The Casimir effect is the force between two uncharged conducting plates caused by quantum vacuum fluctuations.
Hendrik Casimir predicted the effect in 1948 while studying the behavior of electromagnetic fields between closely spaced conducting surfaces. The boundaries allow only certain electromagnetic modes between the plates, while a different set of modes exists outside them. The resulting difference in vacuum energy produces a measurable force that generally attracts parallel ideal plates.
The phrase “vacuum fluctuations” does not mean that empty space contains ordinary particles constantly appearing in a simple classical sense. It refers to the quantum properties of fields and their allowed modes. The effect becomes significant at very small separations.
The Casimir force is relevant to nanotechnology and microelectromechanical systems, where surfaces can be separated by micrometres or less. It is not the same as the Meissner effect, which concerns magnetic-field expulsion by superconductors.