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I Photon propagator

  1. Feb 17, 2017 #1
    Hello! I have a question about the photon propagator in Feynman diagrams. I am looking over a brief derivation (probably there are some details missing), so basically it starts from Proca equation for a mass 0 particle, then it assumes Lorentz condition and in the end it obtains: ##(-p^2g_{\mu\nu})A^\nu=0##, from which the propagator is ##-i\frac{g_{\mu\nu}}{p^2}##. I understand the math, but I am a bit confused about the physics. So it starts with the Proca equation for a mass 0 particle, but the photon that is the propagator is off-shell (hence why p is different from 0) so it has mass. So how can you start from an equation for a massless particle and obtain a particle with mass?
    Thank you!
  2. jcsd
  3. Feb 17, 2017 #2


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    Off-shell means that the four momentum does *not* obey [itex] P^2 =m^2 [/itex] so the fact that [itex] P^2 \neq 0[/itex] does not mean that it is massive, it is still massless.
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