The Casimir effect is a weak force, usually attractive, that acts between two neutral surfaces in close proximity to each other.
The existence of this force was predicted in 1948 by the dutch physicist Hendrik Casimir.
From quantum field theory, Hendrik Casimir concluded that two neutral surfaces would attract each other due to the limited number of wavelengths that can exist in an enclosed space. With fewer wavelengths existing in the space between the surfaces than what can exist in the surrounding space, the energy density of the enclose space would be less. The energy density of the surrounding space would therefore result in pressure on the plates, forcing them together.
Using the formulas found in quantum field theory, the force can be calculated to be infinitely strong. It was therefore a bit of a surprise to discover that the force was in fact quite weak, and can at times be repelling.
Hendrik Casimir’s prediction was correct as far as the existence of a force was concerned. The actual measurements confirmed the existence of something real going on at the zero-point level. But it could hardly be seen as a validation of quantum field theory.
A number of alternative explanations have therefore been proposed. There is a purely relativistic solution, based on the van der Waals force, and there is a coupled ground-state energy solution.
The phenomenon can also be explained in terms of a strict particle model in which there is an aether of zero-point particles.
An electron in an aether of zero-point particles
We can imagine two neutral plates that act as shields, restricting the entry of zero-point particles into the space between them.
Such shields would be far from perfect. Zero-point particles are very small, and have little trouble tunneling through materials. However, some particles will bounce off of atoms in the shields. There will be an overall shielding effect.
This results in an imbalance in the ease with which zero-point particles can enter and leave the enclosed space. The easiest way will always be through the sides where there are no shields.
However, when the opening to the sides get very narrow, only zero-point particles coming in from a very restricted angle are allowed into the enclosed space the easy way. Most of the particles will have to go through the shields.
On the other hand, zero-point particles inside the enclosed space may have little trouble finding their way out. If they come in from the openings, they quickly find their way out again. If they came in through the shields, they may bounce a few times before leaving. On average, the exit may be easier to find than the entrance. The net result of this would be an attracting force.
Such a situation is very similar to various types of traps. In cases where the exit is easier to find than the entry, there will be a general vacancy inside the enclosed space, corresponding to an attracting force. In cases where the exit is harder to find than the entry, there will be a build up of pressure, corresponding to a repelling force.