Metastability of Discrete-Symmetry Flocks

In this talk, I will report a study on the stability of the ordered phase of flocking models with a scalar order parameter. Using both the active Ising model and a hydrodynamic description, I will show that droplets of particles moving in the direction opposite to that of the ordered phase nucleate and grow. This self-similar growth will be characterized analytically and I will demonstrate that droplets spread ballistically in all directions. These results imply that, in the thermodynamic limit, discrete-symmetry flocks -- and, by extension, continuous-symmetry flocks with rotational anisotropy -- are metastable in all dimensions.