4 ms·

Can anyone explain the vertical lines in the space time diagrams? Presumably not infinite velocity?

• yk 12y ago
  They are not space time diagrams, they are Feynman graphs. Technically they are a clever way to write down the terms of a series expansion. In the convention used, on the left hand side are the incoming particles and the outgoing particles are on the right. The internal lines and vertices are just representations of formulas, and the important thing is only the topology of the graph. However one of the really great things of Feynman graphs is, that they admit a easy interpretation about what is happening. Then the interior lines are interpreted as virtual particles that are created during the reaction, but since they are virtual they do not really have a defined speed, so the lines are just drawn like that.
  • jessriedel 12y ago
    In fact, Feynman diagrams are spacetime diagrams. (They are commonly computed in momentum space, i.e. Fourier space, but you can also do them in normal position space.) When you draw a Feynman diagram, the vertices correspond to actual points in spacetime, which are then integrated over a la the path integral.
• jessriedel 12y ago
  Yes, it's infinite velocity. When you compute an amplitude from a Feynman diagram, you sum (integrate) over intermediate particle paths that are superluminal. The math is structured in a way that these contributions cannot actually be used to send signals, but the contributions are crucial and cannot be discarded. Essentially, they represent the relativistic generalization of the effect where one particle in a Bell pair "influences" another over a spacelike separation, but in a subtle and spooky way that cannot produce superluminal signaling.