… Few concepts in physical science have been made as mysterious as the Casimir force. The force, identified in 1948 by the Dutch physicist Hendrik Casimir, is the result of the quantum nature of a vacuum, which is not empty at all but is filled with spontaneously fluctuating electromagnetic fields. These can be regarded as composed of photons created when pairs of particles and antiparticles pop unheralded into existence before annihilating each other in a burst of radiation, suggesting an image of the vacuum as a seething mass of ephemeral activity.
Because the wavelengths of the photons are constrained in the space between two surfaces to be no wider than this gap, the radiation pressure is smaller in this space than in the region outside, creating the (Casimir) force that pushes the surfaces together.
Now, this does represent a valid way of looking at the situation, and does indeed make the Casimir force seem rather remarkable. But there’s another, more mundane way to understand it. All surfaces contain electron clouds that are more or less ‘floppy’ or polarisable, deformable by nearby electric fields. These clouds undergo spontaneous quantum fluctuations, and when two such surfaces are very close together, each ‘feels’ the fluctuating polarisation of the other. The result is that the two may become correlated, creating an electromagnetically induced attraction between them. This is nothing more than the familiar ‘dispersion’ or van der Waals force, which decays as the inverse-cubed power of the distance from a flat surface.
But for one surface to feel the other, a photon has to pass between them. And as the surfaces get further apart, the time taken for the photon to travel across the gap becomes non-negligible, affecting the ability of each surface to respond to the other. This changes the character of the force, specifically by making it decay instead as the inverse-fourth power of the separation. This too is then the Casimir force: a kind of ‘retarded’ form of the dispersion force.
via Column: The crucible.