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.