  The Schwinger limit refers to longitudinal electric fields that are
sufficiently strong to "polarize the vacuum" into electron-positron pairs by a
tunneling mechanism. Laser fields are transverse electromagnetic fields for
which the Schwinger limit has no relevance. Longitudinal and transverse fields
are fundamentally different because of the different values of the
F^{{\mu}{\nu}}F_{{\mu}{\nu}} Lorentz invariant that characterizes the fields.
One aspect of this difference is the zero-frequency limit, that exists for
longitudinal fields, but is ill-defined for transverse fields. The goal of
approaching the Schwinger limit with sufficiently strong lasers is thus not a
possibility. Tunneling transition rates are characterized by an exponential
behavior of the form exp(-C/E), where E is the magnitude of the applied
electric field and C is a system-dependent constant. Searches for such behavior
within a Coulomb-gauge treatment of laser-induced processes are shown to fail.
