  The phenomenon of ball lightning has been observed for a long time, but the
nature of these luminous balls has been unknown. It is proposed here that they
consist of highy excited Rydberg atoms with large polarizabilities that bind
them together. Thus the cohesion of the balls comes from photon exchange forces
(London dispersion forces) rather than the more usual electron exchange
(chemical) forces. The cohesion in plasmas generated at the back faces of
detonating explosives has a similar basis. Estimates are given to justify this
interpretation.
