Empty Spaces Can Exert Forces on Each Other

Empty spaces can exert forces on each other through the action of intervening matter, a US-Germany team has proposed (Aurel Bulgac, University of Washington, Seattle and Andreas Wirzba, Forschungszentrum Juelich).

If experimentally confirmed, this effect would constitute a new kind of force, akin to the traditional "Casimir force," the slight attraction between two metallic plates in a vacuum. The traditional Casimir attraction (Update 300-3) occurs because of the fleeting electromagnetic fields that exist in the vacuum. These fields exert forces on the plates. In between the plates, however, certain electromagnetic waves cannot reside, namely those with wavelengths larger than the plate separation. This imbalance of electromagnetic forces serves to push the plates together.

In the newly proposed force, two or more cavities (empty regions of space) alter the waves associated with surrounding matter in the form of non-interacting fermions, such as a gas of electrons. For a simple example, consider two hollow spheres separated by a sea of electrons which, according to quantum mechanics, can be considered as rippling waves. If the wavelengths of the electrons are comparable to the dimensions of the spheres, then forces between the empty spheres could result. The spheres, even though they're separated, can effectively interact because the electron waves bounce back and forth between them. Whether the spheres attract or repel each other depends on the overall effect of all matter waves between them.

Demonstrating this effect is likely to be very challenging. One approach might be to immerse C₆₀ molecules (buckyballs) in liquid mercury. The buckyballs, effectively hollow spheres, could bind to each other through the action of conducting electrons in the liquid mercury. This new effect could act over an even longer range than the weakly attractive "van der Waals force" between molecules. (Bulgac and Wirzba, Physical Review Letters, 17 September 2001.)