In relativity, the speed of light acts as a fundamental boundary that preserves causality—the idea that a cause must always come before its effect.
Relativity shows that space and time are linked, forming a four-dimensional structure called spacetime. Within this structure, the speed of light defines what is known as the light cone around any event. This cone separates spacetime into regions:
- The past (what could have influenced the event)
- The future (what the event could influence)
- The elsewhere (regions too far for any signal to reach at light speed or slower)
Because no signal or object can travel faster than light, nothing outside the light cone can affect or be affected by the event. This ensures that cause and effect relationships stay consistent for all observers, no matter how fast they’re moving.
If something could travel faster than light, it could, in some frames of reference, arrive before it was sent. That would mean an effect could happen before its cause, breaking the logical order of events. To prevent such contradictions, relativity sets the speed of light as the ultimate speed limit, ensuring that causality is always preserved.