The pressure of nothing
In 1948, Dutch physicist Hendrik Casimir, while working at the Philips Research Laboratories here in Eindhoven, made a startling prediction. He proposed that two uncharged, perfectly reflective plates placed parallel to each other in a perfect vacuum would experience an attractive force. This concept, now called the Casimir effect, originates from the field of quantum field theory. The theory states that a vacuum is not truly empty but is filled with fleeting electromagnetic waves, or "virtual photons". These are pairs of particles and antiparticles that appear and disappear constantly.
Casimir realized that the space between the two plates would act as a resonant cavity, restricting the wavelengths of virtual photons that could exist there. Only photons whose wavelengths fit a whole number of times into the gap can exist. Outside the plates, however, virtual photons of all wavelengths can exist. This imbalance creates a greater energy density, or pressure, on the outside of the plates compared to the inside, pushing them together. Casimir's work was an outgrowth of his research into the van der Waals forces that cause particles to stick together in colloidal solutions like paint and mayonnaise.
Measuring the void
The force Casimir predicted is incredibly small and difficult to measure. For two plates with an area of one square centimeter separated by one micrometer, the force is about 100 nanonewtons, roughly the weight of a water droplet half a millimeter in diameter. The force is also extremely dependent on distance; it increases sixteen-fold if the distance between the plates is halved, scaling inversely to the fourth power of the separation.
It took nearly five decades for technology to become sensitive enough to verify the prediction with precision. In 1997, Steve K. Lamoreaux at Los Alamos National Laboratory conducted a direct experiment that measured the force to within 5% of the value predicted by the theory. Shortly after, Umar Mohideen and his colleagues at the University of California, Riverside, achieved a measurement with 1% accuracy using an atomic force microscope. To avoid the immense challenge of keeping two large plates perfectly parallel, these experiments often use a flat plate and a sphere with a very large radius.
The Casimir effect is a theoretical curiosity. It has practical implications in the world of nanotechnology and microelectromechanical systems (MEMS). At sub-micron scales, the Casimir force can become the dominant force between neutral objects, causing tiny mechanical components to stick together in a phenomenon known as stiction. Understanding this force is important for designing and building the next generation of microscopic machines.