  A four-vector field in flat space-time, satisfying a gauge-invariant set of
second-order differential equations, is considered as a unified field. The
model variational principle corresponds to the general covariance idea and
gives rise to nonlinear Born-Infeld electrodynamics. Thus the four-vector field
is considered as an electromagnetic potential. It is suggested that
space-localized (particle) solutions of the nonlinear field model correspond to
material particles. Electromagnetic and gravitational interactions between
field particles appear naturally when a many-particle solution is investigated
with the help of a perturbation method. The electromagnetic interaction appears
in the first order in the small field of distant particles. In the second
order, there is an effective Riemannian space induced by the field of distant
particles. This Riemannian space can be connected with gravitation.
