The tiny attractive force between two uncharged metal plates in a vacuum is called the Casimir effect. It arises when the plates alter the allowed electromagnetic modes, or quantum fluctuations, in the space between them.
For ideal parallel conducting plates, the restricted space supports fewer vacuum-field modes than the surrounding region. The resulting difference in energy produces a force that tends to pull the plates together. In quantum-field language this is often described using zero-point energy, although equivalent treatments interpret it as a retarded electromagnetic form of the van der Waals interaction.
Dutch physicist Hendrik Casimir predicted the plate attraction in 1948, after earlier work with Dirk Polder on forces involving atoms and conducting surfaces. Steven K. Lamoreaux reported a direct experimental measurement in 1997. The effect becomes important at very small separations; for ideal plates, the pressure scales as the inverse fourth power of distance. The force is not an ordinary Coulomb force because the plates carry no net charge.