One of the groups that I have lunch with likes to discuss physics and astronomy. During one of these lunches, there was a bit of confusion about the Theory of Relativity (ToR), with people not being quite sure whether it means that measurements depend upon one's frame of reference. I was fortunately able to clarify this absolutely (pun intended). The axioms of the ToR are that
* if you do an experiment and I do the same experiment, and
* if the regions of space and time enclosing the experiment are small enough as to appear un-curved (or if they have the same curvature within our measurement precision)
you and I will get the same answers. These axioms are *really* a statement of the absolute and permanent nature of the laws of physics! For "small" experiments, we get the same answers, period, now and forever, with "small" meaning the above.
So why is it called Theory of Relativity and not the Theory of Absolutes? Because if I measure YOUR experiment from a distance, I might get a different answer than you get, depending on our mutual (relative) velocity and curvature differences. I will ALWAYS get the same answer on my copy of the experiment as you get on your copy of the experiment, but I might get a different answer looking at your experiment than you get on your experiment. The two ToR's tell us exactly how to account for the different answers. The Special ToR accounts for our mutual velocities, and the General ToR accounts for differences in spacetime curvature.