Here is a citation from
http://arxiv.org/abs/quant-ph/0609163
Virtual particles?
The calculational tool represented by Feynman diagrams
suggests an often abused picture according to which
``real particles interact by exchanging virtual particles".
Many physicists, especially nonexperts,
take this picture literally, as something that
really and objectively happens in nature. In fact, I have
{\em never} seen
a popular text on particle physics in which this picture was
{\em not} presented as something that really happens.
Therefore, this picture of quantum interactions as processes
in which virtual particles exchange is one of the
most abused myths, not only in quantum physics, but in
physics in general. Indeed, there is a consensus among experts
for foundations of QFT that such a picture should
not be taken literally. The fundamental principles
of quantum theory do not even contain a notion of a
``virtual" state. The notion of a
``virtual particle" originates {\em only} from a
specific mathematical method of calculation, called perturbative
expansion. In fact, perturbative expansion
represented by Feynman diagrams can be introduced even in
{\em classical} physics \cite{thorn,penco}, but nobody
attempts to verbalize these classical Feynman diagrams
in terms of classical ``virtual" processes.
So why such a verbalization is tolerated in quantum physics?
The main reason is the fact that the standard interpretation
of quantum theory does not offer a clear ``canonical" ontological picture
of the actual processes in nature, but only provides
the probabilities for the final results of measurement outcomes.
In the absence of such a ``canonical" picture,
physicists take the liberty to introduce
various auxiliary intuitive pictures that sometimes help them
think about otherwise abstract quantum formalism. Such auxiliary
pictures, by themselves, are not a sin. However, a potential
problem occurs when one forgets why such a picture has been introduced
in the first place and starts to think on it too literally.
