packed hard particles: solidity without elasticity
Anomalous force propagation in anisotropic packs
Anomalous excitation
Ray-like force propagation in isotropic packs?
Minimal connectivity Þ  isostatic network
Þ Delicate stress balance is needed
Þ Forces propagate along rays, not uniformly
Wild response to local perturbations free modes
Wyart: why minimal connectivity entails anomalous slow vibrations:
     marginal modes
Alexei Tkachenko, David Head, Nagel group, Matthieu Wyart
http://jfi.uchicago.edu/~tten/Glasperlenspiel/
Distinctive density of states
Distinctive heterogeneity
Distinctive energy transport