CASIMIR EFFECT

Virtual photons corresponding to vacuum quantum fluctuations are present both in the space between the plates and out of them. On the other hand, through the principle of complementarity (between wave and corpuscular aspects) these photons can be regarded as electromagnetic waves. So it happens that the plates, although placed in a vacuum, and in the absence of explicit sources of electromagnetic fields, they find themselves surrounded in a "sea" of fluctuating electromagnetic waves.

Fonte dell'immagine:

http://www.casimir.rl.ac.uk/casimir-plates.gif

A general characteristic of waves (for example, those corresponding to the vibrations of a guitar string) is that the presence of "boundaries" has the effect of making impossible the existence of certain wavelengths, with respect to the case in which they can propagate freely. In the present case, the existence of plates reduces the "amount" of electromagnetic waves that can exist in the space BETWEEN plates, while there is no such a limit in outer space.