In quantum physics, the Casimir effect produces an attractive force between two closely spaced uncharged conducting plates.
Hendrik Casimir predicted the effect in 1948 while working with Dutch physicist Dirk Polder. In the idealized picture, the plates alter the allowed electromagnetic modes between them. The difference between the mode structure inside the gap and outside it leads to a measurable force pushing the plates together.
The effect is associated with quantum field theory and is often explained using vacuum fluctuations. That explanation is useful, but the more precise treatment compares changes in electromagnetic boundary conditions and the resulting energy or stress. The force becomes important at very small separations, typically on the micrometre scale.
The Casimir effect does not mean that empty space is a simple reservoir of particles that can be freely extracted. Real measurements must account for surface roughness, material properties, temperature, and electrostatic forces. The effect is named for Casimir, not for the similarly named Zeeman effect, which concerns spectral shifts in magnetic fields.