Feynman Diagrams for Exam Prep: Position vs. Momentum Space

[madness]

I'm learning how to do Feynman diagrams for an exam coming up, and have come across diagrams in both position and momentum space. If I'm asked to draw a Feynman diagram without specifying which kind, is it generally assumed that it's in position space? If I label the the incoming and outgoing particles by momentum (as well as spin etc) is it then assumed that the diagram represents momentum space?

 

[ansgar]

yup correct! :)

 

[madness]

What is the difference when drawing diagrams in momentum space and position space? In my notes the 2 lowest order diagrams for Compton scattering look quite different in momentum and position space. In position space the internal electron line in the second diagram points horizontally in position space and vertically in momentum space. What does this mean?

 

[Amalrich]

What is the difference when drawing diagrams in momentum space and position space? In my notes the 2 lowest order diagrams for Compton scattering look quite different in momentum and position space. In position space the internal electron line in the second diagram points horizontally in position space and vertically in momentum space. What does this mean?

bump! I would like the answer to this as well.

 

[saaskis]

What is the difference when drawing diagrams in momentum space and position space? In my notes the 2 lowest order diagrams for Compton scattering look quite different in momentum and position space. In position space the internal electron line in the second diagram points horizontally in position space and vertically in momentum space. What does this mean?

The actual diagrams are the same in every basis, only the mathematical expressions representing the diagrams are different. As for your question about the internal electron line: if two diagrams only differ by some sort of rotation of propagator lines, the diagrams are the same (topologically equivalent), since when you write down the mathematical expression for the diagram, all that matters is the relative positioning of vertices and lines. The orientation of lines does not step into equations.

 

[madness]

Thanks a lot that's exactly what I needed to know.