# Questions tagged [propagator]

propagator gives the probability amplitude for a particle to travel from one place to another in a given time, or to travel with a certain energy and momentum.

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### Product of Lorentz invariant factors may be Lorentz non-invariant

I'm evaluating an integral and I have three cases to consider. The result of that integral must be Lorentz invariant and independent of center-of-mass momentum. One of the cases I'm certain is in fact ...
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### Reconciling special relativity and quantum mechanics

I have been referring to QFT for the gifted amateur, p. 75. To evaluate whether a particle can exist beyond its forward light cone, we check if it has a non-zero amplitude. The amplitude being ...
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### Relevance of perturbation theory at high coupling

Is there any possibility in any field theory model that at coupling >= 1.0, perturbation theory can be used to calculate, say field propagators beyond tree level? What is the room left for ...
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### Why does a pole in the Green function correspond to a bound state?

Consider the many-body (zero temperature) fermion Green function $$G(a,b;t)=-i\theta(t)\langle\psi_a(t)\psi_b^\dagger\rangle$$ Where I'm restricting $t>0$ for causality and that the free ...
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### A question about virtual particle

I am currently learning QFT and there is always a question to haunt me. As I know, virtual particles represent those propagators in the Feynman diagram which could be off-shell. My understanding is ...
• 75
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### Struggling with Peskin and Schroeder equation (12.49) and the constraint of renormalizability

In peskin and schroeder it's written that any renormalizable massless scalar field theory has a 2-point greens function of the form: I don't get how we can know that the 1-loop diagrams have exactly ...
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