It is almost a miracle that the kinetic analogue of the ideal gas works. If we wanted to make a more realistic representation of the impact of molecules on the walls of the enclosure-using information from atomic physics-we might come up with a picture such as fig. 5.10. The double-headed arrows represent oscillations, and the zig-zag lines represent some kind of force-field. The single-headed arrows represent directions of motion. All of that is reduced to a picture such as fig. 5.11 by the elementary kinetic theory of the ideal gas! Why should the analogue work as well as it does? Probably because in our simple representation we look at molecular motions from the perspective of the macro-world. The motion represented in fig. 5.11 is a time-average, or the track of an "average particle"-and an "average particle" is deduced by thinking about the system as a whole. The point is that a "molecule of a gas" cannot be thought of as a single molecule in isolation. A molecule of a gas is a component of a system and, however they do interact, molecules partake of a common field of energy. In modern quantum physics, molecules are ignored as such and the same results obtained concerning pressure and temperature purely in terms of energy.