How to prove that energy travels at group velocity and not phase velocity?
Start here: http://en.wikipedia.org/wiki/Group_velocity
Note that the group velocity is not always the speed for energy transfer!
A general proof is given in this paper: http://journals.aps.org/archive/abstract/10.1103/PhysRev.105.1129 [Broken]
"General Theorems on the Equivalence of Group Velocity and Energy Transport"
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