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Neutrinos and theire speed

  1. Apr 8, 2005 #1
    neutrinos and their speed

    Hi.

    http://hitoshi.berkeley.edu/neutrino/neutrino.html

    Doeas this mean that neutrinos are faster than light?
     
    Last edited: Apr 8, 2005
  2. jcsd
  3. Apr 8, 2005 #2
    Not at all, why would you think that ?

    they move at a speed very close to the speed of light though

    marlon
     
  4. Apr 8, 2005 #3
    I just got confused because the neutrinos arrived 3 hours before the light.
     
  5. Apr 8, 2005 #4
    This just implies that the neutrino's were emitted PRIOR to the actual explosion.


    EDIT : i need to be more exact : the neutrino's came out at the time of the explosion, it's the EM-radiation that 'came later' .

    regards
    marlon
     
    Last edited: Apr 8, 2005
  6. Apr 8, 2005 #5

    Meir Achuz

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    Neutrinos get out of the supernova faster than light, because the photons react more strongly with the matter in the supernova. That is the simple explanation.
    The details get more complicated and speculative.
     
  7. Apr 8, 2005 #6

    mathman

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    Neutrinos came out at the time of the explosion. Gamma rays come out later - they are born afterwards.
     
  8. Apr 8, 2005 #7
    Ah, thank you guys. May I ask one last question? I can't find a proper explanation of what a "right-handed" and a "left-handed" neutrino is.
     
  9. Apr 8, 2005 #8
  10. Apr 8, 2005 #9
    That is not true. Neutrino-velocity does not exceed the speed of light

    I don't even know what this means and where you got such info

    :rolleyes:

    marlon
     
  11. Apr 8, 2005 #10

    Meir Achuz

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    RH and LH for particles are defined by convention.
    The convention is that if the helicity (component of the spin in the direction of the momentum) of the particle is positive, it is called RH, and v-v.
    This applies to all particles. For the photon, this elementary particle definition turns out to be opposite to the optical designation of RH and LH circular poariztion.
     
  12. Apr 8, 2005 #11
    Thank you once again. A very useful link. I suspected that it had something to do with the rotation. :shy:
     
  13. Apr 8, 2005 #12

    Meir Achuz

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    The neutrinos and photons diffuse out of the supernova. The diffusion time depends on their interaction. This knowledge is acquired by reading papers, going to conferences, and working things out.
     
  14. Apr 8, 2005 #13
    No it is not. The reason for the specific left-handedness of neutrino's has to do with parity conservation in beta decay. same goes for the anti-neutrino.

    No it does NOT.

    here is a quote from the site i gave :

    This "left-handed" vs "right-handed" characterization is not meaningful for other particles, like electrons. An electron could have spin to the right and be traveling right and therefore be classified as right-handed. But from the reference frame of someone traveling faster than the electron, its velocity would be to the left, while its spin would be unchanged. This would mean that the electron is a left-handed particle with respect to that reference frame.


    Meir, this is not the first post where you supply erroneous information. you are gonna have to be more careful in the statements that you make.

    marlon
     
    Last edited: Apr 8, 2005
  15. Apr 8, 2005 #14
    Since you have been working things out. Please prove me that neutrino's exceed the speed of light at any time ?

    marlon
     
  16. Apr 8, 2005 #15

    Meir Achuz

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    Two simple things and then I'm outerhere.
    1. LH is not a Lorentz invariant, but does describe the helicity state in any given frame.
    2. The key word is "diffusion".
     
  17. Apr 8, 2005 #16
    I really don't think that you know what you are talking about. I have one question for you : why do you think that elementary particles are massless ??? What is the difference between parity and intrinsic parity ???


    Ofcourse, LH is not Lorentz invariant, that is my entire point. this also proves you wrong because this property cannot be valid for all particles then.


    marlon
     
    Last edited: Apr 8, 2005
  18. Apr 8, 2005 #17
    And what is this nonsense about diffusion ???

    marlon
     
  19. Apr 8, 2005 #18

    dextercioby

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    Scattering would be a better name for the processes in which a photon is involved in a hot dense stelar plasma.I think he would refer,in analogy,to the same kind of processes that occur in our sun and how a photon born in the core would need ~5000 yrs to reach the surface...

    Daniel.
     
  20. Apr 8, 2005 #19
    Marlon - he never claimed that the helicity was Lorentz invariant. However, you can define a helicity for any particle in some given frame, and experimentalists/phenomenologists DO use the concept of helicity even with massive particles. For example, in the pion decay to lepton neutrino, one talks about helicity suppression as being the reason for the muon channel being favored over the electron channel. p 266-7 of Halzen and Martin is a reference for this.

    As for the diffusion, I think that all he was saying was that photons interact more with matter than neutrinos since neutrinos can only interact weakly while photons can interact via EM.
     
    Last edited: Apr 8, 2005
  21. Apr 9, 2005 #20

    SpaceTiger

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    I would say both marlon and Meir Achuz are right in their answer to the poster's question, but it depends on how you look at it. In the initial core collapse, both neutrinos and photons will be created, but each will take a different time to escape from dense core of the supernova. Neutrinos have smaller interaction cross sections than photons (in every case that I'm aware of), so they'll escape the core more quickly than neutrinos. This is what Meir Achuz means when he says that they come out faster than the light, he doesn't mean that they're actually moving faster than light. Also, as he said, this can be thought of like a diffusion process, just like photons in the sun. However, the photons we're seeing after those three hours actually experienced their final scattering well after the neutrinos, so it could be said that marlon is right in asserting that the photons were created after the neutrinos. It's mostly a semantic point, though.

    Anyway, fun fact about supernovae: most of the EM radiation we see is from radioactive decay of nickel-56 and cobalt-56.
     
  22. Apr 9, 2005 #21
    No, i never said he did. point is that helicity is NOT a universal property of all particles, which he claimed.

    marlon
     
  23. Apr 9, 2005 #22
    Well, that is indeed very true but i would never formulate this is as 'moving faster then light' (if that is what was originally meant).

    regards
    marlon
     
  24. Apr 9, 2005 #23
    He did not. You are misinterpreting his post.

    There is nothing in this person's post which is wrong in my view. In a given Lorentz frame, I can call an electron or muon left-handed or right-handed. And practicing experimentalists/phenomenologists do this all the time. That's what he means by "This applies to all particles". He did NOT mean "A muon or any massive particle has a fixed helicity in all frames".

    The only possible quibble you could have is that he did not say explicitly say "in a given Lorentz frame", but that is implicit anyhow since he mentioned momentum.
     
  25. Apr 9, 2005 #24
    Are you his lawer ???

    I still don't agree for two reasons :

    1) LH and RH are defined by convention. This is untrue because this implies that LH and RH are 'fixed' values, which is not the case. The determining factor is conservation of helicity. Point Final....That's what i should have read in that post.

    2) About the 'applying to all particles' i say this : you interpreted that your way, i interpreted it my way. It is very possible that my interpretation is wrong however that clearly proves there must be something missing in the actual explanation. There should have been a clear distinction between parity and intrinsic parity. Besides, the same goes for 'moving faster then light'
    Now, i don't want to continue a discussion that has degenerated into semantics, so why don't we just drop it, ok ?

    marlon
     
  26. Apr 9, 2005 #25

    Meir Achuz

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    I would like to thank a number of the contributors to this thread.
    We do have to keep our cool when discussing hot matter.
    I do want to add one bit from memory. Of course, the optical signal was not seen until that fellow in South America decided to look through his telescope.
    The difference in times for the neutrino burst and the emission of photons depends on theoretical extrapolations of the light curve from the supernova, and so is speculative.

    Astrophysics, abstract
    astro-ph/9711196
    From: Georg Raffelt [view email]
    Date: Tue, 18 Nov 1997 13:17:08 GMT (59kb)

    Photon Dispersion in a Supernova Core
    Authors: A.Kopf (MPA Garching), G.Raffelt (MPP Munich)
    Comments: 8 pages, REVTEX, 7 postscript figures included
    Report-no: MPI-PTh/97-73
    Journal-ref: Phys.Rev. D57 (1998) 3235-3241

    While the photon forward-scattering amplitude on free magnetic dipoles (e.g. free neutrons) vanishes, the nucleon magnetic moments still contribute significantly to the photon dispersion relation in a supernova (SN) core where the nucleon spins are not free due to their interaction. We study the frequency dependence of the relevant spin susceptibility in a toy model with only neutrons which interact by one-pion exchange. Our approach amounts to calculating the photon absorption rate from the inverse bremsstrahlung process gamma n n --> n n, and then deriving the refractive index n_refr with the help of the Kramers-Kronig relation. In the static limit (omega --> 0) the dispersion relation is governed by the Pauli susceptibility chi_Pauli so that (n_refr)^2-1 approx chi_Pauli > 0. For omega somewhat above the neutron spin-relaxation rate Gamma_sigma we find (n_refr)^2-1< 0, and for omega >> Gamma_sigma the photon dispersion relation acquires the form omega^2-k^2=(m_gamma)^2. An exact expression for the "transverse photon mass" m_gamma is given in terms of the f-sum of the neutron spin autocorrelation function; an estimate is (m_gamma)^2 approx chi_Pauli T Gamma_sigma. The dominant contribution to n_refr$ in a SN core remains the electron plasma frequency so that the Cherenkov processes gamma nu <--> nu remain forbidden for all photon frequencies.
     
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