Drawing on a number of threads in this forum, and my very limited understanding of relativity, I am intrigued by the following thoughts: If I have understood correctly, in GR, c is supposed to be only a 'local global constant', and not exactly a Universal constant. Meaning that, two distant observer will each measure c 'locally' to be a given constant value (299,792,458 m/s as per most sources), and this is valid everywhere in the Universe 'locally'. In addition, both observers can conclude that the c at the other observers location is different from his local c (e.g. c is comparatively lower in the vicinity of a large mass, than farther away). My questions are: - How is 'local' or 'locality' defined (i.e. where is the boundary between local and non-local)? - If c can be observed to have different values at various distances from an observer as per GR, does SR have any validity in the real 'gravitational' Universe? - How can c then be considered an 'ultimate speed limit of the Universe', since it can vary at different locations even for a single observer?