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.)