Feynman diagram: incoming and outgoing particles

[Dmitry67]

I always wanted to ask (but was afraid to) about the borders on the famous Feynman diagrams. As I understand, Feynman diagrams have the same age as Copenhagen Interpretation. So at that time it looked very logical: quantum behavior inside, but classical world outside. Outgoing particles are the particles ‘measured’ by some ‘measurement device’. Incoming ones – those coming from a reliable macroscopic source.

Borders of the Feynman diagram separated quantum worlds from a classical environment.

Shouldn’t that be modified when we know that there is no sharp line which separates both worlds: classical and quantum? Shouldn’t we see the Universe as one huge Feynman diagram – without any borders whatsoever? And what value, in that case, has telling if a particle is ‘real’ or ‘virtual’ by looking if it crosses the ‘border’ of the Feynman diagram?

 

[tiny-tim]

Hi Dmitry67!

I always wanted to ask (but was afraid to) about the borders on the famous Feynman diagrams …

Sorry, but Feynman diagrams (either "position representation" or "momentum representation") are just diagrams that help mathematicians to add up the terms (to any required order) of the Dyson expansion of the Hamiltonian.

They aren't pictures (not even symbolic pictures) of actual interactions between particles, and so the internal particles in a Feynman diagram have no relevance to anything physical happening.

The borders of a Feynman diagram are just the input and output particles … they're the only realistic part of the diagram.