== The proposal of the loop-closing definition. == 

 Earlier on this Talk page I proposed to take the loop-closing characteristic as the fundamental characteristic. I propose to add to that another demarcation criterium. 

 A twin scenario can be realized with one dimension of space (plus, of course, the dimension of time). Plotted in a Minkowski space-time diagram it will look like a loop, but the idea is that one dimension of space suffices to enable a twin scenario.  

 I propose to call it a 'Sagnac scenario' if and only if the spatial paths enclose an area. In the case of a ring interferometer, the size of the enclosed area of the counterpropagating beams is an operative factor for the magnitude of the Sagnac effect.  

 By adopting a convention that a spatial area must be enclosed, such a thing as a 'linear Sagnac effect' is excluded from the definition. Again, I think these are matters of conventions of how things are named, not of physics principles.  

 I expect the following: a laser with a linear resonance cavity will not show frequency splitting when it is moving inertially. I expect that a linear cavity laser can have frequency-splitting only when it is being accelerated with respect to the local inertial frame.  
 I take the Canterbury university ring laser setup as the purest realisation of a ring laser rotation sensor. The laser process generates two frequencies of light if the setup as a whole is rotating around an axis.