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Homework Help: Problem with Feynman's iε prescription

  1. Jul 3, 2011 #1
    1. The problem statement, all variables and given/known data

    My Problem is with Feynman's iε prescription. Trying to solve an Integral, it happens that there is a singularity s on the way. s depends on the Energy E of a particle. This integral is not convergent and doenst make any sense. To solve this Problem, one displaces the Energie from E to E - iε, with ε being very small. ( In Case of an anti particle it is E + iε).

    2. Relevant equations

    So the denominator of the dr integral looks like this: [ sqrt(r) - sqrt(2*(M-E' ) ] . now r gets integrated in such a way, that it always would hit the singularity s=sqrt(2*(M-E')). Changing E to E-iε however gets rid of the singularity. My problem is now not how to solve the Integral with the iε (which im glad i found out already by myself), but to argue why we need such an iε in there in the first place.

    3. The attempt at a solution

    I tried to look up several books in Quantum field theory , but as i havent taken such a course yet i dont understand them very well. There is so much stuff in there that im not really sure im missed an explanation. Im very confused :( .

    Can you help to help me and give me a tip where i can find anything that explains it in maybe not a rigourous way? Trying to google feynman iε prescription has not really helped me :(

    thank you very much for reading and maybe answering :)
  2. jcsd
  3. Jul 3, 2011 #2


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    Science Advisor
    Homework Helper

    The short answer is that the choice of the sign of i*epsilon determines whether you have chosen a propagator that propagates into the past or propagates into the future. For causalities sake you want the latter.
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