Kinematics of the Three Moving Space Curves Associated with the Nonlinear Schrödinger Equation

Starting with the general description of a moving curve, we recently presented a unified formalism showing that three distinct space curve evolutions can be identified with a given integrable equation. Applying this to the nonlinear Schrödinger equation (NLS), we find three sets of coupled equations for the evolution of the curvature and the torsion, one set for each moving curve. The first set is given by the usual Da Rios–Betchov equations. The velocity at each point of the curve corresponding to this set is well known to be a local expression in the curve variables. In contrast, the velocities of the other two curves are nonlocal expressions. Each of the three curves is endowed with a corresponding infinite set of geometric constraints. We find these moving space curves by using their relation to the integrable Landau–Lifshitz equation. We present the three evolving curves corresponding to the envelope soliton solution of the NLS and compare their behaviors.