The typical example is of the two uncharged conductive plates in a vacuum, placed a few nanometers apart. In a classical description, the lack of an external field means that there is no field between the plates, and no force would be measured between them. When this field is instead studied using the QED vacuum of quantum electrodynamics, it is seen that the plates do affect the virtual photons which constitute the field, and generate a net force—either an attraction or a repulsion depending on the specific arrangement of the two plates.
This claim of force detection is confirmed on Matt Strassler's Blog so I guess it's generally accepted.
This is the point where I wander into the minefield of my ignorance of string theory and a reliance on popular science books on string theory, so my apologies for what follows.
As far as I remember, 1-D strings require vacuum fluctuations for their stabilisation. I tried to find a source to confirm this, but all I get are references to cosmic strings.
My questions are, if the Casimir force has been confirmed, does it conform to predictions based around 4-D spacetime? If so, then how does this fit with the need for 11 dimensions in string theory? In other words, should the energy related to vacuum fluctuations not be spread across 11 dimensions and therefore be weaker than predicted?
Or does string theory postulate another smaller scale class of vacuum fluctuations, as virtual photons, electrons etc, are the postulated vibration modes of fundamental, many order of magnitudes smaller, 1-D strings and currently we have no chance of detecting these fluctuations, if ever.
My apologies if this question is based on an incorrect understanding of a badly written book and I will withdraw it if that's the case.