Feynman diagrams are drawn on Space-time graphs. So is it possible for any particle path in that diagram to have a slope smaller than 45° or greater than 135°, i.e. can they travel faster than light? Does the slope or or velocity of the particles have any effect on the outcome of the diagram? Virtual particles (photons and the like) should have no limits for their properties, but what about incoming particles, say electrons?
Answer
Incoming/outgoing particles with mass must travel at less than the speed of light. Incoming/outgoing massless particles must travel at the speed of light. The “virtual particles” inside the diagram can “travel” at any speed, but since they don’t really behave like particles this doesn’t matter.
Feynman diagrams aren’t pictures of what is “really happening”; they are just calculational tools. “Virtual particle” is an unfortunate misnomer, and the emphasis should definitely be on “virtual” rather than “particle”.
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