  The principle of equivalence postulating that an acceleration is
indistinguishable from gravity by any experiment, is valid within families of
particles having the same passive gravitational to inertial mass ratio
$m_p/m_i$. Presently experimental observations indicate that we live in a
universe with one single family for which $m_p/m_i=1$, but if we consider the
imaginary case of a universe with several particle families having different
$m_p/m_i$, the principle of equivalence would still apply to each one of them.
On the basis of this generalized formulation of the equivalence principle,
which becomes relative to sets of particles, and that we designate as the
\textit{single-particle equivalence principle}, one demonstrates that inertial
frames can also be implemented for sets of electrically charged particles, with
the same charge-to-mass ratio $q/m_i$, accelerating in homogeneous electric and
/ or magnetic fields (by analogy with the case of particles in free fall in a
homogeneous gravitational field). Experimental evidences in support of the
proposed extension of the principle of equivalence to electric fields are
presented. These consist in the Witteborn-Fairbank experiment which revealed
that electrons do not fall in the Earth gravitational field, and the well know
fact that electric charges do not radiate either when in free fall in a
homogeneous gravitational field, or when being accelerated by a homogeneous
electric field. The London moment in rotating superconductors is also
supporting the proposed extension of the equivalence principle. Prospects for
the future exploration of the consequences of the proposed theoretical scheme
to unify electromagnetism with gravitation in the paradigm of curved
space-time, are also briefly suggested.
