3 ms·
> It seems to me that the statement that Wu's experiment holds globally is equivalent to the statement that the universe has an orientation so I now tend to thi
by skinner_ 8y ago
> It seems to me that the statement that Wu's experiment holds globally is equivalent to the statement that the universe has an orientation so I now tend to think the universe does not admit one of these orientation-reversing loops.
Nice! I tried to play around with this. There's the https://en.wikipedia.org/wiki/Gold_universe https://en.wikipedia.org/wiki/Gold_universe where two inhabitants can honestly disagree about the direction of time. (And the interactions between the two are hilarious.) Can we have a version where one can go far away and come back time-reversed? Or is that excluded by CT=P symmetry breaking? It sounds like it, but it's confusing.
- ocfnash 8y agoThanks. With regard to: > Can we have a version where one can go far away and come back time-reversed? Or is that excluded by CT=P symmetry breaking? I don't know. Having mulled these thoughts further after writing the above I'm now more cautious about guessing. Specifically it seems to me that electromagetic charge, weak isospin, isospin etc. are all only globally well-defined provided we have an orientation of spacetime. So it seems to me that if spacetime is not orientable then there's no guarantee that if we send an instance of Wu's experiment off in a spacecraft on a hypothetical orientation-reversing loop then the apparatus will arrive back at earth unchanged: perhaps its electrons are all now positrons etc. My analogy with the coloured gloves does not permit such a phenomenon because colour is manifestly well-defined. I could make it more elaborate to account for this (e.g., replacing colour with a clockwise/anti-clockwise concept) but I no longer find it useful. In summary: I revert to my original "no opinion" point of view.