The derivation is clean. The conclusion is a little stronger than the maths supports though, and the gap is the interesting part. hashnode.com/posts/why-the-internet-can-never-be-… Two different things are bundled into your 56 ms. The speed of light is a law. The refractive index of 1.5 is an engineering choice, and it is actively being removed: hollow-core fiber guides light through air rather than glass, so its index sits near 1.0, worth roughly a third off transit time. It is already deployed on latency-sensitive financial routes. So that half of the number is not physics, it is the current generation of glass.