Complexity_ A Guided Tour - Melanie Mitchell [78]
All this is relatively recent work and needs to be developed much further. I believe that this approach to understanding computation, albeit unconventional, will turn out to be useful in other contexts in which computation is distributed among simple components with no central control. For example, it is still a mystery how high-level information about sensory data is encoded and processed in the brain. Perhaps the explanation will turn out to be something close to particle-like or, given the brain’s three dimensions, wave-like computation, where neurons are the scaffolding for information-carrying waves of activity and their information-processing interactions.
Brain computation is of course a long jump from one-dimensional cellular automata. However, there is one natural system that might be explained by something very much like our particles: the stomata networks of plants. Every leafy plant’s leaves are covered with stomata—small apertures that open or close in response to light and humidity. When open, the stomata let in carbon dioxide, which is used in photosynthesis. However, open stomata cause water to evaporate from the plant’s fluid stores. Botanist Keith Mott, physicist David Peak, and their colleagues at Utah State University have long been observing the patterns of opening and closing of stomata on leaves, and believe that the stomata constitute a network that is something like a two-dimensional cellular automaton. They also observe that the temporal patterns of opening and closing on the leaves looks very much like two-dimensional versions of interacting particles. They hypothesize that plants perform a distributed, decentralized computation via their stomata—namely, how to optimally open and close stomata in order to best balance carbon dioxide gain and water loss—and that the computation may be explainable in terms of such particles.
CHAPTER 12
Information Processing in Living Systems
EVER SINCE SZILARD’S INSIGHT THAT information might be the savior of the second law of thermodynamics from the attack of Maxwell’s demon, information and its cousin computation have increasingly infiltrated science. In many people’s minds information has taken on an ontological status equal to that of mass and energy—namely, as a third primitive component of reality. In biology in particular, the description of living systems as information processing networks has become commonplace. In fact, the term information processing is so widely used that one would think it has a well-understood, agreed-upon meaning, perhaps built on Shannon’s formal definition of information. However, like several other central terms of complex systems science, the concept of information processing tends to be ill-defined; it’s often hard to glean what is meant by information processing or computation when they are taken out of the precise formal context defined by Turing machines and von Neumann-style computers. The work described in the previous chapter was an attempt to address this issue in the context of cellular automata.
The purpose of this chapter is to explore the notion of information processing or computation in living systems. I describe three different natural systems in which information processing seems to play a leading role—the immune system, ant colonies, and cellular metabolism—and attempt to illuminate the role of information and computation in each. At the end I attempt to articulate some common qualitative principles of information processing in these and other decentralized systems.
What Is Information Processing?
Let me quote myself from chapter 10: “In what sense do natural systems ‘compute’? At a very general level,