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Virtual photons

  1. Feb 20, 2004 #1

    EL

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    Hi everyone! Time has come for my first thread in this nice forum:

    I wonder if someone could explain the concept of virtual photons (and virtual particles in general)? Have I understood it right that they are just a mathematical construction and are not in any way "real"?
    I have a lack of knowledge in particle theory, but am familiar with basic quantum mechanics, so hopefully I will be able to understand an explanation at a rather high level...
     
  2. jcsd
  3. Feb 20, 2004 #2

    selfAdjoint

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    Whether virtual particles are real or not is a moot question.

    Here's the idea. In quantum mechanics nothing is really real unless you can observe it or measure it. In order to be observable, a particle has to have some minimum amount of energy for some minimum amount of time; this comes out of the uncertainty principle that says the product of those two things has to be bigger than a certain number.

    So it's possible to conceive of a particle whose energy is not big enough or whose lifetime is not long enough to permit a true quantum measurement, but still both of them could be greater than zero. The world could be full of such particles, and the measurements would never show it.

    Well, quantum field theory takes those particles seriously. It says they interact with observable particles, for example they make the electron which emits and absorbs them a bit heavier, and a bit more sluggish in motion, than it would be if they didn't exist.

    Furthermore, QFT says that the virtual particles are the ones that carry the forces. For example with photons, the "real" photons make light, and other forms of electromagnetic radiation, but the virtual photons carry the electric force; a charged particle is charged because it emits virtual photons. And the other bosons, that carry the weak and strong forces, behave the same way. Real particles interact with each other by exchanging virtual bosons.

    This is the story quantum field theory tells, and the justification, the reason you should at least consider beliving in it, is that it makes fantiastically correct predictions. That bit above where I said that interacting with virtual particles made the electron sluggish? It's called the anomalous moment of the electron, and the prediction, based on virtual particles, matches experiment to six decimal places.

    For several decades the theorists and experimenters were in a kind of race. The experimenters would rack their brains and come up with a new way to measure down to another decimal place. And the theorists would grind through the next level of calculations (in the same theory mind you, this is not about epicycles) and come up with another decimal place of their own. And the two would match!
     
  4. Feb 20, 2004 #3

    EL

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    Thanks a lot! Everything is now clear (except for the word "moot" =))...
    Does anyone know any good litterature or links about this subject?
     
  5. Feb 21, 2004 #4

    selfAdjoint

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    "Moot" has unfortunately two completely opposite meanings (like "fast"). The first, which I did not intend, is of a completely settled question. The second is of a much discussed question. That is the one I meant.

    The different meanings come from the practice in law schools of using moot (settled) court cases for study and discussion.
     
  6. Feb 21, 2004 #5

    EL

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    Ok, I just never had heard the word "moot", because english is not my main language...
     
  7. Feb 24, 2004 #6
    virtual photons do not conserve energy, they borrow energy and repay.
     
  8. Feb 24, 2004 #7
    If virtual photons borrow energy?..can they also borrow after a repayment has been made? Do they always follow convensional lending practise's?..ie..is repayment possible pior to borrowing?
     
  9. Feb 25, 2004 #8

    arivero

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    In hep physics lingo, they are off-shell
     
  10. Feb 25, 2004 #9

    Njorl

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    It's a shame English is not your first language, because you made a great joke by accident!

    Njorl
     
  11. Feb 25, 2004 #10

    EL

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    Hehe. It was, hmmm, on purpuse...=)

    Just to make something clear...I didn´t ask for litterature about the word "moot"...so, again, anyone who has some texts to recomend about virtual particles?
     
  12. Feb 25, 2004 #11
    start here
     
  13. Feb 25, 2004 #12
    One might think of virtual particles being primarily of probable properties rather than with a definite physical identity. The action (energy-time or momentum-displacement) of a virtual particle is less than Planck's constant, h, and involves complex quantities that interfere toward a resultant probability - say, that an electron has a .5 chance of being within an Angstrom of a particular atom. When measured, the particle is said to be real, and takes on the definite properties bestowed from one possibility of many.

    Forgive my rambling; I gave blood today.
     
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