Model-Making in Physics
Rudolf Ernst Peierls · World Scientific series in 20th century physics · 1997
Physicists tend to use models of various kinds to aid their understanding of complicated physical situations. The models differ by the degree of simplification or exaggeration they involve, according to the purpose for which they are used. It is suggested that one might distinguish seven different types. Examples of each type are discussed, together with the examples of the kind of confusion that can result if the nature of the model used is misunderstood. In reading any papers on current physics, or listening to discussions between physicists, one is almost certain to encounter the mention of models. The extent of this model-making habit is illustrated in table 1. This table lists a number of models familiar in various branches of physics. It is by no means complete-I have included only those that came readily to mind. Many of them are normally called models, but the table also contains some which, while not referred to by that name in common usage , are so similar in nature to accepted models that it seemed logical to include them. It seems worth while to examine the nature and purpose of such models in some detail. It will turn out that different models serve quite different purposes, and they vary in their nature accordingly. To bring out these distinctions, I have divided the models listed in table 1 into a number of types, which will be defined and discussed below. To make the meaning of these types clear I shall discuss a few typical examples of each kind. I shall also consider the confusion that can arise if a model of one type is mistaken for another, or more generally if the limitations inherent in the model are overlooked. The definition of the categories, and the assignment of specific models to them, is of course very subjective, and many readers may prefer to vary the choice of types, or may question my assignments. Few will, however, disagree with the width of the spectrum. Type 1: Hypothesis ('Could be true') The first type to mention is not really a model in the strict sense. It has been included because such hypotheses are often called models. In general they consist of a tentative explanation of a phenomenon, which its author believes to be likely or even certain to be true. Very old examples are the models of the universe of Aristotle and Ptolemy, later abandoned, and that of Copernicus, refined by Kepler, which we now believe to be essentially correct. The advent of general relativity broadened this problem into one of finding a model of the space-time structure of the Universe, such as those of Einstein, t Based on the Cherwell-Simon Memorial Lecture delivered at Oxford on 4 May 1979. W M-7514/80/2101 (1003 SO2(H)( 151811 Tavl,,r & F,xtneis Ltd A2