Written down in a clever coordinate system, vacuum Einstein equations turn out to be a system of quasilinear wave equations on a Lorentzian manifold. A complete understanding of linear wave equations on Minkowski spacetime is the natural starting point to attack the wave problem emerging from Einstein equations. Properties of linear waves on Minkowski can be investigated using different methods. However, there is a fundamental interplay between properties of waves and the geometry of Minkowski which can only be appreciated if we set up the problem geometrically. The aim of the talk is to introduce this geometric setting and then prove a conservation of energy statement and a Morawetz estimate. This will be enough to capture the idea that thinking geometrically is the best approach if we want to go further in the theory.