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Is a virtual particle necessary an off-shell particle? Could a virtual particle be on-shell?

Urb
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Mathieu Krisztian
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  • see also my answer here https://physics.stackexchange.com/questions/185110/do-virtual-particles-actually-physically-exist/185139#185139 – anna v Mar 20 '22 at 14:39

1 Answers1

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A virtual particle, by definition, is an internal line in a Feynman diagram. In most cases, it is off-shell. In some special cases, e.g. when studying factorization channels of the $S$-matrix, it can be taken on-shell.

Prahar
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  • Sorry, my question is necessary. You start by saying yes, and after you say that it cas be on-shell. So your answer is no ? Please correct your answer. – Mathieu Krisztian Mar 20 '22 at 13:59
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    I'm answering the question in your post, not the one in your title. Your title and post have two completely different questions! – Prahar Mar 20 '22 at 14:00
  • In your title you're asking on-shell vs off-shell and in the post you ask about internal vs external. – Prahar Mar 20 '22 at 14:01
  • Sure, I have corrected by putting "necessary" in the main question. Please see. – Mathieu Krisztian Mar 20 '22 at 14:02
  • "it" is not meaning "particle" : it is meaning "virtual particle". So could a virtual particle be on-shell ? – Mathieu Krisztian Mar 20 '22 at 14:02
  • You need to correct it by asking the same question in the title and in the post. As of now, your title and posts are asking different questions. In any case, I have edited my answer. – Prahar Mar 20 '22 at 14:03
  • I have corrected. So could a virtual particle be on-shell (and still called virutal particle), or never ? – Mathieu Krisztian Mar 20 '22 at 14:05
  • "Virtual particles" do not represent ANYTHING physical ever. Individual Feynman diagrams also do not represent anything physical. You should not attempt to take these concepts beyond their realm of validity. However, there is a sense in which virtual particles can go on-shell and it occurs when studying a factorization channel for an $S$-matrix. There is no other physical interpretation of virtual particle going on-shell (because even virtual particles are not physical in any way). – Prahar Mar 20 '22 at 14:07
  • sorry, you do not answer my question : could a virtual particle be on-shell (and still called virtual particle) : yes or no ? – Mathieu Krisztian Mar 20 '22 at 14:08
  • Let us continue this discussion in chat. – Prahar Mar 20 '22 at 14:10