Fig. 2: Example of non-reciprocal interaction arising in soft active systems.

Here, two metal colloids are immersed under light irradiation. The light that reaches the metal colloid A(B) is partly absorbed, and the rest is reflected. The reflected light reaches the metal colloid B(A), giving rise to an optical force FBA(AB). When the absorption/reflection properties of the two colloids are different, the optical forces need not obey Newton’s third law FAB ≠ − FBA.