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  1. The photon is constantly "dressed in a fur coat" of virtual pairs of particles (electrons and positrons, quarks and antiquarks). The lower the photon energy, the weaker the "coat".

  2. The photon spends part of the time "naked", and part of the time is "dressed in a fur coat" of virtual particles. The lower the photon energy, the less time it spends "dressed in a fur coat".

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    $\begingroup$ The photon is always exactly what it is. The "fur coat" is our attempt to approximate the photon in the interacting theory in terms of the photon in the non-interacting theory. $\endgroup$
    – kaylimekay
    Jan 10 at 16:08
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    $\begingroup$ Exactly speaking, all the time. But to a given fixed order in perturbation theory, only a finite number of times, and we sum over all possible sets of times when the interaction takes place. I'm not sure if I'm getting close to your question, or if this counts as an answer. $\endgroup$
    – kaylimekay
    Jan 10 at 16:17
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    $\begingroup$ Well if you've included the field strength renormalization on your external lines in the diagram, that's coming from summing the geometric series of finite-order corrections. So technically that includes an arbitrarily large number of interactions in a single diagram, but each of those interactions is only computed up to some finite order. See my answer here. $\endgroup$
    – kaylimekay
    Jan 12 at 0:16
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    $\begingroup$ Yes, that's fair, if I'm understanding you correctly. $\endgroup$
    – kaylimekay
    Jan 12 at 7:53
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    $\begingroup$ Basically, I wanted to find out, photon all the time, constantly "wearing a fur coat", or just from time to time? Now I understand that "wearing a fur coat" all the time and constantly. Thanks for answers! $\endgroup$ Jan 12 at 7:56

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