Order parameters for the equilibrium solid phase
The traditional order parameters used to distinguish solid from fluid phases are related to microscopic structure, notably crystalline symmetry. However it has been too difficult to prove that any models, even highly simplified ones such as hard spheres, have an ordered phase in such a sense. I will discuss two possible alternative (macroscopic) order parameters, one related to shear and the other to surface tension. If time permits I will discuss the parallel problems in constrained networks and in static granular materials.