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Trying to understand

  1. Nov 4, 2006 #1

    wolram

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    What a virtual particle *is* in theories of quantum gravity.

    http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

    You must understand that a non scientist may want to know how things work, and it is unfair for you guys to use terms like *virtual* without
    giving some explanation as to what it means.
     
  2. jcsd
  3. Nov 4, 2006 #2

    arivero

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    I already told in other thread, I would like understand the psicologocal or semiotical implications of the word "virtual" when used in "virtual particle", because it is probably the most repeated physics question in the internet (check the FAQ of physics in USENET). Not that people asks about particles under Heisenberg uncertainty or about particles in Off-shell Feynman diagrams... they ask about virtual particles.
     
  4. Nov 4, 2006 #3

    wolram

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    I can just about understand Heisenberg uncertainty, but how does this help
    in understanding the real world, it is you guys that use these words, i just want to know how they help in understanding the blossoming of the universe.
     
  5. Nov 22, 2006 #4
    Interesting that I found this post on a search for the precise definition of "virtual". Apparently there is a gap between the "popular" notion of virtual and the "scientific" notion.
     
  6. Nov 23, 2006 #5

    Demystifier

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    Here is a citation from
    http://arxiv.org/abs/quant-ph/0609163

    Virtual particles?

    The calculational tool represented by Feynman diagrams
    suggests an often abused picture according to which
    ``real particles interact by exchanging virtual particles".
    Many physicists, especially nonexperts,
    take this picture literally, as something that
    really and objectively happens in nature. In fact, I have
    {\em never} seen
    a popular text on particle physics in which this picture was
    {\em not} presented as something that really happens.
    Therefore, this picture of quantum interactions as processes
    in which virtual particles exchange is one of the
    most abused myths, not only in quantum physics, but in
    physics in general. Indeed, there is a consensus among experts
    for foundations of QFT that such a picture should
    not be taken literally. The fundamental principles
    of quantum theory do not even contain a notion of a
    ``virtual" state. The notion of a
    ``virtual particle" originates {\em only} from a
    specific mathematical method of calculation, called perturbative
    expansion. In fact, perturbative expansion
    represented by Feynman diagrams can be introduced even in
    {\em classical} physics \cite{thorn,penco}, but nobody
    attempts to verbalize these classical Feynman diagrams
    in terms of classical ``virtual" processes.
    So why such a verbalization is tolerated in quantum physics?
    The main reason is the fact that the standard interpretation
    of quantum theory does not offer a clear ``canonical" ontological picture
    of the actual processes in nature, but only provides
    the probabilities for the final results of measurement outcomes.
    In the absence of such a ``canonical" picture,
    physicists take the liberty to introduce
    various auxiliary intuitive pictures that sometimes help them
    think about otherwise abstract quantum formalism. Such auxiliary
    pictures, by themselves, are not a sin. However, a potential
    problem occurs when one forgets why such a picture has been introduced
    in the first place and starts to think on it too literally.
     
  7. Nov 24, 2006 #6

    Hans de Vries

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    They will basically tell you that a 'virtual' particle in QFT is off the mass shell

    [tex]E^2 - c^2p^2 \neq\ m^2c^4[/tex]

    This hasn't anything to do with Heisenberg's uncertainty principle and the
    idea that a particle "can have a different Energy for a short time"

    On the contrary. The laws of conservation are always observed
    exactly in QFT, for each and every vertex. The fact that they are off
    the mass shell is simply because the particles are not free solutions but
    are interacting. Every real particle which interacts is "off the mass shell"
    One could use the terms "free" and "interacting" particles instead of
    the terms real and virtual

    In more technical terms:

    The equation of motion of a virtual electron contains the interaction term
    from the photon “it has absorbed” while the equation of motion of a
    virtual photon contains the transition current representing the change
    of motion of the electron which has emitted the photon. This is the reason
    why they are off the mass shell.


    Regards, Hans
     
    Last edited: Nov 24, 2006
  8. Nov 24, 2006 #7

    vanesch

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    Great text :approve:
     
  9. Nov 24, 2006 #8

    Demystifier

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    Thanks vanesch! :smile:
     
  10. Nov 24, 2006 #9

    Hans de Vries

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    This may all be written out of some sort of preoccupation that a picture
    is taken to literally for your taste or gut feeling , however, the text has
    become way to dogmatic for my personal taste.

    Your message is:

    Those who consider a literal interpretation seriously are likely somewhat
    dumb (associate them with non-experts). "Kindly" you grant that they are
    not necessary evil though. (no sin). But still you cast doubt that their
    viewpoint should be tolerated at all.....

    There are many physicist who do take the literal interpretation very
    seriously. Nobel price winners like Feynman and Veltkamp (edit: Veltman)
    For the latter the only thing which really makes sense in QFT is the
    diagrammatica.

    Interestingly. These people are/were down-to-earth, no nonsense people
    in a quest for a non mystical, logical picture of Quantum Mechanics. Such
    a picture however does not necessary need to coincide with the most
    common primary reactions like QM should be Brownian motion or Bohmian...


    Regards, Hans.
     
    Last edited: Nov 24, 2006
  11. Nov 24, 2006 #10
    Actually, your previous (and this) answer is the only one with content to it in this thread :approve:

    Careful
     
    Last edited: Nov 24, 2006
  12. Nov 24, 2006 #11

    selfAdjoint

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    Oh yeah? Just because you liked that part and not the latest? Strikes me that this thread is getting pretty heavy on the personal theory side. Let us all remember that the great physicists who built field theory up to the level of the standard model were not fools, and may even have understood more deeply than we do. ALL of us!
     
  13. Nov 24, 2006 #12
    I clearly meant post 6 and 9 written by Hans. There is nothing personal about it, he gives a fairly objective point of view on the issue. I think we misunderstood each other here. Of course I agree with the last post, it is well known that Veltman is quite sceptical towards QFT.
     
    Last edited: Nov 25, 2006
  14. Nov 24, 2006 #13

    selfAdjoint

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    if you disagreed you could have said you disagreed instead of the insulting expression you did use.

    I don't know why so many savants from Europe think that this kind of trash talk is appropriate in discussing controversial issues.
     
  15. Nov 24, 2006 #14
    The thing is that there is no controversy in what Hans said (and neither is there about this topic), some other comments on this thread were much more insulting (as correctly pointed out by him). By the way, I believe Freidel (?) developped a theory of 2+1 quantum gravity with point particles entirely based upon a fairly ``literal'' interpretation of the Feynman diagrams.
     
  16. Nov 24, 2006 #15

    Hans de Vries

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    Martinus Veltman (sorry, my typo) strongly beliefs in Feynman
    diagrams (diagrammatica) but is not so impressed by how they
    are derived, quote:

    And further, in relation to the discussion on this thread:


    Regards, Hans
     
    Last edited: Nov 24, 2006
  17. Nov 24, 2006 #16

    arivero

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    IHans, about this explanation, could you be more concrete for instance in the case of electroweak interaction by exchange of a W or Z0 particle? My understanding is that two electrons, or two neutrinos, can interact by exchange of a Z0 even if their kinetic energy is a lot less than the mass of the Z0.


     
  18. Nov 24, 2006 #17

    Hans de Vries

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    Good remark. For neutral weak processes there are vertices where a
    neutrino emits a Z. (Like in Griffiths 2.4). Now consider conservation
    of momentum and energy at the vertex if the Z has its usual mass
    of 91 GeV...


    Regards, Hans.
     
  19. Nov 24, 2006 #18

    CarlB

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    Thanks for the note. I feel great sympathy for that point of view and will look around for Veltman's writings.

    Okay, I've ordered a copy of Veltman's introduction to QFT through Feynman diagrams:
    http://www.abebooks.com/servlet/SearchResults?&isbn=0521456924&nsa=1

    A great Veltman article:

    Perturbation Theory and Relative Space
    Martinus Veltman, 2006
    The validity of non-perturbative methods is questioned. The concept of relative space is introduced.
    http://arxiv.org/abs/hep-ph/9404358

    By the way, I've just added a section to my book on the elementary particles that has to do with the concept that Feynman diagrams are more fundamental than the Lagrangian. It's section 7.6:
    http://www.brannenworks.com/dmaa.pdf

    Carl
     
    Last edited: Nov 24, 2006
  20. Nov 25, 2006 #19
    Nah, I should have noticed it, the only excuse I have is that it was late in the evening (and I must have thought about ice skating or so):blushing:
     
    Last edited: Nov 25, 2006
  21. Nov 25, 2006 #20
    Unfortunately I'm such a non-expert who takes it too literally.

    To describe the interaction of two unaccelerated charges on each other ( a conservative static force field) we need the concept of virtual particles. Unless we believe in forces-at-distance.

    What am I missing?
     
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