Two mirrors facing each other in vacuum are mutually attracted to each other by the simple presence of quantum vacuum fluctuations. This surprising phenomenon was first predicted in 1948 by the Dutch theoretical physicist Hendrik Casimir, who was working at the Phillips Research Laboratories in Eindhoven on colloidal solutions. The phenomenon is now named after him the Casimir effect, while the force arising between the mirrors is called the Casimir force.
In 1948 Casimir was studying the properties of colloidal solution, which are viscous materials, such as paint and mayonnaise, that contain micron-sized particles in a liquid matrix. The properties of such solutions are determined by van der Waals forces. Such are called attractive long-range forces that exist between neutral atoms and molecules and which are closely related to Casimir forces. One of Casimir’s colleagues, J. Theodor G. Overbeek, realised that the theory that was used at the time to explain van der Waals forces, which had been developed by Fritz London in 1932, did not properly explain the experimental measurements on colloids. Overbeek asked Casimir to investigate the problem. Working with Dirk Polder, Casimir discovered that the interaction between two neutral molecules could be properly described only if it took into account the fact that light travels at a finite speed between two atoms or molecules. Soon afterwards, Casimir noticed that this result could be interpreted in terms of vacuum fluctuations. He then asked himself what would happen if there were two mirrors — rather than two atomes or molecules — facing each other in vacuum. It was this work that led to his famous prediction of an attractive force between reflecting plates.
The Casimir force has been observed quite soon after its prediction in a number of experiments. As early as in 1958, Marcus Spaarnay at Phillips Research Laboratories in Eindhoven measured the Casimir force between two flat metallic mirrors. The experiment turned out to be very difficult for two reasons. First, the mirrors had to be kept neutral as otherwise the electrostatic force would have screened the Casimir force. Second, Marcus Spaarnay had to assure that the plane mirrors were parallel to each other, as the Casimir force is very sensitive to distance changes. Marcus Spaarnay discussed all these effects and concluded that his experimental results did ``not contradict Casimir’s theoretical prediction’’. Many experiments have followed during the following decade and achieved major improvements in the measurement of this force. Although none of them reached a very good precision, they allowed to observe the effect and therefore to convince of the reality of the Casimir force.
Since 1997, the Casimir force has regained a lot of interest and was re-measured already several times with greatly improved precision.
H.B.G. Casimir Obituaries read here