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. https://upload.wikimedia.org/wikipedia/commons/7/7a/Feynman_diagram_general_properties.svg "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.