Physics – General Physics
Scientific paper
2001-02-20
Physics
General Physics
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Scientific paper
This paper examines some of the recent experiments on superluminal propagation. The meaning of information velocity from the perspective of a digital communication system is analyzed. It is shown that practical digital communication systems use bandlimited signals to transmit information consisting of random bits which cannot be predicted by looking at previously transmitted bits and that information is not limited to discontinuous step-functions alone. These random bits are pulse-shaped by using bandlimited pulse (e.g. gaussian pulse), in a causal fashion (meaning waveform during any given bit is obtained by convolution of present and past bits with the causal pulse, but cannot have information about future bits). If these pulse-shaped random bits were not considered as information, if somehow the future random bits could be predicted by, say looking at the shape of the first bit, then there would be no need for elaborate communication systems like cellular systems and data modems. We could transmit the first bit waveform, turn off the transmitter and expect the receiver to correctly detect all future random bits, which clearly is impossible. It is shown that it is possible to achieve information velocity greater than speed of light, in media with zero dispersion and positive index of refraction less than unity in the frequency range of interest, and of sufficient length to make the time gained during medium transmission in comparison to vaccum transmission, more than the duration of several bits. It is shown that while signal causality is preserved from the perspective of an LTI system, Einstein causality is not preserved and hence what this means to relativistic causality violations is analyzed.
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