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Re: wave momentum

James McLean asked:
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?

The sound wave also differs from E&M by its longitudinal nature.
Ask the same question in the context of E&M versus a mechanical
wave along the stretched rope. In this way we compare transverse
waves only.

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.
Ludwik Kowalski