I am a bit mystified by the comments on this thread: There is no real E/M
wave -- we have given up the idea of an aether long ago have we not. There
_is_ real motion in a sound (or to stretch things, water wave) -- ie
molecules do move and thus one can more easily imagine a momentum flow, but
I am a bit nervous about this as well. Am I being to "classical" -- (or
worse, just ancient)???
Thus an e/m wave doesn't "carry" mass thus no momentum -- there is a charge
at the "originating" site which does impose a force on the distant site
thus it may cause some momentum at that site. We just imagine a wave
momentum which we hope will make calculations easier -- just as we talk
about E/M "fields" which are not "real" either but are sometimes helpful
to imagine.
Of course I readily admit that action at a distance is not much more "real"
Jim
At 03:04 PM 3/15/99 -0800, you wrote:
|"James W. Wheeler" wrote:
|> To drive a microphone-e.g. a variable capacitor type, it must exert a
|> force, hence it must carry momentum! The average momentum over a cycle
|> may well average to zero-but it is not zero at every point in the cycle.
|>
|We are in complete agreement. Which brings us back to Carl's question:
|a (continuous) sound wave does not carry (time averaged) momentum, but
|an E&M wave does. What's the crucial difference? How does a shock wave
|fit in?
|
|--
|--James McLean
|jmclean@chem.ucsd.edu
|post doc
|UC San Diego, Chemistry