The more tractable question is: Why this particular maximum speed? None of our theories gives any reason for choosing one value over another. A causal universe with a faster speed of light would be less local than ours; a causal universe with a slower speed of light would be hyperlocal. But each of those universes would still work, and any setting of the speed of light is allowed in physics. It just so happens that we have measured it in our universe and found it to be 300 million meters per second: very fast compared to human experience but very slow compared to the distances one has to
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