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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 that, the concept of virtual particles is artificially given by theoretical physicist to describe physical processes more visually.

My question is that, does it mean that though we are using the description like exchange virtual particle to describe the interaction process, currently we do not know what is actually happening when interaction happened in QFT. Even though the result by using Feynman rule is consistent with experiment data, maybe what we have is just a black box and the actual nature is still unknown.

If you are not clear about what I mean. The comments below could help you to better understand.

David Shaw
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    This depends a lot on what you mean by the phrase "know what is actually going on". Do if think we know what is actually going on in the non-relativistic quantum mechanics of particles? Depending on your answer and what you think understanding really means will determine whether we understand what is going on in QFT by your standard – By Symmetry Apr 26 '22 at 12:04
  • @BySymmetry I know that our theory is always an approximation. I think my question is more close to that is the exchanging virtual particle a real physical process theoretically able to detect? or it only makes sense mathematically. – David Shaw Apr 26 '22 at 12:57
  • What is a "real physical process theoretically able to detect" mean to you? I can use virtual particles to do calculations, which agree with the numbers I extract from my experiments. To me, that means that I can detect their presence, in the sense that the model works. – QCD_IS_GOOD Apr 27 '22 at 03:00
  • @JoshuaLin I admit the mathematical meaning of the virtual particle, but I remain sceptical about its physical meaning as the model still work even through we do not regard the off-shell propagator as virtual particle. Could we actually detect the off-shell particle? If the virtual particle is something we cannot detect, then I think it shouldn't belong to science. The scientific significance of an explanation based on the development of artificially assigned meanings for the exchange of particles depends on the existence of virtual particles, doesn't it? – David Shaw Apr 27 '22 at 07:52
  • This may help: https://physics.stackexchange.com/a/275099/123208 Also see https://www.physicsforums.com/insights/misconceptions-virtual-particles/ – PM 2Ring Apr 27 '22 at 19:08

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