SUPERLUMINAL AND PARELECTRIC EFFECTS IN RUBIDIUM VAPOR AND AMMONIA GAS

Population inversion in an atomic or molecular medium leads to the possibility of superluminal propagation of off-resonance, finite-bandwidth electromagnetic wave packets, whose phase, group, energy, and 'signal' 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 superluminal propagation of laser pulses detuned from a stimulated Raman transition in optically pumped rubidium vapour. The inversion of populations can also lead to a parelectric medium with negative Dc electric susceptibility, thus implying the possibility of the levitation of an electrical charge in the vacuum above this medium. Stable electrostatic configurations of charges placed inside an evacuated cavity surrounded by this medium exist, in seeming violation of Earnshaw's theorem. An experiment is proposed to observe parelectricity in ammonia gas.