ABSTRACT
This chapter shows that the distribution of matter and energy can be described – both in Special Relativity and in General Relativity – in terms of a rank-two tensor field, the stress-energy tensor. It first introduces stress-energy tensor, in flat spacetime, using as an example the simplest physical system, a gas of non-interacting particles. In order to generalize these results to curved spacetimes, the chapter states a fundamental principle of General Relativity, the Principle of General Co- variance. In order to understand the physical meaning of the components of the stress-energy tensor, the chapter discusses the case of a gas of non-interacting particles.
