From: utzoo!decvax!harpo!npoiv!alice!mhtsa!eagle!mhuxt!mhuxi!aluxz!ltn
Newsgroups: net.physics,net.ham-radio,net.followup
Title: Re: faster than light
Article-I.D.: aluxz.37
Posted: Fri Jan 14 08:23:18 1983
Received: Sat Jan 15 05:49:02 1983
References: utcsstat.496

Under certain propagation conditions (not necessarily in a conductor; the
same thing can happen to a wave propagating in a plasma), an electrical
signal *can* travel at a velocity greater than light.
*But* that is only true for the phase velocity.  The phase velocity is the
velocity of a wave of a single frequency, which means that signal has a
constant amplitude for all time.
The key point is that the group velocity *cannot* exceed the speed of light.
The group velocity is the velocity that a modulation envelope (whether
amplitude modulation, on-off cw pulses, or whatever) will travel at.  The
difference arrises because modulation involves propagating signals of many
different frequencies (remember Fourier) which travel with different
velocities (the faster-than-light phenomenon only occurs in dispersive media).
Thus *information* still cannot travel faster than light, and information
propagation is what relativity is concerned with.

Les Niles, Bell Labs Murray Hill (aluxz!ltn)