В данной книге изучается понятие физического закона. Автор полагает. что законы не являются какими-то особыми познавательными объектами, по сути. они относятся к моделям и верны только для них. То, из чего состоят физические объяснения, по сути не слишком истинно - и это. в общем, лежит даже на поверхности. Функция "быть истинным" и "использоваться для объяснения физических явлений" - это разные вещи, и т.н. законы природы должны как-то удовлетворят той и другой функции, что не очень получается. Физические законы описывают упрощенную реальность моделей (которые не реальны), а реальность феноменов отличается от того, что сообщают физические законы. Или можно сказать иначе: создавая картину мира, опирающуюся на физические законы, мы, в общем, принимаем заведомую ложь. Но зато, пользуясь совокупностью этих законов, можно довольно удобно объяснять и предсказывать поведение важных для нас идеальных объектов.
How the Laws of Physics Lie
Nancy Cartwright, Associate Professor of Philosophy, Stanford University, California
Nancy Cartwright argues for a novel conception of the role of fundamental scientific laws in modern natural science. If we attend closely to the manner in which theoretical laws figure in the practice of science, we see that despite their great explanatory power these laws do not describe reality. Instead, fundamental laws describe highly idealized objects in models. Thus, the correct account of explanation in science is not the traditional covering law view, but the ‘simulacrum’ account. On this view, explanation is a matter of constructing a model that may employ, but need not be consistent with, a theoretical framework, in which phenomenological laws that are true of the empirical case in question can be derived. Anti-realism about theoretical laws does not, however, commit one to anti-realism about theoretical entities. Belief in theoretical entities can be grounded in well-tested localized causal claims about concrete physical processes, sometimes now called ‘entity realism’. Such causal claims provide the basis for partial realism and they are ineliminable from the practice of explanation and intervention in nature.
Most scientific explanations use ceteris paribus laws. These laws, read literally as descriptive statements, are false, not only false but deemed false even in the context of use. This is no surprise: we want laws that unify; but what happens may well be varied and diverse.
We are lucky that we can organize phenomena at all. There is no reason to think that the principles that best organize will be true, nor that the principles that are true will organize much.
There is a simple, straightforward view of laws of nature which is suggested by scientific realism, the facticity view: laws of nature describe how physical systems behave. This is by far the commonest view, and a sensible one; but it does not work. It does not fit explanatory laws, like the fundamental laws of physics. Some other view is needed if we are to account for the use of laws in explanation; and I do not see any obvious candidate that is consistent with the realist's reasonable demand that laws describe reality and state facts that might well be true. There is, I have argued, a trade-off between factual content and explanatory power. We explain certain complex phenomena as the result of the interplay of simple, causal laws. But what do these laws say? To play the role in explanation we demand of them, these laws must have the same form when they act together as when they act singly. In the simplest case, the consequences that the laws prescribe must be exactly the same in interaction, as the consequences that would obtain if the law were operating alone. But then, what the law states cannot literally be true, for the consequences that would occur if it acted alone are not the consequences that actually occur when it acts in combination.
If we state the fundamental laws as laws about what happens when only a single cause is at work, then we can suppose the law to provide a true description. The problem arises when we try to take that law and use it to explain the very different things which happen when several causes are at work. This is the point of ‘The Truth Doesn't Explain Much’.
The laws of physics, I concluded, to the extent that they are true, do not explain much. We could know all the true laws of nature, and still not know how to explain composite cases. Explanation must rely on something other than law.
But this view is absurd. There are not two vehicles for explanation: laws for the rare occasions when causes occur separately; and another secret, nameless device for when they occur in combination. Explanations work in the same way whether one cause is at work, or many. ‘Truth Doesn't Explain’ raises perplexities about explanation by composition of causes; and it concludes that explanation is a very peculiar scientific activity, which commonly does not make use of laws of nature. But scientific explanations do use laws. It is the laws themselves that are peculiar. The lesson to be learned is that the laws that explain by composition of causes fail to satisfy the facticity requirement. If the laws of physics are to explain how phenomena are brought about, they cannot state the facts.
I believe in theoretical entities. But not in theoretical laws. Often when I have tried to explain my views on theoretical laws, I have met with a standard realist response: ‘How could a law explain if it weren't true?’ Van Fraassen and Duhem teach us to retort, ‘How could it explain if it were true?’ What is it about explanation that guarantees truth? I think there is no plausible answer to this question when one law explains another. But when we reason about theoretical entities the situation is different. The reasoning is causal, and to accept the explanation is to admit the cause. There is water in the barrel of my lemon tree, or I have no explanation for its ailment, and if there are no electrons in the cloud chamber, I do not know why the tracks are there.
Fundamental laws are supposed by many to determine what phenomenological laws are true. If the primary argument for this view is the practical explanatory success of the fundamental laws, the conclusion should be just the reverse. We have a very large number of phenomenological laws in all areas of applied physics and engineering that give highly accurate, detailed descriptions of what happens in realistic situations. In an explanatory treatment these are derived from fundamental laws only by a long series of approximations and emendations. Almost always the emendations improve on the dictates of the fundamental law; and even where the fundamental laws are kept in their original form, the steps of the derivation are frequently not dictated by the facts. This makes serious trouble for the D-N model, the generic-specific account, and the view that fundamental laws are better. When it comes to describing the real world, phenomenological laws win out.
It is important that the models we construct allow us to draw the right conclusions about the behaviour of the phenomena and their causes. But it is not essential that the models accurately describe everything that actually happens; and in general it will not be possible for them to do so, and for much the same reasons. The requirements of the theory constrain what can be literally represented. This does not mean that the right lessons cannot be drawn. Adjustments are made where literal correctness does not matter very much in order to get the correct effects where we want them; and very often, as in the staging example, one distortion is put right by another. That is why it often seems misleading to say that a particular aspect of a model is false to reality: given the other constraints that is just the way to restore the representation.
Here is a very simple example of how the operation of constraints can cause us to set down a false description in physics. In quantum mechanics free particles are represented by plane waves—functions that look like sines or cosines, stretching to infinity in both directions. This is the representation that is dictated by the Schroedinger equation, given the conventional Hamiltonian for a free particle. So far there need be nothing wrong with a wave like that. But quantum mechanics has another constraint as well: the square of the wave at a point is supposed to represent the probability that the particle is located at that point. So the integral of the square over all space must equal one. But that is impossible if the wave, like a sine or cosine, goes all the way to infinity.
There are two common solutions to this problem. One is to use a Dirac delta function.
These functions are a great help to physics, and generalized function theory now explains how they work. But they side-step rather than solve the problem. Using the delta function is really to give up the requirement that the probabilities themselves integrate to one.
Merzbacher, for instance, says ‘Since normalization of ∫Ψ*Ψ to unity is out the question for infinite plane waves, we must decide on an alternative normalization for these functions. A convenient tool in the discussion of such wave functions is the delta function’.12 I have thus always preferred the second solution.
This solution is called ‘box normalization’. In the model we assume that the particle is in a very, very large box, and that the wave disappears entirely at the edges of this box. To get the wave to go to zero we must assume that the potential there—very, very far away from anything we are interested in—is infinite.
Here is a clear distortion of the truth. The walls may interact with the particle and have some effect on it, but they certainly do not produce an infinite potential.
I think Merzbacher intends us to think of the situation this way. The walls and environment do contain the particle; and in fact the probability is one that the particle will be found in some finite region. The way to get this effect in the model is to set the potential at the walls to infinity. Of course this is not a true description of the potentials that are actually produced by the walls and the environment. But it is not exactly false either. It is just the way to achieve the results in the model that the walls and environment are supposed to achieve in reality. The infinite potential is a good piece of staging.