What aspects of reality must be flexible to accommodate the constant speed of light ($c$) for all inertial observers?

Answer

Space and time measurements (length and time intervals)

The profound consequence of accepting $c$ as invariant is that space and time must adjust (through phenomena like time dilation and length contraction) so that every observer calculates the speed of light to be exactly $c$.

What aspects of reality must be flexible to accommodate the constant speed of light ($c$) for all inertial observers?

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