What is the speed of light (the value of c), and why are we so sure that it is a true and immutable value?
Apparently, the fact is, it is true because we say it is. Einstein pointed this out in 1905, and called it a convention, and we still have no perfect way to say it is true because it is true, and not because we say it is.
We have no demonstrable way to establish that it is a hypothesis, and that our repeated measurements and observations point to it being an established theory.
Derek Muller explains the problem in this entertaining and thought-provoking video, part of his series called Veritasium.
But can a way be found? It’s worth a conversation, and sparks of ideas for creating real doable experiments.