Is the QT "no-cloning theorem" suitable as "Chronological Protection"?

[Tommolo]

TL;DR Summary

Maybe any other scenario different from the "Many Worlds" interpretation of the Quantum Theory would violate the "no cloning theorem". In this case, would this be a suitable way to implement the Hawking's famous "Chronological Protection Conjecture"?

Just a question: how would the wavefunction "collapse" in a time-reversed universe? Let's take Alice. If she's taking a backward time travel to -say- 2021 and finds herself in 2021, wouldn't that be a (prohibited) quantum cloning of an already measured quantum state? Say, the |Alice 2021⟩ ket is measured two times, the first three years before entering the (preposterous) time machine and a second time after entering and reaching her past.

Would this imply that any time travel must create another parallel universe? Any other interpretation would just violate the no-cloning theorem, if I get it right... :)

What do you think? Have I understood it wrong, maybe? :)

 

[PeterDonis]

Maybe any other scenario different from the "Many Worlds" interpretation of the Quantum Theory would violate the "no cloning theorem".

Nothing in QM, regardless of interpretation, can violate the no cloning theorem. It's a theorem.

a backward time travel to -say- 2021

This isn't a "time reversed universe", it's a universe with closed timelike curves. Which are not believed to be physically possible. Nor has anyone formulated a useful model of QM in such a universe. So your question is not answerable since there is no known model to use to answer it.

 

[Tommolo]

Nothing in QM, regardless of interpretation, can violate the no cloning theorem. It's a theorem.

Either this, or the interpretation - in this case "Many Worlds"- requires the (not observed) violation of the "no cloning" theorem. Since we do not observe weird things as other versions of ourselves returning back in time to our universe, the "no cloning theorem" and the "many worlds interpretation" both falls outside logic and are apparently proven wrong. Simul stabunt vel simul cadent :)

This isn't a "time reversed universe", it's a universe with closed timelike curves. Which are not believed to be physically possible. Nor has anyone formulated a useful model of QM in such a universe. So your question is not answerable since there is no known model to use to answer it.

That is precisely what I wanted to prove impossible. It is a classical popperian falsification. Since these universes are not demonstrated to exist, until proven wrong, time travels are impossible and the "CPC", "Chronological Protection Conjecture" by Stephen Hawking holds. As a bynote, also the "Many worlds" interpretation is demostrated wrong IF time travels are shown impossible. Not bad, I'd say :)
Unless there is a loophole somehow I didn't see :p

 

[PeterDonis]

the interpretation - in this case "Many Worlds"- requires the (not observed) violation of the "no cloning" theorem.

No, that's not possible. The no cloning theorem is a theorem of QM. That means it's a proven fact about QM, and therefore about anything that claims to be an interpretation of QM. Something that violates the no cloning theorem must be a different theory, not an interpretation of QM.

Since we do not observe weird things as other versions of ourselves returning back in time to our universe, the "no cloning theorem" and the "many worlds interpretation" both falls outside logic and are apparently proven wrong.

I have no idea why you think this is valid reasoning. It's not.

Since these universes are not demonstrated to exist, until proven wrong, time travels are impossible

If you want to take that position, that's fine. Many physicists would agree with you about that. But they would not agree with you that this constitutes any refutation of the MWI, and they would certainly not agree with you that it constitutes a refutation of the no cloning theorem.

 

[PeterDonis]

the "Many worlds" interpretation is demostrated wrong IF time travels are shown impossible.

Again, I have no idea why you would think this is valid reasoning. It's not. There are countless examples in the literature of scenarios constructed using the MWI which obey all of the laws of QM (including the no cloning theorem, btw) and do not contain any closed timelike curves. Every single one of these scenarios falsifies your claim here. I think you really need to get a lot more familiar with the literature.

 

[PeterDonis]

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[PeterDonis]

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