Originally Posted By: Bill S.
Let me get this straight. Are we saying that light does not always travel at c; it only seems like that to an observer?


The speed of light is always measured as being c due to the fact that the devices employed - clocks and rods - can be physically affected either by relative motion (SR) or respective locations in a gravitational field (GR).

In his book 'Albert Einstein' Banesh Hoffmann (Paladin, 1975) makes no less than six references to Einstein's 'heretical' discovery of the variable speed of light and in 'Relativity, the special and general theory' Einstein wrote that the results of special theory are invalidated by gravity (76, Crown, 1916).

In the introduction to general theory Einstein wrote that the special theory law of the constancy of the speed of light required modification (Annalen der Physik 773, 49 1916).