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Question regarding Feymann Diagrams and other QED things

  1. May 17, 2004 #1
    I have been going through many books on QED, trying to get a general non-mathematical feel for it before I take a course in it and I have come across some questions that I have not been able to answer.

    Nearly all of the sources I have gone to refer to Feymann Diagrams as a easy way to depict the interactions between particles and their virtual particles. The most common example is two electrons moving through time come close together, one electron emits a photon (causing it to repel) and the other electron absorbs this photon causing to to move in the other direction.

    This is hailed as being a wonderful explanation for the Electric Force through the use of virtual particles.

    But what about the attractive force between a positively charge particle and a negatively charged one? Do the particles somehow emit photons in opposite directions so that they move towards each other?

    Second, why do they emit photons in the first place? Is this just a convenient explanation for the phenomenon or is there a deeper reason for it?

    If anybody can offer me some insight into these topics it would be greatly appreciated
  2. jcsd
  3. May 17, 2004 #2


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    You can see (see below)that the Feynman diagram for electron-positron attarction looks very simlair to the one for electron-electron repulsion. Just like in attractive case the elctron and positron still exchange a virtual photon. It isn't a case that one electron emits a photon, the other absorbs it as that would imply an assymmetry in what is a symmetric process, a virtual photon is 'exchanged'.

    edited to add: it's better to think of the electrons both emititng a particle and absorbing a particle

    Here's several Feynman diagrams, including the ones for attraction and repulsion:


    Ask yourself what is a photon? Answer: It is a quantum of electromagnetic radiation. What force causes attraction and repulsion between charged objects? Answer the electromagnetic force! Of course the photon that is exchanged is virtual.
    Last edited: May 17, 2004
  4. May 17, 2004 #3
    I'm sorry but this explanation does not seem quiete satisfactory because the example given shows an electron and positron interaction. I want to know what happens in an interaction between an electron and a proton. I think i have kinda grasped that because a positron can also be depicted as an electron traveling backwards in time then this offers an explanation for the attraction between the two; but this does not explain the attraction between an electron and a proton.

    I have seen diagrams showing a proton and an electron exchanging virtual photons, but they do not offer the same kind of explanation for the electron-electron interaction (electron emits photon and repels in opposite direction, photo hits other electron causing it to go in the other direction). I want to know why this virtual photon interaction can cause two things to be attracted to each other.
  5. May 18, 2004 #4
    gribbin's book Search for superstrings, symmetry and the theory of everything gives a good conceptual account of it, but i'm too close to sleep to regurgitate it right now...maybe tomorrow.
  6. May 19, 2004 #5
    Hi, sharpstones. I have not taken a course of QED. I just studied QED by reading some book. So I'm not confident about my reply. But I hope it is helpful for you.
    You seem to be thinking repel of same charge is caused by repel when virtual photon is emitted or absorbed by electrons. But I think it is not true. Attraction and repel are caused by electromagnetic force itself. An interaction by electromagnetc force can be interpreted as exchange of virtual photons. Interation but not attraction or repel is exchage of virtual photons. Attraction or repel is caused by interation by electromagnetics which is exchange of virtual photon.
    I hope you understand what I mean, which is difficult to put into words.
    Last edited: May 19, 2004
  7. May 19, 2004 #6
    the most concise explanation of feynman diagrams and particle interactions can be found in, you guessed it, QED by feynman himself - a series of lectures prepared for the freshman physics class at caltech. the diagrams are a simplified method of calculating probabilities, and he explains it very directly - it is not a hard concept or methodology.

    as to virtual photons, as per the example given inthe original question - virtual photons and the mechanism by which particles interact at this level is all conjecture, and you are correct that it is merely a convenient way to discuss a process about which we know absolutely nothing - it is a complete mystery, and certainly one of the most intriguing in physics.
  8. May 21, 2004 #7


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    The answer to your question is actually very simple. The amplitudes end up with opposite signs, when you run through the QED rules.

    Now, there are a few subtelties with the proton that is .. problematic. First thing, a proton is not a fundamental particle, its made up of quarks. So really you want to look at some sort of blob (which you can parametrize if you want) at the proton interaction vertex, where other higher order effects might come into play. But at typical qed energies, its not a problem. You can treat it as a point particle.

    Second, to really see the effect you have to actually go through a little bit of spinor math and put in the spins and polarizations and make sure all the signs come out right. But its instructive for beginners in field theory to do just that... So do it!

    In the nonrelativistic limit, you better hope you retrieve coulombs law for that interaction.

    Other thing to keep in mind.. Electron-electron scattering has 2 diagrams that contribute to the amplitude. What you say, different forms for the amplitudes? Well yes, but ew only measure amplitude squared or more precisely cross sections, there are factors of S in fermi's golden rule that will make things work out.

    Its actually easier to compare and contrast electron-positron scattering and electron-electron scattering, as they are very closely related (just the spinors are in different places).
  9. May 23, 2004 #8
    The diagrams are more than just diagrams, they are abstract reps of the equations that describe the interactions.
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