Spiral waves in excitable media interact. Two rotating spirals in a reaction-diffusion system drift toward or away from each other, carve out territories separated by collision interfaces, and influence each other's rotation frequency. The interaction looks gravitational — two bodies, mutual attraction, force scaling with separation. The analogy invites the full Newtonian framework: equal and opposite forces, directed along the line connecting centers, with a well-defined mass for each spiral.
De Coster and colleagues derive the interaction laws for N spiral waves and find that Newton's third law doesn't hold. The forces between spiral pairs are not directed along the line connecting their centers. The interaction is non-reciprocal in direction — spiral A pushes spiral B at an angle that bears no simple relation to the angle at which B pushes A. The phases of rotation, the geometry of collision interfaces, and the wavefront deflections all contribute forces whose directions depend on the local structure of the influence region, not on the relative position of the pair.
The spiral's “mass” — the proportionality between applied force and drift velocity — is not a fixed property. It depends on the shape of the influence region, which changes as the spiral moves and as neighboring spirals reshape the medium. The mass is contextual. A spiral near a boundary has different mass than the same spiral in open space.
This matters because spiral waves in excitable media are not abstractions — they are the rotating electrical patterns that sustain cardiac fibrillation. Understanding how spirals interact determines whether fibrillation terminates or persists. The interaction is non-Newtonian: forces at angles, variable masses, no action-reaction symmetry. The physics of the heartbeat violates the laws we teach in introductory mechanics.