Answer 1:
A virtual particle is an abstract construct
to describe interactions between actual
particles, and arises from perturbation
theory in quantum field theory. It is not
actually a particle at all, but something that
pops up in the math. So, what does that all
exactly mean?
Perturbation theory is a technique to use
known solutions of a simple problem to approximate
a more complicated problem that looks like a
weakly perturbed version of the simple one.
The interaction of actual particles (e.g.,
electrons, photons) is abstracted to be the
exchange of virtual particles. Examples of such
interactions include the Coulomb force,
magnetic field, and strong nuclear force. You are
probably most familiar with the Coulomb force,
which in classical mechanics is:
Coulomb Force (click here)
It turns out you can also cast the fields from
Coulomb interaction in this framework of virtual
particles. Virtual particles are abundant in what
are known as
Feynman diagrams (click here) which are
pictorial representations (click here) of the
integrals
used to calculate these interactions.
This is a pretty good example of a common
phenomenon in quantum mechanics and related fields
where the English word is much less precise about
the mathematical description of some physics. It's
important to decouple what you ordinarily
associate with words colloquially (i.e., in
everyday conversation) from what the physics is
communicating to you. If you wanted to attach
physical significance to the concept of a virtual
particle, it is more useful to think of it not as
a particle at all. Rather, you can think of it
as a disturbance to a field. For example, in
the cased
two interacting electrons, (click here) it
is said that
they "exchange virtual photons." They
don't actually exchange any photons; it is a
disruption to the electromagnetic field of the
electrons (e.g., Coulomb repulsion, magnetic
field
due to moving charge). It turns out that the
mathematical description for this interaction
happens to also describe photons as well, hence
the name.
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