According to Quantum Field Theory, empty space is filled with virtual
particles, which in the case of the electro-magnetic force would be
virtual photons. H.B.G. Casimir predicted that modifying the
properties of these virtual photons, by placing boundaries such as two
parallel mirrors, would lead to an observable fource between the
mirrors. I will discuss some of the puzzling aspects of this force,
its measurement and its role in modern technology.