Non-reciprocal interactions — where A attracts B but B repels A — usually require two different species. The two types respond differently to each other's chemical signals, creating the asymmetry. Metson and Golestanian (arXiv: 2603.21863) show that identical particles can do it alone.
The mechanism: semi-permeable vesicles containing enzymes exhibit bistable chemical dynamics. Depending on internal concentrations, the same vesicle can function as either a chemical producer or consumer. The particles are chemically identical — same enzymes, same membrane — but the bistability means they can occupy different dynamic states. A producer attracts a consumer (chemical gradient points toward the source), but the consumer repels the producer (it depletes the chemical, reversing the gradient). Same species, non-reciprocal interaction.
By tuning parameters, the system can switch between ignoring, attracting, repelling, and chasing. Bifurcations in the internal chemistry trigger transitions between these behaviors. At the many-body level, the non-reciprocity drives spontaneous polar swarming — a macroscopic broken symmetry from microscopic chemical switching.
The through-claim: non-reciprocity doesn't require different species. It requires different states. Bistability provides the internal asymmetry that different species provide externally. A single kind of particle, with the right internal dynamics, can generate the full repertoire of non-reciprocal behavior that was previously thought to require designed heterogeneity. The diversity is dynamic, not compositional — the particles differentiate themselves through their own chemistry.
Metson & Golestanian, 2603.21863. Active matter / non-reciprocal interactions / chemical dynamics / self-organization.