  We compute the electromagnetic field created by an ultrarelativistic charged
particle in vacuum at distances comparable to the particle Compton wavelength.
The wave function of the particle is governed by the Klein-Gordon equation, for
a scalar particle, or the Dirac equation, for a spin-half particle. The
produced electromagnetic field is essentially different in magnitude and
direction from the Coulomb field, induced by a classical point charge, due to
the quantum diffusion effect. Thus, a realistic computation of the
electromagnetic field produced in heavy-ion collisions must be based upon the
full quantum treatment of the valence quarks.
