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I Virtual particles in Feynman diagrams and not

  1. Mar 8, 2016 #1
    I know some people here hate the subject of virtual particles. So I will try to narrow my questions. Granted, virtual particles are just attempts to describe the math and are not real in and of themselves.

    But there seem to be a couple of different kinds of virtual particles (in the math). There are the kind that appear in Feynman diagrams that are internal states. And then there are those that are said to exist everywhere, the kind used in descriptions of Hawking radiation, Lamb shift, vacuum polarization, etc. The virtual particles in Feynman diagrams are connected to things. For example, a photon turns into a pair of virtual electron/positrons, and then these combine into a photon again. But then there are the other kind that are said to pop into and out of existence. There are no lines going into them or coming out of them. The question is what property do the Feynman's virtual particles have that the others don't?
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  3. Mar 8, 2016 #2

    A. Neumaier

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    We don't hate virtual particles. We just take them as what they are - virtual, not real. This is why one can equip them with all sorts of marvellous properties otherwsie never heard of, without ever running into contradiction with reality. It is like virtual reality - made up reality without any true reality content.
  4. Mar 8, 2016 #3


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    Why do people never get tired of certain subjects and keep trying to get everyone writing the same things on these forums over and over again?
    There are at least 150 threads on this topic here. It's more writing than in "War and Peace" by Tolstoy. If people believe a line in a certain diagram is an element of reality, so be it. I believe only in mathematics.
  5. Mar 8, 2016 #4
    Right! Let's keep it about the mathematics. There seems to be some calculation for the vacuum energy, adding up the effect of virtual particles, as I recall. These don't seem to use in or out states, like in the Feynman diagrams. So what's different about them in the math?
  6. Mar 8, 2016 #5


    Staff: Mentor

    Yes, they do. Both the "in" state and the "out" state are the vacuum--no particles. That means all of the Feynman diagrams being evaluated have internal lines and loops only--no external lines. But it's still the same mathematical method being used.
  7. Mar 9, 2016 #6
    Thanks. Interesting way to look at it. I'd like to see the math. Can someone post the math used in the Feynman diagram where a virtual photon turns into an electron/position pair and then decays into a photon again. I want to see the term responsible for making sure it turns back into a photon again. Thanks.
  8. Mar 9, 2016 #7


    Staff: Mentor

    It's in any textbook on quantum field theory. If you have not looked at one, I strongly recommend doing so. This is much too complex a topic for a PF post; you need to have a good familiarity with the material already.
  9. Mar 12, 2016 #8
    Then let's take the simplest property of virtual particles. They have no in or out lines. They come in pairs, particle and antiparticle. They both appear at the same point, and they both exist for the same amount of time, and they both disappear at the same point. This suggests that their wave functions cancel each other out, which means they have the same magnitude, and they are 180° out of phase. What complex factor in the exponent of their wave functions would guarantee that the antiparticle is 180° out of phase with the particle? It can't be simply that one is the complex conjugate of the other, since the angle could be small. And for the same reason it can't be that the antiparticle has negative time, or that the antiparticle has negative mass. These don't guarantee 180° phase shift. So what could it be that guarantees that their wave functions are 180° out of phase? Thanks.
  10. Mar 12, 2016 #9


    Staff: Mentor

    Pretty much everything you're saying is wrong. Some of it isn't even a property of virtual particles.

    In and out lines (external lines) are a property of Feynman diagrams, not virtual particles.

    No, they don't. A virtual particle is a single internal line in a Feynman diagram.

    All wrong. See above.

    As you can see from the above, your question is based on an incorrect understanding of what virtual particles are. You need to spend some time getting a correct understanding; as I said, that's too big a topic for a PF thread.
  11. Mar 12, 2016 #10
    I didn't mean that all virtual particles have those properties. I meant let's take the case of a virtual particle pair that does not have any in or out lines, and that are created in a pair, particle and antiparticle. In that case (only), as I understand it, they are created at the same place, exist for the same amount of time (if any) and annihilate at the same place. Doesn't this mean that their wave functions cancel? Then what in the math accounts for that cancellation? Or were you trying to deny that virtual particles have wave functions?
  12. Mar 12, 2016 #11


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    They don't. Then again, in quantum field theories, neither do non-virtual particles.
  13. Mar 12, 2016 #12


    Staff: Mentor

    Yet you asked a question saying all the same incorrect things that I already pointed out (plus one other incorrect thing that I hadn't pointed out, but Nugatory did). Once more: you need to spend some time with textbooks getting a correct understanding of virtual particles, it's too big a topic for a PF thread.

    Thread closed.
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