Saturday, April 23, 2016

Physics as a mode of thought – critical points in biology (and mind?)

Philip Ball, in Nautilus: Why Physics Is Not a Discipline. Rather, it's a mode of thinking that knows no disciplinary bounds.
The habit of physicists to praise peers for their ability to see to the “physics of the problem” might sound odd. What else would a physicist do but think about the “physics of the problem?” But therein lies a misunderstanding. What is being articulated here is an ability to look beyond mathematical descriptions or details of this or that interaction, and to work out the underlying concepts involved—often very general ones that can be expressed concisely in non-mathematical, perhaps even colloquial, language. Physics in this sense is not a fixed set of procedures, nor does it alight on a particular class of subject matter. It is a way of thinking about the world: a scheme that organizes cause and effect.

This kind of thinking can come from any scientist, whatever his or her academic label. It’s what Jacob and Monod displayed when they saw that feedback processes were the key to genetic regulation, and so forged a link with cybernetics and control theory. It’s what the developmental biologist Hans Meinhardt did in the 1970s when he and his colleague Alfred Gierer unlocked the physics of Turing structures. These are spontaneous patterns that arise in a mathematical model of diffusing chemicals, devised by mathematician Alan Turing in 1952 to account for the generation of form and order in embryos. Meinhardt and Gierer identified the physics underlying Turing’s maths: the interaction between a self-generating “activator” chemical and an ingredient that inhibits its behavior.

Once we move past the departmental definition of physics, the walls around other disciplines become more porous, to positive effect. Mayr’s argument that biological agents are motivated by goals in ways that inanimate objects are not was closely tied to a crude interpretation of biological information springing from the view that everything begins with DNA. As Mayr puts it, “there is not a single phenomenon or a single process in the living world which is not controlled by a genetic program contained in the genome.”

This “DNA chauvinism,” as it is sometimes now dubbed, leads to the very reductionism and determinism that Mayr wrongly ascribes to physics, and which the physics of biology is undermining. For even if we recognize (as we must) that DNA and genes really are central to the detailed particulars of how life evolves and survives, there’s a need for a broader picture in which information for maintaining life doesn’t just come from a DNA data bank. One of the key issues here is causation: In what directions does information flow? It’s now becoming possible to quantify these questions of causation—and that reveals the deficiencies of a universal bottom-up picture.
Biological systems seem to operate close to critical points, where the system changes from one phase to another:
By operating close to a critical point, Bialek and Mora said, a system undergoes big fluctuations that give it access to a wide range of different configurations of its components. As a result, Mora says, “being critical may confer the necessary flexibility to deal with complex and unpredictable environments.” What’s more, a near-critical state is extremely responsive to disturbances in the environment, which can send rippling effects throughout the whole system. That can help a biological system to adapt very rapidly to change: A flock of birds or a school of fish can respond very quickly to the approach of a predator, say.

Criticality can also provide an information-gathering mechanism. Physicist Amos Maritan at the University of Padova in Italy and coworkers have shown that a critical state in a collection of “cognitive agents”—they could be individual organisms, or neurons, for example—allows the system to “sense” what is going on around it: to encode a kind of ‘internal map’ of its environment and circumstances, rather like a river network encoding a map of the surrounding topography. “Being poised at criticality provides the system with optimal flexibility and evolutionary advantage to cope with and adapt to a highly variable and complex environment,” says Maritan. There’s mounting evidence that brains, gene networks, and flocks of animals really are organized this way. Criticality may be everywhere.
I have written variously about behavioral mode. Those modes may be considered different phases of mind.

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