Complexity_ A Guided Tour - Melanie Mitchell [80]
In the previous chapter I proposed that particles and their interactions are one approach toward such a high-level language for describing how information processing is done in cellular automata. Information is communicated via the movement of particles, and information is processed via collisions between particles. In this way, the intermediate steps of information processing acquire “meaning” via the human interpretation of the actions of the particles.
One thing that makes von-Neumann-style computation easier to describe is that there is a clear, unambiguous way to translate from the programming level to the machine code level and vice versa, precisely because these computers were designed to allow such easy translation. Computer science has given us automatic compilers and decompilers that do the translation, allowing us to understand how a particular program is processing information.
For cellular automata, no such compilers or decompilers exist, at least not yet, and there is still no practical and general way to design high-level “programs.” Relatively new ideas such as particles as high-level information-processing structures in cellular automata are still far from constituting a theoretical framework for computation in such systems.
The difficulties for understanding information processing in cellular automata arise in spades when we try to understand information processing in actual living systems. My original question, “In what sense do natural systems ‘compute’?” is still largely unanswered, and remains a subject of confusion and thorny debate among scientists, engineers, and philosophers. However, it is a tremendously important question for complex systems science, because a high-level description of information processing in living systems would allow us not only to understand in new and more comprehensive ways the mechanisms by which particular systems operate, but also to abstract general principles that transcend the overwhelming details of individual systems. In essence, such a description would provide a “high-level language” for biology.
The rest of this chapter tries to make sense of these ideas by looking at real examples.
The Immune System
I gave a quick description of the immune system way back in chapter 1. Now let’s look a bit more in depth at how it processes information in order to protect the body from pathogens—viruses, bacteria, parasites, and other unwelcome intruders.
To recap my quick description, the immune system consists of trillions of different cells and molecules circulating in the body, communicating with one another through various kinds of signals.
Of the many many different types of cells in the immune system, the one I focus on here is the lymphocyte (a type of white blood cell; see figure 12.1). Lymphocytes are created in the bone marrow. Two of the most important types of lymphocytes are B cells, which release antibodies to fight viruses and bacteria, and T cells, which both kill invaders and regulate the response of other cells.
FIGURE 12.1. A human lymphocyte, whose surface is covered with receptors that can bind to certain shapes of molecules that the cell might encounter. (Photograph from National Cancer Institute [http://visualsonline.cancer.gov/details.cfm? imageid=1944].)
FIGURE 12.2. An illustration of a lymphocyte (here a B cell) receptor binding to an antigen.
Each cell in the body has molecules on its surface called receptors. As the name implies, these molecules are a means by which cells receive information. The information is in the form of other molecules that chemically bind to the receptor molecules. Whether or not a receptor binds to a particular molecule depends on whether their physical shapes match sufficiently.
A lymphocyte