In the last chapter we have found that physical laws in Special Relativity Theory have the same form in every reference system if they are expressed as four-vector equations. The physical laws of spacetime thus behave like the classical laws of Newtonian physics: They are invariant under Galilean transformations when formulated in (three-dimensional) vector form, cf. Sect. 12.1. The four-vectors, which we will often simply call “vectors” from now on, are geometric objects that have different components in different coordinate systems, but remain unchanged in spacetime.