🚀 go-pugleaf

In article <25368@dog.ee.lbl.gov> sichase@csa3.lbl.gov (Scott Chase) writes:
>My point here was to attempt to illustrate the distinction between
>phase velocity and group velocity.  The phase velocity can and does
>often exceed the speed of light.  But the group velocity may not
>do so.  Another oft-quoted example is a slightly oblique wave hitting
>a beach.  The wave rolls in at a few miles per hour, but the 
>point of contact between the wave and the beach can run along the 
>beach at many times the speed of the wave.  For an angle of attack
>arbitrarily close to 90 degrees, an arbitrary wavefront moves laterally
>across the beach at arbitrarily large speed - in principle it could
>be faster than the speed of light.
>

Point well taken, but strictly speaking, what you say is false.  The
group velocity CAN exceed the speed of light.  No information may be
transmitted at that speed, and Sommerfeld & Brillouin showed in particular
that the "edge" of any disturbance will never travel faster than c,
even though in regions of anomalous dispersion, the point of stationary
phase (which defines the group velocity) may.  

aephraim

-- 
Aephraim M. Steinberg                    | "WHY must I treat the measuring
UCB Physics                              | device classically??  What will
aephraim@physics.berkeley.edu            | happen to me if I don't??"
                                         |       -- Eugene Wigner