It is shown that a unitary theory of interacting phonons with orthogonal polarization vectors can be described by a pure SU(2) Yang-Mills theory. The three orthogonal modes: the two transverse and one longitudinal mode for each value of the momentum must be dotted into the SU(2) generators for interactions to occur in a conveniently unitary form. Without this the commutation relations and the antisymmetry of the Field Strength Tensors results in all scattering vertices cancelling. This suggests that the electron states in strongly anharmonic crystals, which are the phonon source terms, may be non-trivial and consist of a many-body structure which is not describable in terms of single quasiparticles.
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