  A new concise method is presented for the calculation of the ground-state
energy of the electromagnetic field and matter field interacting system. With
the assumption of squeezed-like state, a new vacuum state is obtained for the
interacting system. The energy of the new vacuum state is lower than that given
by the second-order perturbation theory in existing theories. In our theory,
the Casimir effect is attributed neither to the quantum fluctuation in the
zero-point energy of the genuine electromagnetic field nor to that in the
zero-point energy of the genuine matter field, but to that in the vacuum state
of the interacting system. Both electromagnetic field and matter field are
responsible for the Casimir effect.
