On possible origin of an anisotropy in the speed of light in vacuum
  We assume the spacetime foam picture in which vacuum is filled with virtual
wormholes. In the presence of an external field the distribution of wormholes
changes. We consider an anisotropic distribution of wormholes and analyze its
relation to the speed of light. We show that speed of light acquires an
anisotropic character and save the normal dispersion a gas of virtual wormholes
may possess also an anomalous dispersion, i.e., when the light velocity exceeds
that in the vacuum.
