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"The Special Theory of Relativity makes it clear that there are no
circumstances under which a signal can propagate at a speed greater than
c. Yet we have already seen that under certain circumstances (section
3.5.1) the phase velocity can exceed c. The contradiction is only an
apparent one, arising from the fact that although a monochromatic wave can
indeed have a speed in excess of c, it cannot convey information. In
contrast, a signal in the form of any modulated wave will propagate at the
group velocity, which is always less than c in normally dispersive media."