Book review: The Dappled World

The Dappled World:  A Study of the Boundaries of Science
by Nancy Cartwright (1999)

Cartwright proposes that we are led astray by our expectation that the world is united under a small number of laws.  She thinks it more likely that the world is a patchwork of distinct natures, each with its own regularities.  Predictive laws can only be applied when one has a “nomological machine”, meaning (so far as I can tell) an isolated system where all forces take one of the expected forms.  If one takes seriously the Aristotelian claim that the world is governed by natures rather than a fundamental law, arguably this is what one would expect.

Usually, when one imagines laws of physics breaking down, the idea is that they fail in some untested regime, such as very high energy.  Cartwright appears to be more interested in the idea that they would fail for untested functional forms.  For example, one’s dynamical laws may work for a quadratic but not a quartic potential.  It would be hard to rule out such a possibility, but none of the cases considered in the book pose particular difficulty for the “fundamentalist”, as Cartwright calls believers in unified laws.  Take her example of predicting where a thousand dollar bill carried by the wind will land.  Because we don’t know the exact initial conditions of bill or air, we should consider our initial state as a distribution in some vast state space evolving according to a sort of Liouville equation.  Such a calculation would correctly predict that the bill may fall anywhere in a wide area.  (A full calculation of this sort would be unfeasible, but a sense of the range of possible behavior could easily be gotten by numerically evolving the fluid+bill surface equations for, say, a dozen initial states within the range of uncertainty.)  Just because Newtonian mechanics is uninformative in such cases doesn’t make it wrong.  A more central example in the book is the BCS theory of superconductivity.  Cartwright makes a great deal of the the fact that the BCS Hamiltonian operator is a combination of what one might call “stock components”.  I agree that this illustrates well something about how modeling really works in physics, that in practice one does not start with the most general, fundamental action principle and then start approximating or dropping terms to get to one’s desired system.  However, the message is not that quantum mechanics has some problem with more general Hamiltonians, but that the goal is to capture the essence of superconductivity, so inessential and particularizing details have been dropped.  And it’s easy to see why certain components (esp. quadratic potentials) would be expected to appear in many contexts, as Cartwright acknowledges.  None of this is to say that she is wrong, but the fundamentalist position remains viable.  (Making it explicit and putting it into question, though, can only be good.)qw

Cartwright argues that the way scientists go about their business assumes an Aristotelian multitude of natural capacities and does not fit with Hume’s model of laws as a summary of observed regularities.  She points out that charges moving according to, say, the Coulomb electrostatic force never exactly happens.  The electrostatic force rather describes a tendency, a “what would have happened” if other effects had not been present.  The book then gives an intriguing discussion of Goethe’s criticisms of Newton, arguing that the success of Newton’s optical experiments is more consistent with an Aristotelian world of discrete natures than with Goethe’s more holistic world.

The book ends with some speculations on the relation between classical and quantum mechanics.  Cartwright proposes a sort of property dualism for the physical world, in which systems might have both quantum and classical properties, which presumably would have to interact somehow.

 

4 Responses

  1. […] apply outside of the highly controlled situations in which scientists test them. This is Nancy Cartwright’s position, but her examples are unconvincing. Feser flirts with this idea, but the main thrust of his […]

  2. […] apply outside of the highly controlled situations in which scientists test them. This is Nancy Cartwright’s position, but her examples are unconvincing. Feser flirts with this idea, but the main thrust of his […]

  3. […] because different things have different natures and hence different principles of motion.  (cf. Nancy Cartwright’s “dappled world”)  The idea of general laws of motion is much more natural in a Platonic/transcendent […]

  4. […] because different things have different natures and hence different principles of motion.  (cf. Nancy Cartwright’s “dappled world”)  The idea of general laws of motion is much more natural in a Platonic/transcendent […]

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