The mutual energy ow satises the mutual energy ow theorem. The retarded wave is the action the emitter gives to the absorber. The advanced wave is the reaction the absorber gives to the emitter. In this article this author will derive the Huygens principle from the mutual energy ow theorem....
In this work, we demystify the use of total time derivatives — as well as terms proportional to the linear equations of motion — and show that they can lead to artificially large contributions cancelling at different orders of the in-in operator formalism. We discuss the treatment of total ...