[iainmerrick]

While you're explaining individual words here (!) can you help me understand that use of "virtual" in high energy physics?

"Virtual particles" implies they're somehow not real, but I assume the theory says they do have a real physical existence in some sense -- they have an influence on the real particles. Are they "virtual" just because they're extremely hard to isolate, so all our practical experiments are indirect, or are they different in principle from the normal, "real" particles that our experiments work with? Is it a qualitative or just a quantitative difference?

[yoklov]

It’s been a long time (and this was never my strongest area), but in case someone who really knows what they’re talking about doesn’t get back to you, it’s basically a particle who may or may not exist based on quantum uncertainty, but in practice IIUC it’s more like an excitation of the underlying quantum field.

[Koshkin]

Both real and virtual "versions" of a type of particles are described as excitations of the same quantum field; the difference is that only the real particles can be detected experimentally, whereas the virtual ones are used to model interactions between (real) particles and thus only appear in calculations.

[iainmerrick]

Thanks, that helps, I think...!

Would it be roughly correct to say that virtual particles by definition always vanish (mutually annihilate, etc, whatever) before the measured outcome of an experiment, therefore they definitely cannot be directly observed? But they're an integral part of the model, therefore if the model is correct, they really do have a real physical existence (in some complex quantum sense)?

[Koshkin]

I do not believe so. Generally speaking, mathematical models often include what I call "scaffolding," which is something that, while being part of the model, does not represent anything real. (Different models are likely have different scaffolding, and some future theory might do away with virtual particles as a mechanism of interaction.)

[deepnotderp]

IIRC, those are also called quasiparticles.

[qubex]

As I understand it, ’virtual’ particles are those that are not observed ”on shell” (i.e. going into or coming out of a specific interaction), and hence whose existence cannot be affirmed if not by the effect their evanescente presence had upon what did actually come out, or rather, the statistics of what comes out of many identical inputs interacting.