In an aquarium, two schools of fish swim in adjacent compartments separated by a transparent wall. They cannot touch each other, but they see every movement. When one school decides to turn left, the other does the same a moment later. It is as if two pendulums hanging behind glass begin to swing in time.

Fish in a school constantly change direction, but not randomly. Their collective motion settles into one of two stable states: either all swim in one direction or in the opposite. This is like a swing that can rock left or right but does not linger in the middle. Switches between states occur spontaneously, driven by the random movements of individual fish. When schools see each other, these switches begin to coincide.

The scientists measured how the strength of this synchronization depends on the width of the gap between compartments. It turned out that the connection decays with distance more slowly than one might expect. This means that fish respond to a distant group not as individuals but as an averaged movement of the whole school. At short distances, details matter; at long distances, the overall rhythm does. This result helps understand how groups of animals remain connected even when there is a physical gap between them.