A simple mixture of beads and water could boost one property of sound waves past light speed. Experiments in 1982 demonstrated that the group velocity of a light pulse—based on the position of the peak of the pulse, not the individual waves—could exceed light speed. (Relativity remains unscathed because group velocity transmits no information in this case.) Now Joel Mobley of the University of Mississippi contends that an ultrasound pulse will disperse in a mix of water and plastic beads, so that the pulse's different frequencies vary greatly in speed. The sum of these components would create a faster-than-light, or superluminal, group velocity. Mobley's initial subluminal experiments support his calculations, he reported at the Acoustical Society of America meeting on October 19, 2005.
This article was originally published with the title "Underwater Speeding Violation" in Scientific American 294, 1, 33 (January 2006)