appearance of virtual particles.

Is the creation of virtual particles a random or predictable event? I used to have a great, worked out theory regarding free will, but just now in a conversation, I remembered about virtual particles. They might introduce a random variable. So, which are they?

They’re predictably random I think. It’s a QM thing. I don’t quite get it. Or maybe I do, I haven’t looked recently. q;}

I am not an expert but aren’t quantum events supposed to be the only events that are truly random? My understanding (taken from other people) is that the appearence of virtual particles is completely random in all senses of the definition.