The Casimir Effect

The Casimir effect is a measurable force resulting from quantum fluctuations of the electromagnetic field in empty space. When two parallel uncharged conductive plates are brought very close together, there should be an observable attractive force between them. Theoretically, this is due to electromagnetic properties of the plates stopping virtual photons of certain wavelengths from occurring between the plates. Everywhere else, though, these restrictions don’t exist. This results in a (radiation) pressure difference between the inside and outside regions, pushing the plates together. Although this force is extremely small, it has been detected experimentally, and may one day even be used in micromachines or highly sensitive devices.


Because of the requirement that the conductive plates be exceedingly parallel (within 10^-5 radians for plates that are 1 cm in diameter), it proved quite difficult to measure the force using two flat conductive plates. A much simpler setup involves using one flat plate and a conductive sphere. This removes the need for parallelism. It also makes adjustments to the distance between the conductors much easier to do.


Lamoreaux was the first to use a sphere and plate to detect the Casimir force. A torsion pendulum made of a very fine, long Tungsten wire suspending an aluminum plate was used to detect any force between the conductors. The conductors were placed on one side of the pendulum, and another pair of circular conducting plates was placed on the other side in order to balance the electrostatic forces exerted on the pendulum. Because the electrostatic forces between the plates are much greater than the Casimir force, the forces needed to be distinguished from one another by observing the different distance dependencies involved with each force. The measured Casimir effect in this experiment agreed to within 5% of the theoretical value.