Re: wave momentum

[James McLean <jmclean@CHEM.UCSD.EDU>]

Ludwik Kowalski wrote:
> The crucial difference is that dB/dt vector (which is proportional
> to E) is perpendicular to B while for the rope wave the dx/dt vector
> (v of particles) is in the same plane as their displacements, x.
True for circularly polarized E&M waves, but not linearly polarized.

How about this:
Take any wave supported by a (massive) medium, and which leaves the
medium unchanged by it's passage.  That is, any piece of the medium ends
up in the same position where it started, so that it's average velocity
(and hence average momentum) is zero.  This in turn means that the
momentum flux (which passed through the piece) must time average out to
zero.

Thus, an oscillatory wave through a massive medium cannot transmit net
momentum.  (Sound transmits momentum *fluctuations*, but not *net*
momentum.)  A domino or shock wave does transmit net momentum, because
it doesn't leave the medium undisturbed.

I'm guessing that a wave in a 'field' (a non-massive medium?) will
always carry momentum.  Can a field-theory-type person verify that?

--
--James McLean
jmclean@chem.ucsd.edu
post doc
UC San Diego, Chemistry