I know some people here hate the subject of virtual particles. So I will try to narrow my questions. Granted, virtual particles are just attempts to describe the math and are not real in and of themselves. But there seem to be a couple of different kinds of virtual particles (in the math). There are the kind that appear in Feynman diagrams that are internal states. And then there are those that are said to exist everywhere, the kind used in descriptions of Hawking radiation, Lamb shift, vacuum polarization, etc. The virtual particles in Feynman diagrams are connected to things. For example, a photon turns into a pair of virtual electron/positrons, and then these combine into a photon again. But then there are the other kind that are said to pop into and out of existence. There are no lines going into them or coming out of them. The question is what property do the Feynman's virtual particles have that the others don't?