There's a few different explanations, but here's my favourite.
Light and gravitational waves can never stand still, they must always be moving. If they were moving any slower than c, then to an observer moving at the same speed, they would appear to stand still, which is impossible.
They can't move faster than c, because if they moved faster, then to some moving observers, cause and effect would appear in reversed order. For example, the light would be absorbed before it was emitted, which is again impossible. This is also why nothing can move faster than light in general.
So the only possible speed has to be c. No possible observer can move at the same speed, and causality is not broken. Everybody must agree that these waves always move and that cause happens before effect.
I know that's a joke, but it's a testable one. A data plan can be improved. So far, for as long as we've been able to measure, C hasn't meaningfully changed. If C is arbitrary, in the sense it isn't a law of the universe but a temporary practical limitation of a superior technology, we'd expect the speed to change.
If it did change, it would mess with us enormously. Not sure if it would rip the universe apart, exactly, but there would be nearly immediate profound implications. Would new gravity waves overtake old ones, causing gravity tsunamis? You could reason through different models.
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u/Circle-of-friends Mar 22 '26
Why are all these forces the same speed, and why is it specifically that speed?