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In a dispersive medium, different monochromatic modes of light have different phase velocities. Under special circumstances, a superposition of these modes results in an interesting effect wherein the group velocity (the velocity at which the peak of the wavepacket propagates) could be greater than c or even negative although the phase velocities of the modes are all less than c. Can this superluminal group velocity be used for information transfer? We show here that the ~superluminal‘ effect is due to a coherent optical wave superposition effect. Whatever the velocity of the ‘peak‘, the whole pulse cannot travel with a speed greater than the fastest phase velocity of its component modes. Thus the maximum speed for information transfer, which involves the sending of a finite pulse, cannot be greater than the maximum phase velocity in the medium. 相似文献
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