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Where are all the neutrinos?

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  1. Mar 21, 2014 #1
    Current main stream theory is neutrinos must have mass. Recent paper claims its on the order of 5 to 50 eV (I've forgotten exactly). So, they must be slowed down (as well as sped up) by gravitational interaction, correct? As well as the expansion of space. Hence they must be attracted "just like" any other space dust (allowing for their high, but slowing, speed) and so wouldn't we expect some of them to be in orbit around Sag A*, the Sun or even the Earth? If this is correct, what happens to this swarm of 'cold' neutrinos?
     
  2. jcsd
  3. Mar 21, 2014 #2

    Chronos

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    Neutrinos are antisocial. They rarely and barely interact with anything - including their own kind. This is why it took so long to prove they even existed, despite theoretical proof of their existence was demonstrated by Pauli back in 1930. As a non-interactive particle they have no effective way to shed momentum. They can pass through light years of lead unscathed and, until recently, were believed to be massless because they travel at virtually the speed of light. They share many of the same properties as dark matter, which may turn out to be a close relative of the neutrino. It is certainly possible a small number of neutrinos may orbit black holes just like it is possible a small amount of dark matter may orbit a black hole. The window of opportunity is, however, quite small and these 'orbits' are probably unstable, much like the photon sphere of a black hole. For a good discussion of neutrinos, see; http://arxiv.org/abs/1210.3065, Neutrino. History of a unique particle.
     
  4. Mar 23, 2014 #3
    You claim that a massive particle, the neutrino, behaves like the photon. That combined with your claim that they travel "at virtually the speed of light" implies they do NOT slow down. I don't understand that at all. Could you expand on that, please? It is my understanding that the photon sphere is unstable precisely because photons travel at one speed, c. Since neutrinos have an energy spectrum, I assume they travel at more than one speed...Have I made an error? Solar neutrinos are created at 10e10 per second, or 10e17 per year.There are 14e9 years since the BB. There are 10e22 stars in the Observable Universe. 17+9+22 is 48, there are crudely ~10e48 (excluding those from supernovae and other mechanisms) neutrinos out there plus primadorial ones (estimated at 150 per cc). How can there only be a "small number" in orbit? Thanks.
    BTW, the article you cite contains no information on the orbital characteristics of neutrinos nor any on gravitational interaction. Was this citation a mistake?
     
    Last edited: Mar 23, 2014
  5. Mar 23, 2014 #4

    Bandersnatch

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    Imagine, you're a particle of some non-zero mass. Like all of your kin, you were emitted from some nuclear process at close to the speed of light. Say, 99.99% c. If you were to encounter a black hole in your travels through the universe, you'd fall towards it, accelerating in its gravitational field up to something even closer to c, say, 99.99999% c. Being a non-interacting particle, you're unable to shed any of that speed, so you follow a hyperbolic trajectory and eventually end up as far as away from the hole as you had been earlier, with the same speed of 99.99% c(unless you happen to fall into it).

    Now, since our detection methods are limited in accuracy, both 99.99 and 99.99999 could very well be indistinguishable, and you'll always detect them travelling at 'virtually the speed of light'.

    At such high speeds, you're way beyond the escape velocity of any stellar body barring a very close encounter with a black hole, so you will always travel in hyperbolic orbits. There will be no neutrinos in orbit around Earth or the Sun, or any other star.

    So, if you begin your life travelling at very close to c, and with no way to shed the speed, you'll practiaclly always travel at close to c.

    You could be a neutrino emitted in a supernova explosion, very close to the event horizon of a black hole, in which case you would be slowed down significantly for all outside observers to see.
    IF they could detect you.
    The thing is, we can only detect the energetic ones:
    http://en.wikipedia.org/wiki/Neutrino#Detection

    Similarly, there may be processes that emit low-energy neutrinos(wiki says nuclear reactors make a lot those), so the above reasoning doesn't apply any more - they don't start at close to c. But then again, we can't detect them.

    By the way, the speed of a neutrino as a function of its kinetic energy can be easily calculated as shown here:
    http://en.wikipedia.org/wiki/Measurements_of_neutrino_speed#Overview
     
  6. Mar 23, 2014 #5

    mfb

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    The upper limits are somewhere at .5 eV, and the lower limits for the heaviest (mass eigenstate) are in the range of a few meV. All known production mechanisms for neutrinos today give them an energy of the order of (at least) keV. This is so far beyond their mass that they travel extremely close to the speed of light (99.9999% c is a low number here).
    The cosmic neutrino background could have slower neutrinos (that depends on the precise masses), but "slow" is still a significant fraction of the speed of light. There is no reasonable probability that such a particle would enter an orbit around anything, if it cannot be slowed down in collisions.
     
  7. Mar 23, 2014 #6

    Chronos

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    Relic neutrinos from the big bang have certainly been slowed by expansion. Whether or not that translates directly into a lower velocity is an interesting, but, as of yet unanswered question. 'Cold', or non-relativistic neutrinos would be devilishly hard to detect [as noted by bandersnatch]. Could they cluster around massive bodies? The question has been considered by a number of researchers and the best guess is yes - e.g.; http://arxiv.org/abs/1310.6459, Neutrino clustering around spherical dark matter halos. Recent studies suggest the sterile neutrino, the right handed cousin of garden variety left handed neutrinos could account for some or all of dark matter. It is postulated to be a 'warm', or sub-relativistic velocity neutrino.
     
  8. Mar 23, 2014 #7

    phyzguy

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    From the standpoint of numbers, there should be large numbers of relic neutrinos streaming through space. According to this presentation from the Max Planck institute, the number density of relic neutrinos is on the order of 100 /cm^3 /flavor, so if there are three flavors (as there probably are), there are about 300 relic neutrinos per cm^3. Since a cubic meter of the universe contains on the order of 1 hydrogen atom and 10^9 CMB photons, this would mean that the same cubic meter of space would contain on the order of 3x10^8 neutrinos, meaning there are nearly as many neutrinos in the relic neutrino background as there are photons in the cosmic microwave background. So the answer to the OPs question is that they are all around us, passing through us as we speak.
     
  9. Mar 24, 2014 #8

    ChrisVer

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    I think the massive right handed neutrino theory is dead....(?) if not, disfavored from experimental data.
     
  10. Mar 24, 2014 #9

    Chronos

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  11. Mar 24, 2014 #10

    phyzguy

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    As Chronos pointed out, it is far from dead. IMHO, it is one of the leading possibilities for the nature of dark matter.
     
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