Galileo rolled spheres off the edge of a table and plotted their trajectories to study how gravity accelerates bodies toward the earth in a parabolic arc. He used the presumption of constant horizontal motion in free-fall as the 'clock'. Importantly, there was no explicit measure of time in this particular experiment. Galileo used one kind of motion to measure another kind of motion.
Newton imagined that time existed as a universal and independent process by which all change can be measured objectively, with little justification. Einstein, with some help from others, showed that simultaneity and duration were not absolute, but relative to an observer's frame of reference. This also means that dependent measures of energy, mass, and gravity are also relative.
Julian Barbour, in his book "The End of Time," suggested that instead of time being the measure of change, perhaps change is the measure of time. In other words, a good clock might be defined as something that changes in regularized step with all communicating matter in the universe, and in particular, in terms of the overall rate of change of state of all of that matter.
Any two or more processes that undergo change in the environment may be plotted against each other in a coordinate system. Some of these will show rates of change, some will show curvature, and so on, but only with respect to each other. Time is not available in any direct way. Today, the most ideal clocks are those that oscillate, and that count those oscillations compared with a wide variety of other oscillations...but there is no absolute measure of accuracy.
The only phenomenon that does not appear to change, within any frame of reference, is the speed of light in a vacuum.
Perhaps light is the final arbiter of all other rates of change.
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