Nonlinear Schr\"odinger equations: Symmetries, superposition, and classicality from a Bohmian perspective

Interference is commonly regarded as the most direct manifestation of the superposition principle.

This association is natural for the linear Schrödinger equation, where coherent alternatives combine at the level of probability amplitudes.

However, the situation becomes less transparent when nonlinear couplings are present, or when the field is only partially coherent.

In this work, we argue that a more robust organizing principle is provided by the local flow generated by phase variations.

In this sense, phase-induced flow acts as a unifying mechanism for interference-like dynamics in nonlinear and partially coherent Schrödinger systems.

The discussion is developed from a hydrodynamic, or Bohmian, perspective, understood here as a practical probing tool rather than as an additional ontology.

Three representative situations are considered: interfering Bose--Einstein condensates described by the Gross--Pitaevskii equation, nonlinear Schrödinger dynamics obtained by modifying the quantum-potential contribution, and partially coherent Airy beams described through their cross-spectral density.

Although these systems differ in physical origin and mathematical implementation, they share a common dynamical structure: density-related observables are shaped by velocity fields determined by phase, or ensemble-phase, information.

From this viewpoint, interference-like traits, localization, self-acceleration and coherence loss can be interpreted in terms of the preservation, deformation or breaking of the symmetries displayed by the underlying flow.

This provides a compact way of connecting interference, nonlinear dynamics, classicality, coherence loss, and structured-light propagation within a single trajectory-based framework.