Mass and angular momentum, left ambiguous by Einstein, get defined
The most eye-opening thing I've learned about relativity in the past few years is that the notion that space has no preferential direction is an axiom in the theory.There's nothing about the way we measure the speed of light that would disambiguate if light traveled instantaneously in one direction and at half the measured speed of light in the other. We just don't have a way to know, because time measurements require information to re-converge at the original site of the experiment's beginning.It's a pretty good axiom, because we also have no reason to believe there's a preferred direction in space... But it's an axiom.