New Vacuum State of the Electromagnetic Field-Matter Coupling System and the Physical Interpretation of Casimir Effect

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 Uasimir 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.