In this talk I’ll describe the hydrodynamic theory of the motion of incompressible flocks: that is, collections of self-propelled entities (“birds”) that are packed so tightly together that their density cannot change as they move. In two dimensions, this problem can be mapped onto an equilibrium magnet with a peculiar constraint. This problem, in turn, can be shown to be equivalent to a 2d smectic (“soap”), with the flow lines of the flock playing the role of the smectic layers.
