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Attractive Forces from Particle Exchange

  1. Jul 20, 2014 #1


    Staff: Mentor


    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. Jul 20, 2014 #2

    Simon Bridge

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    Science Advisor
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    Last edited: Jul 20, 2014
  4. Jul 20, 2014 #3


    Staff: Mentor

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