3 ms·

I think left and right could not just be swapped seamlessly. Radioactive decay (beta) would behave differently. See Wu's experiment and parity violation.

ramshorns 8y ago
Meanwhile, a mirror organism, made of antimatter, who travels back in time like Benjamin Button, would be exactly the same as a regular organism. At least I think that's how CPT-symmetry works.
ocfnash 8y ago
Great point, I agree! 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. At first I was surprised that we could apparently draw a global conclusion based on local observations but I've mostly convinced myself now based on the following analogy (which I hope is not misleading). Imagine we live in a two-dimensional universe and we want to know whether or not it is orientable. E.g., do we live on the surface of a torus or on the surface of a Klein bottle? We have defined a local orientation in the topologically-trivial patch where we live and so are used to speaking of "left-handed" and "right-handed" gloves. The question of whether these words are globally meaningful is exactly the question of whether our universe is orientable. One day we learn about "colour" and are stunned to discover that all left-handed gloves are red and all right-handed gloves are blue. It seems very unlikely to us that red somehow transitions into blue as we traverse a loop and so we conclude we must live in an orientable universe.
- 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 ago
    Thanks. 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.

ramshorns 8y ago

Meanwhile, a mirror organism, made of antimatter, who travels back in time like Benjamin Button, would be exactly the same as a regular organism. At least I think that's how CPT-symmetry works.

ocfnash 8y ago

Great point, I agree! 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. At first I was surprised that we could apparently draw a global conclusion based on local observations but I've mostly convinced myself now based on the following analogy (which I hope is not misleading). Imagine we live in a two-dimensional universe and we want to know whether or not it is orientable. E.g., do we live on the surface of a torus or on the surface of a Klein bottle? We have defined a local orientation in the topologically-trivial patch where we live and so are used to speaking of "left-handed" and "right-handed" gloves. The question of whether these words are globally meaningful is exactly the question of whether our universe is orientable. One day we learn about "colour" and are stunned to discover that all left-handed gloves are red and all right-handed gloves are blue. It seems very unlikely to us that red somehow transitions into blue as we traverse a loop and so we conclude we must live in an orientable universe.

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 ago
  Thanks. 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.