Turing’s paper described a theoretical mechanism based on 2 substances — an activator and an inhibitor that diffuse across an area at different rates. The interaction between these 2 “morphogens,” as Turing called them, allows one to interrupt the effect of the other — creating a pattern of colored lines on a tropical fish, for example, rather than a solid color. Turing’s mechanism was indeed responsible for the stripes in the bismuth. And it demonstrated once again how robust and powerful Turing’s original insight was. Here, the stripe-forming process is driven by the forces at play among the bismuth atoms and the metal below. Bismuth atoms want to fit into particular spots on the molecular lattice of the metal. But these spots are closer together than the bismuth atoms find comfortable. Like a photograph that gets shoved into a frame that’s too small for it, the sheet of bismuth atoms buckles. The strain creates a wavy pattern that leaves some atoms raised, forming the stripes. The vertical shift — movement away from the plane of the crystal — acts as the activator in the Turing equations, while the shift within the plane acts as the inhibitor. The morphogens here are displacements, not molecules. When part of a Turing pattern is wiped out, it grows back. You might not assume that inorganic materials like bismuth crystals would be able to heal as animals do,but indeed, his team’s simulated bismuth crystal was able to mend itself.
Gregor J. Rothfuss 