Why are quasi-particles interesting?

Last week I had a phone call from a journalist Andrew Grant at Science News, asking about quasi-particles. Why are they interesting?

1. Quasi-particles exist. It is not at all obvious why they should exist in strongly interacting quantum systems. Yet they are rather robust and found in diverse systems, ranging from atomic nuclei, to magnets, to metals, to neutron stars… Thus, they are an important organising principle in quantum many-body physics.

2. Occasionally quasi-particles have different quantum numbers to the constituent particles of a system. The most striking example is the fractional charge and statistics of quasi-particles in the fractional quantum Hall effect.

3. Many electronic materials of current interest [e.g. high-temperature superconductors] are “bad metals” that do not seem to have quasi-particles [except at low temperatures].

I find all of these are profound and surprising. They illustrate emergence.