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Basic QED Question

  1. Jan 12, 2009 #1
    The QED vertex has two electrons lines joining with a photon line. However, 4-momentum conservation does not allow this process to occur. So why is this process studied?

    Does it really mean anything to calculate the g-factor of an electron for a process that cannot happen? Only the vertex correction contributes to the g-factor right? But if the other end of the photon line has to connect to something else to conserve 4-momentum, then don't you have to have a diagram with the photon self-energy?
  2. jcsd
  3. Jan 12, 2009 #2

    Vanadium 50

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    You misunderstand what a Feynman diagram is. It's not a description of the sequence of events (this electron is over here, then it emits a photon and moves over here...). It's a mnemonic for writing down the proper terms in doing a field theoretical calculation.
  4. Jan 15, 2009 #3
    Would it make sense to do a field theoretical calculation for the process of incoming electron, outgoing photon, and outgoing electron? The process can't happen because 4-momentum conservation won't allow it.

    I understand that such a calculation is useful if at least one of the three participating particles is off-shell, because then that would be a calculation of the basic QED vertex, which can be used to construct processes (such as 2-electron scattering) that actually can happen by 4-momentum conservation.
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