In case you didn’t follow the code-alongs, we provide a video of Langton’s loops below. Please take a moment to reflect that this simple pattern took 30 years to discover after cellular automata were first discovered. Notice also that the cells not only self-replicate, but after a period of time, they enter a “death” state in which they no longer have cells that change state. In this sense, they mimic life itself.

One of the enduring qualities of Langton’s loops is how lifelike they seem. We would not be unreasonable to notice that the loop itself vaguely resembles a cell, or that its three-cell wide outer shell is reminiscent of a biological molecule like DNA that has a two-sided exterior with information represented inside the molecule.

In fact, Langton’s work in the mid-1980s helped launch an entire field of research called artificial life that examined whether we can use models to mimic the complex systems of life and asked some deep questions about the nature of intelligence, such as “Can we build a computational model that simulates the conditions on Earth four billion years ago and that produces early life forms only from chemical reactions?”

You likely have never heard of artificial life because after some early excitement, the field largely failed to produce any substantive research results and mostly flamed out in the 1990s. Artificial life offers an excellent example of what can happen to a perfectly reasonable area of scientific interest when it becomes oversold. Today we have the field of “artificial intelligence”, which was often ridiculed for years only now to demonstrate immense hope and progress.

Finally, while philosophizing, we would return to the start of this chapter and the Fermi paradox. Perhaps we have not seen self-replicating space probes not because alien life isn’t out there, but because other civilizations find self-replicating robots just as difficult to build as we do. If it took 30 years just to discover a simple self-replicating cellular automaton, maybe the fact that the trillions of cells in your body go about their business of self-replication every day is something of such depth and elegance that we take it for granted.

Or maybe the resolution to the Fermi paradox is something much simpler. Maybe our lust for exploration makes us a peculiar species. Maybe the aliens know how to build von Neumann probes, but they are too busy enjoying existence to be bothered.

Chapter Exercises

You are now ready to apply what you have learned to this chapter’s exercises, in which we will guide you through building your first larger coding project. If you enjoyed this chapter, you will love the exercises, which consider a related version of cellular automata first hypothesized by the father of modern computer science, Alan Turing.


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