Several days ago on HN others and I were discussing the relationship between the electric constant and magnetic constants (vacuum permittivity and magnetic permeability) and the speed of light (a debate† that's still ongoing). It seems to me this would have serious consequences for physics and the universe in general if the following relationship didin't hold under all conditions:
c = 1/(μ0 ε0)^0.5
If the speed of light were instantaneous in one direction, then the value of (μ0 ε0)^0.5 would have to be zero when light was travelling in that direction. Surely that has to be a serious problem?
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† https://news.ycombinator.com/item?id=24876598