Cornell University
LABORATORY OF ATOMIC AND SOLID STATE PHYSICS
CLARK HALL
ITHACA, NEW YORK 148~3
Office of the Director
Dear Mr. Gordon,
The speed of light in vacuum is C, even when one takes into account vacuum polarization (the "excitation" of virtual electron-positron pairs) On the other hand your question is a good one, and I really can't think of any simple reason why the preceding sentence is correct.
Sincerely yours,
N. David Mermin