The Maxwell stress tensor is used to introduce an expression for the radiation pressure of the vacuum fields on
a slab. This expression is obtained in terms of the imaginary part of Green function of system and temperature
by getting expectation value at thermal state and using fluctuation dissipation theorem and Kubo?s formula.
After determining the required Green functions, we obtain the force between two slabs in Fabry- Perot cavity in
terms of the distance between two slabs and the slab from the mirror. Finally, the formula for force is presented
in some states of high and low temperatures