Breaking action–reaction symmetry Non-reciprocal interactions occur when one particle influences another more strongly than it is influenced in return. This figure illustrates how unequal electrohydrodynamic attraction between differently sized particles generates non-reciprocal interactions and self-propelled particle pairs. (Courtesy: Yutaka Sumino, Tokyo University of Science, Japan)" title="Click to open image in popup" href="https://physicsworld.com/wp-content/uploads/2026/09/nonreciprocal.jpeg"> Breaking action–reaction symmetry Non-reciprocal interactions occur when one particle influences another more strongly than it is influenced in return. This figure shows how unequal electrohydrodynamic attraction between differently sized particles generates non-reciprocal interactions and self-propelled particle pairs. (Courtesy: Yutaka Sumino, Tokyo University of Science, Japan)

Attractive interactions between particles do not necessarily lead to these particles aggregating.…