We've seen by experiment that the speed of light c appears to be constant for each observer (leading to all well-known consequences of relativity).
I'm wondering if this appearance of constancy of c might be due to the observer's way of measuring it: All observers are bound to compare c to something else which itself is also based on c. A clock based on a photon bouncing between two mirrors (and taking the time it takes to bounce) for instance uses that speed of the photon to measure everything. A clock like a watch based on springs uses tension forces buried in the spring material (electromagnetic forces are based on c). Quartz crystal oszillators, sand clocks (hourglasses), water clocks — all facilitate some mechanism like friction or piezoelectricity which fundamentally are electromagnetism.
Nevertheless it is said that the time appears to be going slower, not just all clocks we can build.
My questions now are:
Is there a reasoning (which I just didn't find in my research) why the time as a whole is supposed to be influenced by relativity, not just all events based on the forces based on c? Maybe there even is a word or a term to google for in order to find more about this?
I understand that physicists managed to unite three of the four basic forces, wrapping up electromagnetism with the strong and the weak force. I guess then that these additional two forces also are based on c. Is there any such connection of c to the remaining force, the gravitation?
I could understand that if all existing forces are hinged on c then there is no real difference between saying "all clocks we can build are going slower" and "the time itself is going slower".