16 minutes ago, Genady said:

I mean input and output in particle reactions. External legs on Feynman diagrams. Virtual particles are, by definition, internal lines in the latter.

OK that's what I thought. That seems to suggest there is no experiment one could ever devise that would be evidence of any property of these virtual particles. In that respect they are unlike, say, the Higgs boson, whose existence was predicted and then evidence was found. 

7 minutes ago, exchemist said:

OK that's what I thought. That seems to suggest there is no experiment one could ever devise that would be evidence of any property of these virtual particles. In that respect they are unlike, say, the Higgs boson, whose existence was predicted and then evidence was found. 

OK. To me, the fact that reactions involving virtual particles occur as predicted, is as good evidence as any.

More generally, to me, virtual particles are as real as any, with a defining distinction that they appear temporarily and disappear during reactions.

I would say something like, virtual particles actually contribute to the mass, charge, and other quantum numbers of real particles. So, in a way, we do have indirect evidence of them playing a role. It's just that they never show up. They are modes of the quantum field. It's just they generally keep a low profile, so to speak. I would try to stay away from any metaphysical language here.

5 minutes ago, joigus said:

I would say something like, virtual particles actually contribute to the mass, charge, and other quantum numbers of real particles. So, in a way, we do have indirect evidence of them playing a role. It's just that they never show up. They are modes of the quantum field. It's just they generally keep a low profile, so to speak. I would try to stay away from any metaphysical language here.

I can't come up with any caveat to add here .

1 hour ago, Genady said:

I can't come up with any caveat to add here .

Yes maybe that's the best way to look at it. 

1 hour ago, Genady said:

I can't come up with any caveat to add here .

 

2 minutes ago, exchemist said:

Yes maybe that's the best way to look at it. 

Now I realise I meant to emphasise "indirect", not "evidence."