![The speed of light, visualized [GIF - 48KB]](https://pic8.co/sh/Ujwhbd.gif){kind=link}
"Currently physically measurable Speed of Light under these specific conditions" would be more accurate. Currently, the actual speed of light is not known. If it is, the PTB are not sharing it with the rest of us. The definition we are currently using for the speed of light is both circular in reasoning (Distance is Defined using the Speed of Light, The Speed of Light is Defined using Distance) and defined as being measured in an impossible condition (Complete Vacuum).
Distance was not defined relative to the speed of light, until c was precisely known. Before that, it was defined relative to a bar in France.
C is not precisely known. It has been measured in some specific conditions with imperfect methods, and the currently accepted speed is a prediction of how it would perform in a total vacuum based upon these imperfect measurements.
It is incredibly precisely known, and has been measured by an enormous variety of techniques, and many different conditions.
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