Nowadays, in SI, they're defined as constants: c is exactly 299,792,458 m/s, µ_0 = 4π×10^−7 H/m, ε_0 = 1/(µ_0*c^2). But these were chosen, based precise measur

Nowadays, in SI, they're defined as constants: c is exactly 299,792,458 m/s, µ_0 = 4π×10^−7 H/m, ε_0 = 1/(µ_0*c^2). But these were chosen, based precise measurements, to ensure the (old) metre and the Ampere remained relatively unchanged. As for your question, homogeneity is considered the simplest assumption you could make: you can't get simpler than a no-parameter model. It's also easy to disprove, if you can get data, making it a good scientific hypothesis. There's some astronomers trying to measure the various fundamental constants in other regions of the galaxy via maser emissions. There was some interesting results a few years ago, but I don't think they're considered definitive. Still, lots of fame if they can do it!