Re: Group velocites greater than the speed of light

[Larry Woolf <larry.woolf@GAT.COM>]

> "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."

I can provide some nice references for this question:

1. Wave Propagation and Group Velocity by Leon Brillouin, Academic Press,
1960, chapter 2: "About the Propagation of Light in Dispersive Media."

He discusses the various speeds of light: phase velocity, group velocity,
energy propagation velocity, signal propagation velocity.

2. Absorption and Scattering of Light by Small Particles by Craig Bohren
and Donald Hoffman, Wiley, 1983, p. 235-239.

( I am a big fan of anything written by Craig Bohren)



Dr. Lawrence D. Woolf; Phone: (858)-455-4475; www.sci-ed-ga.org
General Atomics; 3550 General Atomics Court; San Diego CA 92121-1194