Do virtual particles interact with each other?

[Nugatory]

Of course they're not defined as observables, since they are virtual. I think the problem is that virtual particles are not defined, even for the brief moment they might appear, to have a specific location in time and space. So things like trajectory are not applied to them. But perhaps the probability of virtual particles interacting is calculable. Isn't that what we are doing inside complicated Feynman diagrams?

It's time to close this thread, and the last words should come from vanhees: "So better start studying a good quantum field theory textbook."