Roy:

[snip]
"Roy Lewallen" wrote in message
...
The property of "vibrations" or "oscillations" that
have a direction different from the direction of the field is new to me.
[snip]

That's fine, there are lots of things new to me as well. :-)

These are called longitudinal or compressive-dilutive waves. Such
vibrations do not [usually] occur
with electromagnetic fields, and in the early days of em theory scientists
did wonder if such were
possible, as I have noted in prior postings. However there are numerous
physical systems described
by wave equations which do support both transverse and longitudinal
vibrations of the constituient fields.
I gave several examples in prior postings and you will find lots of such
examples in "Physics of Waves".

[snip]
I notice that you're now qualifying your statement to free space and
isotropic media. Does this perhaps leave open the possibility that waves
in a lossy medium, or bounded within a hollow waveguide, could have
"vibrations" that *aren't* transverse to the direction of propagation?
[snip]

The point is not that the waves have "vibrations" that are transverse to the
direction
of propagation. Of course, guided em waves have vibrations which are not
perfectly
perpendicular to the direction of propagation. The TEM mode would not exist
and
TEM waves would not propagate if there were not some potential driving the
waves
forward, meaning that both the E and H fields have some tiny component in
the direction
of propagation. All of this to establish that I do understand em wave
propagation, and to
say that... this has nothing whatsoever to do with longitudinal or
compressive-dilutive
vibrations. The fact that E and H fields "lean" slightly in the direction
of propagation
in a wave guide is not a compressive-dillutive effect on the fields. For
longitudinal
field vibrations to occur the wavelength of the propagating fields has to
change as
it propagates. This does not occur in "normal" em propagation. I
conjecture that
there may however be some exceptions to this, e.g. plasmas, etc... I just am
not
aware of them. Perhaps some other newsgroup reader/poster is more familiar
with any possible longitudinal vibrations of em waves.

[snip]
My original question was in response to your statement that EM waves
were always transverse, regardless of the medium.
[snip]

Roy, I believe that you may be reading too much into the word "transverse",
it
can be used in several contexts. Tansverse vibrations are not compressive
vibrations. With compressive vibrations, the wavelength of the waves
actually
changess it propagates. While in transverse vibrations no such wavelength
changes occur. In this usage the word "transverse" does not refer to
directionality
with respect to direction of propagation, but rather to the fact that the
waves
maintain their wavelegth during propagation. As a "real" example, some
seismic
waves [the so called "S-Waves" in the earth actually change their wavelength
as
they propagate..

[snip]
Do you perhaps have Krus' _Electromagnetics_, or electromagnetics texts
by Holt, Johnk, Skilling, Magid, Magnusson, or Jordan & Balmain? If so,
perhaps you could direct me to a section which addresses this.
[snip]

Roy, yes indeed I have two editions of "Kraus" and I took a course from
Keith Balmain
using his first edition text when I was at U of T.

And...

I can tell you here and now that neither of those two august gentlemen
address the issue
of longitudinal vibrations anywhere in their texbooks! Simply because, as I
have stated in
other postings, Maxwell-Heaviside equations do not support longitudinal
vibrations, and so
why would a text on em waves even discuss such vibrations? The fact that
Kraus and Balmain
do not discuss such things does not surprise me, nor should it you, since
electromagnetic wave
propagation and the Maxwell-Heaviside equations are a particularly simple
example of wave
motion.

Roy if you wish to deeply understand wave equations and wave motions and to
understand
the wider ramifications of wave motion, you just gotta read more widely in
the "Waves"
literature.

Kraus and Balmain are very narrow in scope, being confined strictly to em
waves!

If they had attempted to include any "early" history of em research from
around the middle
of the 1800's then they would have outlined some of the early speculations
by contemporaries
of Maxwell, such as Kelvin, Heaviside and others as to the possibility that
Maxwell might have
left longitudinal terms that might have proved significant, of course they
were found never to be
needed. However even back in those times most Natural Philosophers [They
were'nt called
Physicists in those days] and Electricians like Heaviside were more widely
schooled than
today's Engineers and they knew and studied wave equations in their full
glory... longitudinal
vibrations included. These days however our electrical engineering
education is far too narrrow
and does not expose folks to the wider view of the world. Thus we often
find em wave
mechanics who don't understand longitudinal waves. Until I became involved
in underwater acoustics and seismic propagation problems and saw the wave
equations in
their full glory, I too had a narrow view of wave mechanics.

I don't know if Kraus or Balmain ever encountered the "full" wave equations,
but in any
case their texts are directed at em specialists and so their narrow view is
not surprising.

As I posted before, if you are interested in such things, check out:

Elmore and Heald, "Physics of Waves" and say, Kennett's, "The Seismic
Wavefield" among
others to help you to broaden your horizons on these issues of longitudinal
waves.

--
Peter K1PO
Indialantic By-the-Sea, FL.