Maybe a more precise definition of "inertial observer" would help, but is there one? My assumption was the two characteristics given elsewhere on the forum: (a) the observer's frame does not accelerate or rotate, and (b) the observer has mass.
The term "dual" can be used in the mathematical...
If an inertial "observer" or state has mass and no rotation, then a massless state with rotation (i.e. having maybe a generalized rotation such as "spin," e.g. a photon) seems to be dual to that state.
Would this viewpoint then take the photon as a "matching" channel or "process" for...