  The energy momentum tensor is used to introduce the Casimir force of the
massive scalar field acting on a nonpenetrating surface. This expression can be
used to evaluate the vacuum force by employing the appropriate field operators.
To simplify our formalism we also relates the vacuum force expression to the
imaginary part of the Green function via the fluctuation dissipation theorem
and Kubo formula. This allows one to evaluate the vacuum force without
resorting to the process of field quantization. These two approaches are used
to calculate the attractive force between two nonpenetrating plates. Special
attention is paid to the generalization of the formalism to D + 1 space-time
dimensions.
