Answers are grounded in this paper and cite their sources.
This paper is a review and development of recent ideas on group velocity and energy propagation by Lighthill [l, 21, Landau and Lifshitz [3, $5 66 and 671, Ursell [7], and the present writer [a]. Some of the ideas seem to be new but it is not always clear how much is implicit in the above references.
No takes yet. Share an insight, caveat, or question.
Gerald Beresford Whitham (1961) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: