Introduction to General Relativity
General relativity (GR) can be seen as the gauge theory of local coordinate transformations. The basis of GR is the equivalence principle, which, in some form, states that physics should be independent of the choice of coordinate system. This is already present in Newtonian dynamics, where that the inertial mass of a particle is equal to its gravitational mass. Consider a thought experiment due to Einstein where we have an observer in a box. The observer should not be able to distinguish between gravitational and inertial acceleration by means of local experiments i.e. if the box is small enough no observer will be able to distinguish between these two types of acceleration.