  We give a vacuum description with zero-point density for virtual
fluctuations. One of the goals is to explain the origin of the vacuum
permittivity and permeability and to calculate their values. In particular, we
improve on existing calculations by avoiding assumptions on the volume occupied
by virtual fluctuations. We propose testing of the models that assume a finite
lifetime of virtual fluctuation. If during its propagation, the photon is
stochastically trapped and released by virtual pairs, the propagation velocity
may fluctuate. The propagation time fluctuation is estimated for several
existing models. The obtained values are measurable with available technologies
involving ultra-short laser pulses, and some of the models are already in
conflict with the existing astronomical observations. The phase velocity is not
affected significantly, which is consistent with the interferometric
measurements.
