counter-propagating field to provide the necessary conservation of momentum [@hrdiss; @hr62; @hr71; @burke]. The fact that photons convey momentum is incompatible with the concept of a critical electric field for laser-induced processes.

Section IV concludes with the practical problem of communicating with submerged submarines. This has been done under circumstances that emphasize how different plane-wave fields are from QSE fields

Section V explores the notion of a hierarchy of physical quantities. Potentials are directly related to energies, so they are identified as primary quantities. Fields are derived from potentials, so they are secondary. Forces are determined by fields and so forces are also secondary. The hierarchy concept is related to classical mechanics in that Newtonian physics is couched in terms of forces, and so it is secondary to versions of classical mechanics based on energy-based system functions like the Lagrangian and Hamiltonian. Mechanics formulated with system functions infer Newtonian mechanics, but the converse is not true.

Symmetry violation
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The two examples presented have an important qualitative difference. The first example – a constant electric field – is so elementary that the proper choice of potentials is obvious, and there is no motivation to explore the properties of the symmetry-violating 