Fermi National Accelerator Laboratory
Mitchell Golden
Fermilab MS 106
P.O. Box 500. Batavia. Illinois .60510

Dear Mr. Gordon,
I just got your question letter which you sent to Fermilab a year ago. For most of that year, it sat on someone's desk, and he didn't find it again until recently.
Your question is a good one. It is true that virtual processes can indeed change the mass of a particle by slowing it down when it's in a virtual state. When this happens one says that the mass of the particle has been "renormalized". For example, the mass we measure of the electron includes the correction from the time the electron spends dissociated into an electron plus a photon. That the photon can travel at the speed of light implies that it is massless. It remains massless even though it is sometimes dissociated into an electron-positron pair. This is because the electron and positron are virtual, and therefore are not constrained to travel at speeds less than the speed of light. A real electron can never go faster than light; no physical effect can propagate with speed greater than C. But a subtle cancelation happens in the case of light, and the average speed of the virtual electron-positron pair is still c, and the photon is not slowed down. A word of warning about this: the philosophy of quantum mechanics is that it is meaningless to talk about quantities which are not measured. Thus, strictly speaking, we are not allowed to talk about the velocity of a virtual particle, since, by definition, we never detect it. The correct treatment of these things requires an understanding of quantum mechanics. If you want to learn a bit about it, you could have a look at the third volume of Feynman's Lectures in Physics. I hope this answers your question. Sorry for the long delay.
Sincerely,
Mitchell Golden