SUPERLUMINAL AND PARELECTRIC EFFECTS IN RUBIDIUM VAPOR AND AMMONIA GAS

Population inversion in an atomic or molecular medium leads to the pos sibility of superluminal propagation of off-resonance, finite-bandwidt h electromagnetic wave packets, whose phase, group, energy, and 'signa l' velocities, as defined by Sommerfeld and Brillouin, all exceed the vacuum speed of light c. Einstein causality is not violated, since the front velocity is c. Here we propose an experiment to observe superlu minal propagation of laser pulses detuned from a stimulated Raman tran sition in optically pumped rubidium vapour. The inversion of populatio ns can also lead to a parelectric medium with negative Dc electric sus ceptibility, thus implying the possibility of the levitation of an ele ctrical charge in the vacuum above this medium. Stable electrostatic c onfigurations of charges placed inside an evacuated cavity surrounded by this medium exist, in seeming violation of Earnshaw's theorem. An e xperiment is proposed to observe parelectricity in ammonia gas.