Wikipedia:Reference desk/Archives/Science/2018 October 9

= October 9 =

Neutrinos interacting with each other?
Suppose we see a supernova that went off in a galaxy billions of light-years away. The neutrinos from that star have been travelling parallel to one another for all that time, and the supernova might have given them a limited range of energies, and the ones travelling near each other now should all be sorted for one particular velocity. So as far as I can figure in my mind, there ought to be a soup of neutrinos that are all pretty near rest relative to one another, chilling out in space for billions of years.

What do they do with each other in that time? I asked a question about "neutrino nuggets" a long time back but that was a fairly unusual idea about accelerons. Is there a way to measure or predict if they really interacted? Wnt (talk) 02:48, 9 October 2018 (UTC)
 * There is way to much empty space in our universe for the human mind to "figure". We are build to figure 100 miles at best, when we stand on a high mountain in ideal weather conditions for far sight. You can memorize that sight and recall it pretty exactly. Go try that with the picture at the right sight and then calculate how many times more your billion(s) lightyears is (tip ONE year is 31,557,600 seconds and the distance Earth moon is 1.3 seconds). Next try to calculate how many neutrinos (0.3 attometer) you could line up in a straight line in between. --Kharon (talk) 03:45, 9 October 2018 (UTC)


 * That's a pretty useless answer. Doroletho (talk) 21:04, 9 October 2018 (UTC)


 * Its better than your comment about it atleast, User:Doroletho. But since you really seem the fresh new wikipedian your fresh account insinuates, we will patiently remain, to see if your aim improves in the future. --Kharon (talk) 21:59, 9 October 2018 (UTC)


 * Neutrino: "Observations of the cosmic microwave background suggest that neutrinos do not interact with themselves". Rojomoke (talk) 06:12, 9 October 2018 (UTC)
 * The citation for that quote is a SciAm article which doesn't say much more about it than is in the Wikipedia article, so it's hard to be sure exactly what it's talking about, but my guess is that when it says they don't interact with themselves, it means exactly that &mdash; each individual neutrino does not interact with itself. That doesn't mean they don't interact with each other.
 * That's different from, say, electrons; each individual electron does interact with itself. What does that mean exactly?  To be honest, I don't understand that in detail.  But if you Google "electron self-interaction" you'll get a lot of hits from which you can start to investigate, and it's pretty clear that they're talking about an individual electron interacting with itself.
 * So in any case I don't think this rules out Wnt's hypothesis of neutrinos interacting with one another on their long journey. Intuitively I would expect that interaction to be very very tiny, but I'm not an expert. --Trovatore (talk) 07:15, 9 October 2018 (UTC)
 * No, self-interaction in this case almost certainly means interaction among themselves, one neutrino interacting with another neutrino (as in self-interacting dark matter); the abstract of this paper may serve as an example where "self-interaction" means neutrino-neutrino scattering. The terminology is admittedly rather mushy. What you're thinking of in connection with electrons is presumably self-energy. --Wrongfilter (talk) 07:47, 9 October 2018 (UTC)
 * I did look at the self-energy article, but it wasn't very explicit about electron self-interaction. Here's an example of the kind of thing I'm talking about, I think:  [Quantum Electron Self-Interaction in a Strong Laser Field].  From the abstract: The quantum state of an electron in a strong laser field is altered if the interaction of the electron with its own electromagnetic field is taken into account. --Trovatore (talk) 17:49, 9 October 2018 (UTC)
 * Actually, in the context of electrons, the concept of "self-interaction" usually refers to a common interpretation "one-electron-at-a-time" version of the double slit experiment, and similar phenomenon. When electrons are used in the double slit experiment, they form an interference pattern at the detector even if the electrons are allowed to go through the experiment one-at-a-time so that each electron is detected before the next electron is fired.  Since the resulting interference pattern is indicative of interaction between two different waves, a common interpretation of that result means the single electron must be passing through both slits and interacting with itself before being detected.  I have no idea how this relates to the neutrino problem above (and reading this apparently not at all), but in the context of self-interaction and electrons, that's what it usually means.   -- Jayron 32 14:11, 9 October 2018 (UTC)
 * I have my doubts regarding "usually" (and feel pressured to voice my doubts given the rather dismissive "no" from your edit summary). In Wikipedia, Self-interaction redirects to Renormalization because of Self-interactions in classical physics. In the double slit, the wave function interferes with itself. That is not quite the same as saying "the electron interacts with itself". Google gives results where "self-interaction" and "double slit" both occur, but not very many. Be that as it may, relevant references for Wnt's question are and . --Wrongfilter (talk) 15:37, 9 October 2018 (UTC)
 * Good point. -- Jayron 32 16:12, 9 October 2018 (UTC)
 * Is it possible to set up Young's Double-slit experiment (1801) using Neutrinos ? DroneB (talk) 15:39, 9 October 2018 (UTC)
 * As a thought experiment, certainly. In practice, it'll be hard to find a sufficiently absorbant screen in which to cut the slits... --Wrongfilter (talk) 15:52, 9 October 2018 (UTC)


 * Sorry my earlier answer went into a direction that has little or no use for you. I am so fascinated by the gap between human imagination capability and reality that this carries me away every time someone comes near that with a question. Since the Double-slit experiment is about interference, maybe Neutrino oscillation is helpful. --Kharon (talk) 22:39, 9 October 2018 (UTC)


 * Neutrinos interact with each other directly via the exchange of Z-bosons. The cross section for neutrino-neutrino scattering becomes large at the resonance energy for creating Z-bosons. This effect has been invoked in the past to evade explain ultra-high energy cosmic rays above the GZK limit, the so-called Z-burst model, also mentioned here.


 * At low energies, interactions due to higher order effects yielding a finite neutrino magnetic moment will be more important. The neutrino magnetic moment is given by $$\frac{3 G_F m_em_{\nu}}{4\pi^2\sqrt{2}}\mu_B\approx 4\times 10^{-20}\mu_B$$ where $$G_F$$ is the Fermi constant and $$\mu_B$$ is the Bohr magneton. This is due to only known Standard model physics, extensions of the Standard model could give rise to far larger magnetic moment, experimental constraints limit to be smaller than about $$10^{-12}\mu_B$$, but it's likely going to be less than $$10^{-14}\mu_B$$ as for larger magnetic moments the parameters from the extension of the Standard model needed to get you there, must be fine tuned to conspire with each other so as to not yield neutrino masses that become way too large.


 * Interactions due to the magnetic moment can be detected due to e.g. effective neutrino decay. Such interactions can induce spin flips, causing left-handed neutrinos to become sterile right-handed neutrinos that don't interact with matter at all. Count Iblis (talk) 15:19, 10 October 2018 (UTC)

Outstanding but perplexing answer. To begin with, our article on the Z-burst says that "This process proceeds via a (virtual) Z-boson..." and I'm already off track. How can a reaction of real particles proceed via a virtual particle intermediate? How do you get one virtual particle at all? I hope they're wrong, or before long I'm going to have to wonder whether the transition state of a Krebs cycle intermediate bound to an enzyme is "virtual". Then there is the neutrino magnetic moment you're calculating, which I agree reflects this ... I suppose I knew a neutron has a magnetic moment, but it still seems surprising to think a fundamental "point particle" with no charge would have one. (though our article makes it sound uncertain) I don't feel confident to calculate whether there can be a meaningful interaction based on this; and if there is, can energy be released as they approach one another to leave them in a bound system? Wnt (talk) 10:11, 12 October 2018 (UTC)