![The speed of light, visualized [GIF - 48KB]](https://pic8.co/sh/Ujwhbd.gif){kind=link}
It is incredibly precisely known, and has been measured by an enormous variety of techniques, and many different conditions.
No, it is not precisely known. It has been measured in some very limited conditions with imperfect methods. How fast is it when completely removed from the influence of Gravity? How fast is it in a true vacuum? How fast is it where there is a lot of Dark Matter? How fast is it if there is no Dark Matter present? What if Dark Matter theory is completely wrong? The fact is that we have a very limited knowledge of this. You are simply repeating what you have been told without giving the matter any actual thought.
You're just stunning yourself with your own ignorance. Every single one of those questions is approachable by studying cosmology. Depending on what conditions you're talking about, there are various levels of achievable accuracy.
It's not inconceivable that we'll eventually detect something interesting under extreme conditions. We're just talking about incredibly tiny modifications, at that point.
No. We have a useful definition of the speed of light, but not an exact one by any stretch of the imagination. Useful != perfect. And, the definition of the speed of light is definitely circular.
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