🚀 go-pugleaf

In article <CKAMAw.Kpp@netnews.jhuapl.edu> Robert Jensen
>>        
>>I think that this should be the "group velocity".  No energy or
>>information moves at the phase velocity.

By the usage I am accustomed to, this is "phase velocity."

For an extreme example, imagine a plane wave moving up through the earth at
a slight angle to the surface.  When different parts of the wave reach
different parts of the surface at slightly different times, it appears to
a person on the surface like a wave traveling incredibly quickly across
the surface.  This apparent velocity exceeds the actual speed of the wave
through the earth.  It is the coincidental appearance of the leading edge
(same phase) of different parts of the wavefront at different locations that
gives this phase velocity its "impossible" speed.  There is no energy
transfer between points on the surface by this wave, merely energy transfer
from the origin to two different points at almost the same time.

If an earthquake occurs beneath the surface, the wavefront will not reach the
surface epicenter much before it reaches nearby areas on the surface.  There
is less time than one might expect to get a warning from the center to the
outlying areas because the shockwave gets to follow the hypotenuse of the
triangle.  The time is the same as would be obtained by using the phase
velocity as the velocity of the shock wave outward from the surface epicenter.