Consider the space between two closely spaced flat conducting parallel plates. The boundary conditions give a maximum wavelength that can fit between the two plates along the normal direction. The maximum normal wavelength between the plates is twice the plate separation. This restriction is not found outside the plates. Therefore, there are more field modes outside the plates than inside. This implies that the vacuum zero point energy density due to the virtual particles outside the plates is bigger than it is inside the plates. The result is a measurable Casimir attractive force between the plates found by experiment. These zero point energy forces can be used in the operation of delicate nanodevices.