We're inside the system (I don't mean this in the 'we're in a simulation' sense) - changes fundamental to how the system works are not necessarily measurable to

We're inside the system (I don't mean this in the 'we're in a simulation' sense) - changes fundamental to how the system works are not necessarily measurable to us. It's one thing if the speed of light is localized and the changes are visible at different scales/distances. It's another if it's universal and changes - you have to understand that the speed of light is fundamental to how things works. It influences things like electromagnetic forces, which define atomic structure. The speed of light changing would mean that even a physical object like a meter long ruler would also change. Which is why, to my layperson understanding, this is such an exciting field of study - the different fundamental forces in play with these nuclear clocks might enable us to catch relative changes to the fine structure constant, which includes the speed of light as a component.