Models in physics
In its most common use, the term ‘model’ refers to a simplified and stylised version of the socalled target system, the part or aspect of the wor ld that we are interested in. For instance, in order to determine the orbit of a planet moving aro und the sun we model the planet and the sun as perfect homogenous spheres that gravitationa lly i teract with each other but nothing else in the universe, and then apply Newtonian mech ani s to this system, which reveals that the planet moves on an elliptical orbit. Views dive rge about what sort of entity such a model is. Those focussing on the formal aspects of models regard them either as equations or settheoretical structures, while those opposed to such an approach take them to be descriptions or abstract (yet non-mathematical) entities. A further question concerns the relation of models and theories. In some cases models can be derived f rom theory simply by specifying the relevant determinables in a theory’s general equati ons. But many models cannot be obtained from theory in this straightforward way, and some e v n involve assumptions that contradict the fundamental theory. The relation of models to t heir respective target systems is equally complex and fraught with controversy. Two influenti al proposals take the relation between a model and its target to be isomorphism or similarit y, respectively. This, however, has been criticised as too restrictive as many models do not seem to fit this mould. 1. Introducing Models In physics ‘model’ is used in different ways. Phras es like ‘it is just a model’ indicate either that physicists take a cautious attitude towards a particular piece of physics which they regard as particularly speculative and provisional, or tha t t e piece of physics is known to be false and is entertained only for heuristic reasons. On s ome occasions theories are referred to as ‘models’, for instance in particle physics where th best and currently widely accepted theory of elementary particles is called ‘the standard mod el’. However, in the most common use of