Observables in General Relativity

The notion of observable in general relativity differs profoundly from the corresponding concept in special relativity and in Newtonian physics. This is because in general relativity fixation of a frame of reference and gauge transformations are intertwined in a manner not encountered in any other area of physics. In other field theories an observable is any (local) component of a field that is gauge-invariant, such as a component of the electric field strength at a stated instant in time and at a given space point. Of course, functions, or functionals of such observables are themselves observables.