Granular matter as a measurable model system for macroscopic state evolution: A transition-centred perspective

Granular matter provides an experimentally accessible system for studying how macroscopic behaviour emerges from constrained rearrangement in discrete many-particle assemblies. Quantities such as the angle of internal friction, dilatancy, particle rearrangement, relaxation, and flow onset are often treated as related but partly separate descriptors of shear response. In this article, we argue that they can be interpreted as coupled manifestations of macroscopic state evolution. We propose a review-based framework organized around five concepts: configuration space, admissible trajectories, rearrangement cost, relaxation pathway, and metastability or regime boundary. Rather than introducing new state variables or a new theory of jamming, the contribution lies in assigning distinct analytical roles to five established concepts and in mapping measurable experimental and discrete element method (DEM) resolved observables onto these roles within a common transition-centred description. The framework links measurable quantities from shear experiments, bulk-solids characterization, and DEM simulations to the evolution of particulate systems between stable, metastable, yielding, and flowing regimes. Its purpose is not to introduce a universal constitutive law, but to provide a physically grounded vocabulary for describing how granular assemblies lose stability, reorganize their internal structure, dissipate energy, and enter new modes of motion. The strongest extension of this framework concerns self-gravitating granular bodies, such as rubble-pile asteroids and regolith-dominated aggregates, where contact mechanics, friction, porosity, and rearrangement remain physically relevant. Broader comparisons with planetary, stellar, or galactic systems are treated only as weaker organizational analogies and are used to clarify the limits of transferability. We therefore identify granular matter as a measurable model system for particulate state evolution and as a reference system for cautious comparison with related discrete many-body systems.