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zeratul98 t1_j6k3k36 wrote

Basically it goes like this:

Our current understanding of physics says there's a maximum speed that anything can travel. This is the speed of causality, which we call c.

One result of the equations we've discovered that gave us this information is that the more mass something has, the harder it is to accelerate, but also that that acceleration gets even harder as it gets going faster and faster. A corollary of this is that if something has no mass, it's super easy to accelerate. In fact, so easy that something without mass literally cannot be still, and it can only travel at one speed: the speed of causality, c.

So we know a) there's a maximum speed limit in the universe, b) anything without mass must always travel at that speed, and c) light has no mass. From that we conclude that light must also travel at that speed, which is why we often call it the speed of light.

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is_this_the_place t1_j6k553l wrote

How fast does gravity travel?

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mesonofgib t1_j6k5q1u wrote

At c as well. For example, if the sun suddenly disappeared we wouldn't know about it in any way for 8 minutes or so. Then the lights would go out and the Earth's path through space would change.

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