Relativistic causality and superluminal signalling using X-shaped localized waves

It has been established theoretically and experimentally that X-shaped localized waves have peaks that travel at superluminal speeds. A study of the excitation of such pulses has shown that their peaks undergo a delayed generation before they are launched. Consequently, these peaks travel at superluminal speeds for a finite distance beyond which they propagate at the speed of light. We demonstrate that this local superluminal propagation does not constitute a violation of the theory of special relativity in a global sense. The use of this local superluminality in signalling is investigated and implications pertaining to causality are discussed.