so few detected extrasolar neutrino sources?

  1. Feb 28, 2012 #1
    Hi, I've just registered in the forum because I have a couple of Physics questions. I'm not a specialist, and furthermore English is not my mother tongue, so please be indulgent with me.

    As far as i know (please correct if not), only two sources of natural 'extraterrestrial' neutrinos have been detected by now, the Sun and the supernova SN1987A. But (as far as I know too) they also are the only known particles that are not significantly attenuated by their travel through the interstellar medium (and that's one of the reasons because they're so interesting...)

    So my question is, shouldn't we be detecting neutrinos from a great amount of extrasolar sources? Why not, please?

    Thank you very much in advance, and please relocate this question if it isn't in the appropiate section.
  2. jcsd
  3. Feb 28, 2012 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Neutrinos interact only rarely. We need a source which is very bright - and through a numerical coincidence, "bright" means roughly "bright enough in photons to see in daylight". That leaves exactly two sources: the sun, and neutrinos from nearby SN.
  4. Feb 28, 2012 #3
    Thank you very much, Vanadium. So I understand from your answer:
    a) The number of neutrinos arriving to the Earth from an emitting source is roughly proportional to the number of photons arriving from that same source.
    b) The "density" of neutrinos arriving to the Earth from other extrasolar sources is way under the minimum amount required to interact with our detectors in a meaningful way.

    Is this right or am I messing everything up?

    (BTW, in such a case, does any kind of Olbers' paradox apply?)

    Thanks again!
  5. Mar 1, 2012 #4
    SN1987a wasn't bright enough to see in daylight. It was only possible to detect the neutrino's because the neutrino's are produced in a burst only seconds long, while the visible light emission was spread out over months.
    For sn1987a, only 24 neutrino's were detected (at 3 different detectors) in 13 seconds.
    These would never have been noticed if spread out over months.

    For ordinary stars, the number of neutrino's is equal to the number of nuclear reactions that produce them, and this is of course proportional to the total amount of energy radiated by the star.
    There are various reactions that produce neutrino's of different energies, and wich reactions occur most often depend on the temperature of the star. (look up proton-proton chain and cno-cycle)
    Higher energy neutrino's are easier to detect.

    Since the nearest star is about 2.6 * 10^5 times as far away as the sun, and the intensity of all kinds of radiation, including neutrino's is inversely proportional to the square of the distance, the neutrino emissions of other stars would be weakened by at least a factor 6.7 * 10^10, so it's not strange we can't see those.
  6. Mar 1, 2012 #5

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Hence the "roughly". Also, we barely detected it in neutrinos. One experiment saw 11, one 8 and one 5. A factor of two dimmer, and today we would be using adjectives like "possible" to describe the neutrinos.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook