Phase dynamics for incommensurate nonequilibrium systems

We discuss the long-wavelength, low-frequency properties of pattern-forming incommensurate nonequilibrium systems, and we show that the additional macroscopic variable is either hydrodynamic or associated with a finite gap in the long-wavelength limit (k→0), depending on whether or not there is an interaction between the two phases involved. In the case of interpenetrating spirals in the Taylor instability the additional variable might be associated with an additional propagating mode.