What crucial concept in reality is strictly enforced because nothing can travel faster than the speed of light in a vacuum?

Answer

The cause-and-effect structure (causality)

Because $c$ is the maximum speed of information transfer, it strictly preserves the cause-and-effect structure of reality, preventing an event at one location from instantaneously affecting a distant location.

What crucial concept in reality is strictly enforced because nothing can travel faster than the speed of light in a vacuum?

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