It is named after the Dutch physicist Hendrik Casimir, who predicted the effect of electromagnetic systems in 1948.

In the same year, Casimir and Dirk Polder described a similar effect experienced by a neutral atom in the vicinity of a macroscopic interface called the Casimir-Polder force. Their result is a generalization of the London-van der Waals force and includes retardation due to the finite speed of light. The fundamental principles leading to the London-van der Waals force, the Casimir force, and the Casimir-Polder force can be formulated on the same footing.

In 1997, a direct experiment by Steven K Lamoreaux quantitatively measured Casimir force to be within 5% of the value predicted by the theory.

The Casimir effect can be understood by the idea that macroscopic material interfaces, such as electrical conductors and dielectrics, alter the vacuum expectation value of the energy of the second-quantized electromagnetic field. Since the value of this energy depends on the shapes and positions of the materials, the Casimir effect manifests itself as a force between such objects.

Any medium supporting oscillations have an analog of the Casimir effect. For example, beads on a string and plates submerged in turbulent water or gas illustrate the Casimir force.

In modern theoretical physics, the Casimir effect plays an important role in the chiral bag model of the nucleon; in applied physics, it is significant in some aspects of emerging microtechnologies and nanotechnologies.

Physical Properties

The typical example is of two uncharged conductive plates in a vacuum, placed a few nanometers apart. In a classical description, the lack of an external field means that no field exists between the plates, and no force connects them. When this field is instead studied using the quantum electrodynamic vacuum, it is seen that the plates do affect the virtual photons that constitute the field, and generate a net force - either an attraction or a repulsion depending on the plates' specific arrangement. 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.

The treatment of boundary conditions in these calculations is controversial. In fact, "Casimir's original goal was to compute the van der Waals force between polarized molecules" of the conductive plates. Thus it can be interpreted without any reference to the zero-point energy (vacuum energy) of quantum fields.

Because the strength of the force falls off rapidly with distance, it is measurable only when the distance between the objects is small. This force becomes so strong that it becomes the dominant force between uncharged conductors at submicron scales. In fact, at separations of 10 nm – about 100 times the typical size of an atom – the Casimir effect produces the equivalent of about 1 atmosphere of pressure (the precise value depends on surface geometry and other factors).