The Casimir effect was first calculated and predicted by physicist Hendrik Brugt Gerhard Casimir in 1948. The experimental proof followed in 1956. The effect states that two parallel metal plates (or conductive plates) attract each other in vacuum, respectively that a force, the Casimir force, acts on them, pressing the plates together     .
To explain the Casimir effect, a short explanation what vacuum is made of is necessary. Vacuum is defined as a quantum field, i.e. as a superposition of different field configurations with large and small wavelengths. According to quantum field theory every vacuum contains energy, the so-called zero point energy (ZPE) or vacuum energy. This kind of energy however can not be extracted, because the vacuum itself is already the lowest energy state. Although the quantum vacuum seems to consist of „nothing“, it is actually a very complex structure, making it impossible to completely deplete a space of all particles. Particles in this apparently empty space always exist only for a short time, leading to fluctuations in the quantum vacuum , .
The Casimir effect is based on the fact that an electric field on parallel metal plates can only be perpendicular. This severely limits how these fields can form in the region between the plates. Configurations with wavelengths longer than the plate spacing are no longer possible. In this region the vacuum is in a different state than in the „undisturbed“ vacuum. Due to such a restriction of possibilities, the zero-point energy decreases there. If the plates are now pushed even closer together, this effect is further intensified, the plates attract each other and a small but measurable energy is gained , .
In recent years the Casimir effect has become more important and more relevant again. By means of modern measurement techniques such as atomic force microscopy, it is possible now to measure smaller and smaller forces with much higher accuracy. There is also an increased interest from a technological point of view, as the Casimir force becomes particularly important for miniature machines in the micro- and nanometer range. In fundamental research and especially with respect to the effort to unify gravity (i.e. theory of relativity) and quantum mechanics, the Casimir force also plays an important role in experimentally proving the fundamental forces assumed by physicists. Since these forces change Newton’s law of gravity for distances in the submillimeter range, it is important to be able to predict and calculate the Casimir force . Spektrum: Lexikon der Astronomie: Casimir-Effekt; (Link: https://www.spektrum.de/lexikon/astronomie/casimir-effekt/61), accessed 14 May 2020  Physik Cosmos indirekt; (Link: https://physik.cosmos-indirekt.de/Physik-Schule/Casimir-Effekt), accessed 14 May 2020  Spektrum: Lexikon der Astronomie: Quantenvakuum; (Link: https://www.spektrum.de/lexikon/astronomie/quantenvakuum/376), accessed 18 May 2020  Plunien, Günter; Müller, Berndt; Greiner, Walter: The Casimir effect; Physics Reports Volume 134; 03/1986  Casimir, H. B. G.: Mathematics. – On the attraction between two perfectly conducting plates; Gems from a century of science 1898-1997; 29.05.1948  Lambrecht, Astrid: Das Vakuum kommt zu Kräften; Physik Unserer Zeit, 2005
Further reading: Milton, Kimball A.: The Casimir Effect: Physical Manifestations of Zero-Point Energy; University of Oklahhoma, 02/2008  Antonini, P.; Bimonte, G.; et. Al.: An experimental apparatus for measuring the Casimir effect at large distances; 12/2008  Quach, James Q.: Gravitational Casimir effect, 02/2015  Mostepanenko, V. M.: Experiment, theory and the Casimir effect, 03/2009  Bachmann, Sven; Kempf, Achim: The Transplanckian Question and the Casimir Effect, 04/2005  Bordag, M.; Mohideen, U.; Mostepanenko, V. M.: New developments in the Casimir effect; Physics Reports Volume 353; 10/2001