Place a polymer in a bath where self-propulsion varies in space and time as a traveling wave. The intuition says the polymer should be carried along — entrained by the local active fluctuations like a leaf on a current. Valecha, Sommer, and Sharma show the response depends on two properties: how long the polymer is and what topology it has.
Long polymers and structures with ring or star topologies ride the wave, displaying a positive drift in the direction of the wave's propagation. Short polymers and fully connected structures drift against the wave. The same driving signal produces opposite motion depending on the object's internal architecture.
The mechanism works through how the polymer couples to the spatiotemporal pattern. A long chain samples multiple phases of the wave simultaneously — its segments span regions of high and low activity — and the asymmetry in how these forces distribute along the contour produces net drift with the wave. A short chain fits within a single wavelength and responds to local gradients differently, experiencing the spatial derivative of the activity field rather than its average. Topology matters because ring and star architectures distribute internal tension differently from fully connected networks, changing how the wave's force propagates through the structure.
The through-claim is about what determines response to a signal. The signal — the self-propulsion wave — is the same for all polymers in the bath. The direction of motion is a property of the polymer, not the wave. Two polymers in the same bath, at the same location, experiencing the same driving, move in opposite directions because their internal architectures couple to the wave differently. The environment selects nothing. The structure determines everything.