I still do not understand the Casimir effect, more precisely what causes this. In the usual example of a parallel square plate kept very close to each other in vacuum, the plates feel an attractive force. This is a purely quantum field theoretic effect. My doubts are: 1. It is said that in vacuum there is no photon in between the plates but all the possible vibrational modes are present. We build an expression out of the quantities h, c and a (plate separation) but the amplitude of any wave do not occur since we are dealing with vacuum. I am not able connect vacuum fluctuation and vacuum polarization with this effect. Can anyone explain. 2. Why should only the electromagnetic field cause casimir effect? why not scalar/spinor/nonabelian gauge fields?