The problem that it is very difficult to obtain exact solutions to systems involving interactions between more than two bodies – using either classical mechanics or quantum mechanics. To understand the physics of many-body systems it is necessary to make use of approximation techniques or model systems. For some problems, such as the three-body problem in classical mechanics, it is possible to obtain qualitative information about the system. If there are a great many bodies interacting, such as the molecules in a gas, the problem can be analysed using the techniques of statistical mechanics.
Subjects: Chemistry — Physics.