Mysterious Casimir Effect

The Casimir force is the mutual attraction of two closely spaced, parallel, and uncharged conducting planes that persists even at absolute zero temperature. This force results from a change in the zero point energy of the electromagnetic field between the plates, due to the modification of the field modes as the plates are brought together.
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 quantum electrodynamic vacuum, 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. Although the Casimir effect can be expressed in terms of virtual particles interacting with the objects, it is best described and more easily calculated in terms of the zero-point energy of a quantized field in the intervening space between the objects. This force has been measured and is a striking example of an effect captured formally by second quantization.
Possible causes:

•  Vacuum energy-The causes of the Casimir effect are described by quantum field theory, which states that all of the various fundamental fields, such as the electromagnetic field, must be quantized at each and every point in space. In a simplified view, a "field" in physics may be envisioned as if space were filled with interconnected vibrating balls and springs, and the strength of the field can be visualized as the displacement of a ball from its rest position. Vibrations in this field propagate and are governed by the appropriate wave equation for the particular field in question.
•  Relativistic vander waals force-Alternatively, a 2005 paper by Robert Jaffe of MIT states that "Casimir effects can be formulated and Casimir forces can be computed without reference to zero-point energies. They are relativistic, quantum forces between charges and currents. The Casimir force (per unit area) between parallel plates vanishes as alpha, the fine structure constant, goes to zero, and the standard result, which appears to be independent of alpha, corresponds to the alpha approaching infinity limit," and that "The Casimir force is simply the van der Waals force between the metal plates."
• Coupled ground state energy- A third way of understanding Casimir forces has been suggested, based on canonical macroscopic quantum electrodynamics. In this interpretation, there exists a ground (vacuum) state of the coupled system of matter and fields, which determines the ground-state properties of the electromagnetic field, giving rise to a force. The Casimir force is fundamentally a property of the coupled system of matter and fields, in which the interaction between the plates is mediated by the zero-point fields. In more traditional interpretations, however, the emphasis has fallen either on the electromagnetic field or the fluctuating material in the plates.

Text Courtesy -  Wikipedia.