In special relativity speed of light is constant for all observers at any frame of reference but as Einstein proved later in some of his papers. This is not the same case in general relativity, when you have an accelerated frame of reference i.e. a deference in space-time curvature then the speed of light will be constant only for local observers and different for a non-local observers. For example, a distant galaxy away from us by billions of light-years due to the expansion of the universe moving at a velocity higher than the speed of light in relative to us, then if we try to measure the speed of light in that galaxy we should find it higher than the speed of light on our local perspective but it will be equal to the ordinary speed of light if it measured from a local observer in that distance galaxy. For another example a spacecraft using the hypothetical warped space-time drive, then the speed of light inside this spacecraft for the persons inside (i.e. local observers) will be unaffected, but for an outside observer in flat space-time (i.e. nonlocal observer) then the speed of light inside this spacecraft will be unobservable but it’s higher than the local speed of light. So by following this line of work I manage to confirm and develop Einstein's work.