  The need for a cutoff in the Lamb shift calculation suggests that high-energy
virtual photons do not interact with real particles. In this paper, we assume
that the creation of virtual photons follows a canonical distribution. As a
result, the Coulomb potential is modified to $V(r)=- ze^2\arctan (m_c
r)/(2\pi^2 r)$, and the zero-point energy density of the electromagnetic field
becomes finite.
