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Questions about neutrinos

  1. Jul 18, 2003 #1
    Well. Do they have mass or not? How is the investigation nowadays? Anybody knows?
     
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  3. Jul 18, 2003 #2

    mathman

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    Current evidence is that neutrinos (there are 3 kinds) have mass, since the observations seem to show that neutrinos change from one kind to another. This could not happen if they were massless.
     
  4. Jul 18, 2003 #3
    Any data about the upper mass bound? it should be very small,I suppose.
     
  5. Jul 18, 2003 #4

    chroot

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    The sum of the masses of all neutrino species (most likely three) is less than or equal to 0.79 eV, with 95% confidence. (WMAP 2003 in conjunction with 2dFGRS 2003.)

    - Warren
     
  6. Jul 18, 2003 #5
    Thank you chroot, you have understand me even my bad english! :wink:
     
  7. Jul 19, 2003 #6
    Does somebody know the upper mass bond of a photon?

    Due to Louis de Broglie the photon also has a mass, and I believe that this is a reasonable assumption.
     
  8. Jul 19, 2003 #7

    Tom Mattson

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    The upper bound is 2×10-16 ev.

    Source: http://pdg.lbl.gov/2003/s000.pdf

    No, de Broglie's hypothesis was made in regards to matter waves, not photons.

    I believe that it is an unreasonable assumption.

    A photon mass would imply, among other things, that the electrostatic force is not really an inverse square law, but rather derived from a Yukawa potential. It would also destroy gauge invariance. Both of these things are well tested.
     
  9. Jul 19, 2003 #8

    marcus

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    Warren can you give a brief indication of how WMAP and 2dF data can serve to establish that upper bound, or how the bound is arrived at?

    Also I had another question you may know something about. Lineweaver said that the predicted temperature of the cosmic neutrino background (left over from baryon-genesis at a certain stage in the cooling of the early universe) was 1.9 kelvin.
    I don't know how far people are from being able to detect neutrinos well enough to determine if there is a cosmic background of neutrinos(and if so what temperature it is).
    Or even if such a thing is in principle measurable given all the other neutrinos in the foreground. Do you know anything about this?


    edit: I just did a google search for cosmic neutrino background and came up with a recent article by a german physicist at DESY
    arxiv.org/hep-th/0301157
    "How to detect the cosmic neutrino background?"

    Also for what its worth a 1999 AIP abstract:
    Number 425 (Story #1), April 28, 1999 by Phillip F. Schewe and Ben Stein


    THE COSMIC NEUTRINO BACKGROUND can in principle be detected. There are almost as many neutrinos loose in the universe as photons, and almost as much energy vested in neutrinos as in photons. Yet, owing to the extreme reticence of neutrinos to interact with other particles, detecting the neutrino background is not easy as detecting the cosmic photon (microwave) background. Indeed, dedicated neutrino detectors struggle just to record a handful of incoming neutrinos from potent nearby sources like the sun. Nevertheless, there might be a chance to map the background indirectly. The pattern of lumps in the microwave background, which will be measured by the upcoming MAP and PLANCK orbiting detectors, encodes information about the neutrino background. Scott Dodelson of Fermilab (630-840-2426), Michael Turner and Robert Lopez of the University of Chicago, and Andrew Heckler of Ohio State show these measurements will accurately establish the time at which slow-moving matter (protons and later atoms) became predominant over fast-moving radiation (photons and neutrinos), and that this in turn determines precisely how much early annihilation energy (arising from electrons and positrons smashing up) was apportioned among photons and neutrinos. (Lopez et al., Physical Review Letters, 17 May 1999.)
     
    Last edited: Jul 19, 2003
  10. Jul 19, 2003 #9
  11. Jul 19, 2003 #10

    marcus

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    Meteor, thanks for the reference to the South Pole neutrino detector "Amanda". This led me to several interesting articles about Amanda and the improved version called "Ice Cube".

    But, disappointingly, Amanda cannot detect neutrinos of the cosmic background. The article I mentioned discusses the reasons for this

    (How to detect the cosmic neutrino background, by Ringwald of DESY)

    and gives estimates of the original energy when the neutrinos were released and their presentday energy and number density.

    According to Ringwald the neutrinos were released when the universe was about 1 second old and they had energies on the order of 1 MeV.

    Now they have energies on the order of 10-4 eV
    says ringwald. It is much too low for the detector to see.
     
  12. Jul 20, 2003 #11
    De Broglie wrote in Comptes rendus (1923):

    "I showed elsewhere ... that the atom of light should be considered as a moving object of a
    very small mass (< 10^-50 g) that moves with a speed very nearly equal to c (although slightly less)."

    This assumption makes it easy to understand that a photon has a momentum and a (relativistic) mass as well.

    Whether one finds a conflict to the inverse square law in a measurement depends on the numeric deviation from mass=0 or v=c.
     
  13. Jul 20, 2003 #12

    Tom Mattson

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    OK, I wasn't aware of that. How did he "show" this?

    Right, so what evidence is there that the inverse square law or gauge invariance are not accurate? See, what I consider a "reasonable assumption" is something that is necessary to account for experimental results. Postulation of quarks and neutrinos were "reasonable assumptions" by that standard. The only reason you've offered here is that it clears up the conceptual difficulty of "massless momentum", but I don't think that alone warrants the assumption. There has got to be *some* necessitation from experiment.
     
  14. Jul 22, 2003 #13
    De Broglie was the first one who created the idea of the wave properties of a particle. And he developed a model to descibe this phenomenon mathematically. From his model his assumption about the photon followed as a consequence.

    The appoach of de Broglie to QM was discarted by Bohr and Heisenberg during the Solvay conference 1927 in Brussels. De Broglie gave up at the time, but later he restarted his version of quantum theory (wich has some relation to the one of David Bohm).

    To my knowledge the approach of de Broglie was never reevalued again by the physical community. But it has certain abilities. For instance is it quite easy to determine the parameters of the electron in a classical way if the model of de Broglie is used. Normally the electron is used as the chief witness for the (believed) fact that elementary particles can only be descibed by QM.



    They are proven accurate to the extend the experiments can provide. You can only give some kind of an upper bond by an experiment.

    The other pre-condition for this conclusion is that the photon is in fact the exchange particle representing the electric field. This is of course a convenient assumption. But it may also be true that both are not the same.
     
  15. Jul 22, 2003 #14

    Tom Mattson

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    But my question was how did he show this. And how can the photon mass be simultaneously an "assumption" and a derived "consequence". That makes no sense to me.

    You don't have to type it all out here; a reference will do.

    Yes, I know that. My point is that there is currently no observational falsification of Classical ED or QED, such that a photon mass is required. In fact, a photon mass would make a right mess of things, so again I say that it is unreasonable to assume it. That is unless de Broglie has shown that the photon mass is a necessary implication of his theory of matter waves. You claim that he did, but as I said I'm still waiting on the details of the proof.

    It may be true that both what are not the same? Real and virtual photons?
     
  16. Jul 22, 2003 #15
    I don't know if this has been asked,
    but does anyone know why originally
    they were thought to have no rest mass?
     
  17. Jul 22, 2003 #16

    Tom Mattson

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    Are you talking about neutrinos or photons? I ask because there are actually 2 conversations going on here: The actual topic, and this tangent discussion about light.
     
  18. Jul 23, 2003 #17
    Nuetrinos....... sorry I didn't
    have time to read all of the thread
     
  19. Jul 23, 2003 #18
    Sorry if I was not careful with words. I mean the following: His model is of course an "assumption" as every model. If his model is accepted than the conclusion is a "consequence".

    He gave in the mentioned paper the following reference:

    Journal de Physique, 6c serie, t.3, 1922, p.422.

    I did not try to read it as I do not understand French.

    But if you follow the whole idea of how de Broglie understood the existence and the properties of particles (which are not waves in his theory) this statement about the photon fits to it.

    I think we have meant real photons here. Virtual photons a subject to QED. And QED is in fact open in respect to its physical meaning. - I have read a statement in a textbook of Richard Feynman:
    (double translated to German and now back)
    " The laws of QED are presented in the following, but we do not have a justification for them at present"
    That means for my understanding that QED works well (like QM), but we do physically not know, why.

    To come back to your original point: The exchange particle of the electric field may be different from real photons.-
     
    Last edited: Jul 23, 2003
  20. Jul 23, 2003 #19

    Tom Mattson

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    Still makes no sense, and now for a different reason. You say it is that the photon mass logically follows from de Broglie's hypothesis on matter waves. Acceptance by other people has nothing to do with whether or not the former logically follows from the latter.

    Does that mean that you really do not know that deBroglie showed what you claimed he showed?

    If particles are not waves in his theory, then why does he postulate that particles have a wavelength?

    So you keep saying.

    My question is the same: How? How? How?

    Notice a pattern here?

    With Feynman's remark taken out of context like that, I am hard pressed to infer what he means, so I withold agreement or disagreement on your interpretation for now.

    They are certainly different from real photons. Real photons have k&mu;k&mu;=0, whereas virtual photons do not.
     
    Last edited: Jul 23, 2003
  21. Jul 23, 2003 #20

    Tom Mattson

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    Neutrinos were thought to be massless because a massive neutrino implies a phenomenon called neutrino oscillations. I can post more about that later, but the answer to your question is that this phenomenon had not been detected until very recently. When it was detected, the massless neutrino hypothesis was falsified.
     
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