Sunday, October 26, 2008

Downward Causation in Fluids?

Bishop claims to have found a case of downward causation in physics based on the existence of what is known as Rayleigh-Benard convection in fluids. In the simplest case we have a fluid like water that is heated from below. What can result, as this image from Wikipedia shows, is a series of cells, known as Benard cells, where the dominant large-scale structure is fluid flowing in interlocking circular patterns.

The claim is that these patterns require new causal powers over and above what can be ascribed to the smaller scale fluid elements: "although the fluid elements are necessary to the existence and dynamics of Benard cells, they are not sufficient to determine the dynamics, nor are they sufficient to fully determine their own motions. Rather, the large-scale structure supplies a governing influence constraining the local dynamics of the fluid elements" (p. 239).

There is no doubt that this is an interesting case that should receive more scrutiny. As with McGivern's article, the tricky interpretative question is how closely we should link the workings of the mathematical model to the genuine causes operating in the system. Bishop's conclusion seems based on taking the representation of fluid elements very seriously, but I am not sure that the link between the representation and reality at this level is well enough understood. Still, I would concede his point that many features of downward causation from philosophical accounts appear in this example.

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