4. RET of Rarefied Monatomic Gas

In this chapter, we give a survey of the main results of RET concerning rarefied monatomic gases, some of which are explained in the Müller–Ruggeri book of RET (Müller and Ruggeri, Rational Extended Thermodynamics, Springer, New York, 1998). We start from the phenomenological RET theory with 13 fields and prove that the closure of RET coincides with the one obtained by Grad using kinetic arguments and with the MEP procedure. The theory with N-moments is also presented with the proof of nesting theories that emerge from the concept of principal subsystem. The problematic of bounded domain in RET is also considered, and a simple example of heat conduction is explained to show a significant difference of the results between RET and NSF. A lower bound for the maximum characteristic velocity is obtained in terms of the truncation tensor index N. This quantity increases as the number of moments grows and it is unbounded when \(N \rightarrow \infty \). The relativistic counterpart is also described briefly. In this framework, the maximum characteristic velocity is bounded for any number of moments, and converges to the light velocity from the below for \(N \rightarrow \infty \). The chapter contains also comparison between the RET theory and experiments in sound waves and light scattering.