  In the fat brane model, also known as the split fermion model, it is assumed
that leptons and baryons live in different hypersurfaces of a thick brane in
order to explain the proton stability without invoking any symmetry. It turns
out that, in the presence of a gravity source $M$, particles will see different
four-dimensional (4D) geometries and hence, from the point of view of
4D-observers, the equivalence principle will be violated. As a consequence, we
show that a hydrogen atom in the gravitational field of $M$ will acquire a
radial electric dipole. This effect is regulated by the Hamiltonian
$H_{d}=-\mu\mathbf{A}\cdot\mathbf{\delta r}$, which is the gravitational analog
of the Stark Hamiltonian, where the electric field is replaced by the tidal
acceleration $\mathbf{A}$ due to the split of fermions in the brane and the
atomic reduced mass $\mu$ substitutes the electric charge.
