Main Question or Discussion Point
It seems fascinating to me that quasi-particles appear everywhere, and can appear at multiple scales within the same system. Why is this? Is there a good intuitive reason for why entities behaving like free particles should exist at various scales in a system that is otherwise strongly interacting? Are there any `large' quantum systems that have no quasi-particles at any scale? If so, how would we find them experimentally (or perhaps more ambitiously, predict their existence)?