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  • $\begingroup$ Thank you! I think I get it now. However I want to know your "opinion" about my last question: what does it mean physically when a propagator diverge? $\endgroup$ Commented Aug 22, 2015 at 7:28
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    $\begingroup$ @Crazydemon: No opinion to give, it's a well-known fact (called the Källén-Lehmann spectral representation) that the poles of the propagator correspond to the $n$-particle states of the theory. In other words - the value of $p^2$ where the free propagator diverges is the mass. $\endgroup$ Commented Aug 22, 2015 at 13:35
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    $\begingroup$ Not well known for me, I'm currently doing QED as a student. Thank you very much! $\endgroup$ Commented Aug 22, 2015 at 17:33
  • $\begingroup$ @ACuriousMind is there any intuition behind why the poles of the propagator correspond to the on-mass-shell particle states of the theory? $\endgroup$ Commented May 11, 2021 at 14:14