Scope and classification of genuinely nonreciprocal phenomena

Characterize which emergent phenomena should be regarded as genuinely nonreciprocal and distinguish them from phenomena that merely share formal similarities with nonreciprocal systems while arising from different physical mechanisms.

Background

The authors identify a broader conceptual challenge concerning the boundaries of nonreciprocal matter. Phenomena such as flocking, Hall responses, and other non-Hermitian or nonequilibrium effects may display mathematical or physical features resembling those of nonreciprocal systems without being generated by interaction asymmetry of the type considered in the Perspective.

Resolving this classification problem is presented as necessary for identifying universal principles of nonreciprocal matter and clarifying its relation to the wider field of nonequilibrium statistical physics. The paper leaves unresolved which shared signatures reflect genuine nonreciprocity and which are only formal analogies.