Casimir Effect

When two parallel uncharged metal sheets are placed in a perfect vacuum, they attract each other with a tiny force that is not gravitational. What is the source of this effect?

Answer

Although the classical vacuum is a void, the quantum mechanical vacuum is a soup of virtual particle-antiparticle pairs that interact with the real atoms in the metal plates, these pairs being created and annihilated in extremely short time intervals in accordance with the Heisenberg uncertainty principle. That is, the more the total energy ΔE in the pair, the less time duration Δt is its existence so that ΔEΔt ≥ h/4π. This vacuum pair “soup” pushes inward at both plates when the plates are very close to each other because certain particle-antiparticle pairs are practically forbidden from momentarily appearing between them. Essentially, if their deBroglie wavelength exceeds the plate spacing, these pairs have a much lower probability to be between the plates. But these same pairs appear outside the plates and provide the additional forces, whence the net inward force. Known as the Casimir effect, it was first measured in 1958.

The Casimir force is too small to be observed for plates that are not within microns of each other. Two mirrors with an area of 1 cm² separated by a distance of about 1 μm have an attractive Casimir force of about 10^–7 N. Although this force seems very small, at distances of less than a micrometer the Casimir force becomes the strongest force between two neutral objects! At separations of 10 nanometer roughly 100 times the size of an atom the Casimir effect produces a force that is the equivalent of 1 atmosphere of pressure. The resurgence of interest in the Casimir force is because micromechanical devices on the scale of tens of nanometers must accommodate its effects!