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I accidentally came across the words "soft photon" today after reading a few blogs. There was some discussion of special situations involving gauge redundancies and a theorem by Weinberg.

  1. What is a soft photon?

  2. What is the simplest clearest possible description of Weinberg's soft photon theorem is?

  3. How is it motivated and then derived?

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    $\begingroup$ or here, section 6. $\endgroup$ – AccidentalFourierTransform Apr 15 '16 at 14:56
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    $\begingroup$ and comments here. $\endgroup$ – Mike Apr 15 '16 at 14:59
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    $\begingroup$ In traditional QFT literature, a soft photon is a photon with very small energy compared to the typical energy scale of the scattering problem. Another point of view that is also getting some attraction these days is that a soft photon is really to be understood as a exactly zero energy photon - so that instead of giving a soft photon a particle interpretation it becomes synonymous with "a degenerate vacuum state of the theory". $\endgroup$ – Prahar Apr 15 '16 at 18:13

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