Attractive Forces from Particle Exchange

  1. anorlunda

    anorlunda 432
    Gold Member


    Above is Figure 65 from Feynman's book QED. It shows the bidirectional stream of photons between an electron and a nucleus, that Feynman says "keeps the electron within a certain range." I can visualize repulsive forces, but I'm having trouble with attractive forces.

    I understand that force comes from the gradient of fields, and that fields result from the presence of particles, so I should be able to answer my own question, but I'm having trouble putting it all together.

    "Feynman diagram general properties" by Maschen

    My thinking reflects the Feynman diagram above that I found on Wikipedia. As long as a right moving photon carries only right pointing momentum, (ditto for left) it can only produce repulsive forces. It sounds like I am describing radiation pressure, which indeed can only be repulsive between two emitting neighbors. But I fail to include in the analysis the magnitude and signs of the charges of the two particles, which is the elephant in the room.

    As I see it, Feynman's Figure 65 would be the same if I substituted a positron for the electron, and Maschen's Feynman diagram would be the same regardless of charges. So there's got to be more to the story.

    What am I missing, and where is a source I can go to learn more about it?

    p.s. I have the same conceptual problem with gluons, Z bosons, W bosons, and gravitons. If they are simply carriers of momentum, exchanges of particles should create only repulsive forces. Clearly I'm thinking wrong.
  2. jcsd
  3. Simon Bridge

    Simon Bridge 14,639
    Science Advisor
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    Last edited: Jul 20, 2014
    1 person likes this.
  4. anorlunda

    anorlunda 432
    Gold Member

    Wow; that's quite an answer. It will take me some time to absorb it. Thank you Simon Bridge.
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