  The aim of the present article is to give physical meaning to the ingredients
of standard gauge field theory in the framework of the scale relativity theory.
Owing to the principle of the relativity of scales, the scale-space is not
absolute. Therefore, the scale variables are functions of the space-time
coordinates, so that we expect a coupling between the displacement in
space-time and the dilation/contraction of the scale variables, which are
identified with gauge transformations. The gauge fields naturally appear as a
new geometric contribution to the total variation of the scale variables. The
gauge charges emerge as the generators of the scale transformation group
applied to a generalized action (now identified with the scale relativistic
invariant) and are therefore the conservative quantities which find their
origin in the symmetries of the scale-space. We recover the expression for the
covariant derivative of non-Abelian gauge theory. Under the gauge
transformations, the fermion multiplets and the boson field transform in such a
way that the Lagrangian, which is here derived instead of being set as a
founding axiom, remains invariant. We have therefore obtained gauge theories as
a consequence of scale symmetries issued from a geometric fractal space-time
description, which we apply to peculiar examples of the electroweak and grand
unified theories.
