Cartwright: Do the Laws of Physics State the Facts?

Transcription

1 Cartwright: Do the Laws of Physics State the Facts? Introduction Facticity view of laws: Laws of nature describe the facts Paradigm: fundamental laws of physics (e.g., Maxwell s equations) Dilemma: If they are meant to be descriptions, they are false. They do not describe actual behaviour, if other forces are present. If they are amended to be true, they are useless for explanation. Restricted laws do not apply in cases where other forces are present. Example: charged and massive particles. The problem is worse in physics than in biology. Laws in biology may be approximate, but they are about real objects and they describe them more or less accurately. Laws in physics are not about their objects they don t even approximately describe their behaviour. Note: same conclusion as Dretske, but for opposite reasons. Dretske: laws are descriptively accurate but can t be about objects because they are not just about actual behaviour. Cartwright: laws aren t even descriptively accurate for actual objects.

2 1. Composition of Causes; trade-off of truth and explanatory power Cartwright: fundamental laws can either be principles of explanation or true descriptions of behaviour, but not both. Reason: each law describes the operation of one basic causal factor, but actual behaviour is the resultant of multiple causes. Example: F = Gm 1 m 2 / r 2 Unrestricted laws are descriptively false. False if taken as a description of how bodies behave. if charged: there is electric force and they move differently if friction is present: they move differently even confining attention to this law: if multiple bodies are present, each application to m 1 and m 2 gives a false description of behaviour Restricted version: If no other forces are present [and no other bodies], then F = Gm 1 m 2 / r 2. Restricted laws are true but explanatorily useless. So restricted, the law is not available for explanations of complex circumstances (where gravity and other forces are present). Correct picture. A world full of complex phenomena, whose behaviour is the result of multiple forces. Fundamental laws describe the simplest component processes. Fundamental laws tell us only how things would act separately. Actual behaviour is the resultant of several simpler processes. Actual explanation is by composition of causes. Rather than being surprised that fundamental laws cannot truly describe actual behaviour, we should expect this.

3 2. Vector Addition Introduces Causal Powers Objection to Cartwright s claim that unrestricted laws are false. Forces superpose as vectors add. Two or more forces are produced in accordance with two or more laws, and then added together to get a resultant force. This solves the problem: the unrestricted laws are descriptively accurate. In more detail, we amend our laws as follows. Any two bodies produce a gravitational force between them that is described by F = Gm 1 m 2 / r 2. Ditto for electrical force (with appropriate changes). Each law truly describes what happens: a component force is produced. The resultant force is the sum of these produced forces. Cartwright s criticism: We (not nature) add the forces. All that occurs is a single force. The components are not really there vector addition is just a metaphor. A body moving northeast is not moving due north and due east. A body pulled equally in two directions is not moving in both of them. Correct way to understand vector addition (and to restore the idea that laws provide true descriptions ). Laws are not factive (about bodies), but about causal powers. Bodies have causal powers to produce the two forces, but the force that they actually produce is some kind of compromise (described by vector addition). We have no good story about causal powers and their place in explanation and prediction.

4 Discussion: 1) Is it correct to say that the component forces are not really there? What about the electrical field and the gravitational field? 2) If we admit causal powers, why not component forces?

5 3. A Real Example Ground state of carbon. What are the possible energy levels of the six electrons in a carbon atom? Three answers. 1) Single energy level. An approximation (average value). 2) Three energy levels. The 1s, 2s and 2p levels. These reflect the exact Coulomb potential: repulsion between inner-shell electrons and outer pair of electrons. 3) Five energy levels. The 2p splits into three possibilities when we take in spin effects, which create a magnetic field. Significance: the forces here are not additive. The vector addition strategy fails. We can t claim that the Coulomb factor and the spin factor are separately present and somehow additive. But we still have separate, true, counterfactuals about what would happen if only one of these causal factors were present. Problem: how do we state the relevant laws? We have no model that takes counterfactuals for the simple case and combines them to describe the actual case.

6 4. Composition of causes vs. covering-law explanation The point: Here is one more strategy for salvaging the idea that laws are simultaneously explanatory and true ( factive ) but it also fails. Initial observation: A single covering law may be descriptively true. For example: a super-law that combines Coulomb and gravitational force into a single equation. This will be descriptively accurate when these two forces (and nothing else) are present. It succeeds as a covering law. Cartwright s criticism: Super-laws are not the answer. i. Not always available. We can often explain, even without super-laws. Our account of laws and explanations should not always require a super-law. ii. Not always explanatory. A bland covering that does not exhibit the relevant causal factors does not explain anything. Example: The net force on any rope pulled by Jules and Jim is zero. Provides a D-N explanation for The rope did not move. But intuitively non-explanatory: we need to understand that there are two equal and opposite causal powers (because Jules and Jim are equally strong). iii. Not the full explanation. Even if there is a super-law, and it provides some sort of explanation, it results from combined action of separate laws. We need these for a full explanation. Upshot: We need a philosophical account of laws that are not factive.

7 5. Conclusion Facticity view: does not work because it does not fit explanatory laws, the fundamental laws of physics. Fundamental laws must be factive for the simplest cases (which may be purely counterfactual situations they may never occur). That implies that they can t be factive for complex cases involving multiple factors. So the law can t be literally true. If we restrict such laws to cases where only the relevant factor is present, then they become factive but can play no role in explaining complex cases. Explanations should work in the same way when only single causes are present, and when composite causes are present so our final theory of explanation must find a way to incorporate these fundamental laws. We need an account of explanation that does this.