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)