yak

@lain You could still measure a difference with only approximately synchronized clocks. If they are 1 ns out of sync, then you can measure the difference to within 1 ns/distance. There doesn't seem to be any compelling reason to believe that light would behave this way, either.

@lain Why wouldn't it be possible to time it both directions? Are you saying you have to measure the "same" photon going first one way and then the other?

@lain you mean forwards and backwards in time?
Time-reversibility was also a very well established theory long before Einstein (it's a basic consequence of Newtonian, Lagrangian, and Hamiltonian mechanics, which are all equivalent on the intersection of their well-posed problems).

@lain
>you can get all of special relativity without fixing light speed
This is true, but the finite speed of light was already well established by the time special relativity was come up with. In many ways, special relativity was just a better explanation of already-confirmed theories such as electrodynamics.

As for QM, I'm less well versed in that, but if I remember correctly it's very difficult to explain experimental results using deterministic models. And it's a common misconception that experts in the field are unanimous in how they interpret quantum mechanics; the Copenhagen interpretation is the most popular, but not a majority. https://www.cs.umd.edu/~gasarch/pys-quantum.pdf