Irreversibility vs reversibility

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No. I believe (but cannot demonstrate) that the necessary "additional structure", that allows you to pick out the alive/dead basis as the one that is physically relevant, is already present in the universal wave function; it doesn't have to be added in "by hand".
Einselection (https://en.wikipedia.org/wiki/Einselection) is supposed to be the "additional structure" already present in the universal wave function that doesn't need to be added by hand (which Demystifier hasn't seemed to address) and the most powerful counter-Einselectioner is Kastner. She argued Einselection is not even enough and can't take off. Do you have any critical reasoning or argument that can counter Kastner?

http://philsci-archive.pitt.edu/10757/1/Einselection_and_HThm_Final.pdf

When I'd share about Einselection in a hall telling how it can select the Preferred Basis that doesn't have to by added by hand. And there is someone who would mention Kastner arguments. I want to be ready what to say.
 
  • #52
PeterDonis
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What does he mean it is the "monitoring" of the apparatus by the environment which results in the apparent reduction of the wave packet??
He means the constant interaction of the apparatus with the environment which entangles the states of the two.

Do you have any critical reasoning or argument that can counter Kastner?
No. This is an unsolved problem. Kastner does not prove that something like Zurek's scheme is impossible; she only proves that he has not proven it is possible. In other words, it's still an open question.
 
  • #53
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I said that in practice it can only happen if there is no decoherence.



In principle any unitary evolution can be reversed. That is inherent in the definition of unitary evolution.
Going back to this reversibility thing. When glass falls to floor.. it can't go up by itself and reform because of entropy. In quantum, stevendaryl mentioned about this Entropy angle yet Tashi countered in message #19 there was no entropy in quantum system... and the two haven't followed up their discussions.

I seemed unable to disentangle Born Rule from this reversibility thing.

But first thing first. You said in principle unitary evolution can be reversed. Did you mean in practice it can't happen due to Entropy? And broken glasses can't reform being the same reason unitary evolution can't be reversed in pactice? But in principle broken glasses can reform if you can reverse entropy in the same way unitary evolution can be reversed by reversing entropy?

Or is the reason unitary evolution can't be reversed in practice because you can't tract the many degrees of freedom in the environment and disentangled the wave function? Here we assume the particle is not destroyed. I assume this reasoning doesn't work if the electron is destroyed like in the double slit detection event where it is destroyed (same situation of Entropy unable to reverse burned paper back to normal)

Or in practice unitary evolution can't be reserved due to the former case of not able to reverse entropy or in the latter case simply not able to tract the environment and find where the two entangled pairs is located (and simply disentangling each one manually without taking into account entropy)?

Now connecting this to Born rule. It is random where the electron will hit the detector or in situation where the particle is not destroyed. So even if you can track the particles in the environment and reverse all the entanglement.. how do you know which branch is entangled to which branch or how do you reverse the Born rule (reversing random thing you still get random)?
 
  • #54
PeterDonis
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You said in principle unitary evolution can be reversed. Did you mean in practice it can't happen due to Entropy?
In practice it can't happen for large systems because we can't keep exact track of all of the degrees of freedom. That is generally taken to be related to entropy, yes: there is information about the exact microstate of the system that we do not have.

However, we have to be careful about what "reversing" actually means in this context. Consider the broken glass. What would it mean to "reverse" that process? We would have to be able to take every single little piece of glass and give it exactly the reverse of the motion it had while falling to the floor, so the pieces would come together in exactly the right way: and then we would have to take each of the individual atoms in each of the broken surfaces and re-attach them in exactly the right way. In other words, we would have to be able to exert a kind of detailed control over each individual atom that we simply do not know how to exert, at least not with our current technology. So even if we did know the exact microstate of the broken glass, it wouldn't help because we don't know how to control all the pieces and all the atoms the right way to "unbreak" it.

Trying to reverse the unitary evolution of any large system will be the same sort of thing: even knowing the exact final microstate of system plus apparatus plus environment (for example, cat plus everything else) won't help because we won't be able to control all the individual quantum degrees of freedom in the right way. So it's not just a matter of not knowing the exact state. The concept of "entropy" doesn't really include all that, at least not as it's usually defined.
 
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  • #55
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In practice it can't happen for large systems because we can't keep exact track of all of the degrees of freedom. That is generally taken to be related to entropy, yes: there is information about the exact microstate of the system that we do not have.

However, we have to be careful about what "reversing" actually means in this context. Consider the broken glass. What would it mean to "reverse" that process? We would have to be able to take every single little piece of glass and give it exactly the reverse of the motion it had while falling to the floor, so the pieces would come together in exactly the right way: and then we would have to take each of the individual atoms in each of the broken surfaces and re-attach them in exactly the right way. In other words, we would have to be able to exert a kind of detailed control over each individual atom that we simply do not know how to exert, at least not with our current technology. So even if we did know the exact microstate of the broken glass, it wouldn't help because we don't know how to control all the pieces and all the atoms the right way to "unbreak" it.

Trying to reverse the unitary evolution of any large system will be the same sort of thing: even knowing the exact final microstate of system plus apparatus plus environment (for example, cat plus everything else) won't help because we won't be able to control all the individual quantum degrees of freedom in the right way. So it's not just a matter of not knowing the exact state. The concept of "entropy" doesn't really include all that, at least not as it's usually defined.
Ok. Thanks.. the above is very clear. To make my questions complete. Whenever you hear they say decoherence can be reversed in principle in MWI or unitary evolution. Does it mean all branches need to be reversed at same time. For example. Supposed there is another branch where Clinton won the election and in this branch it was Trump. To reverse the unitary evolution.. can we just reverse Trump branch without affecting Clinton branch or do all branches have to be reversed at same time when they are talking about reversing is possible in principle?
 
  • #56
PeterDonis
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Whenever you hear they say decoherence can be reversed in principle in MWI or unitary evolution. Does it mean all branches need to be reversed at same time.
Obviously, since they were all produced by unitary evolution, reversing the unitary evolution will reverse all of them.
 
  • #57
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Obviously, since they were all produced by unitary evolution, reversing the unitary evolution will reverse all of them.
Ok. Let me ask this critical question here instead of making another thread as don't want to belabor it (and want to rest next week from sleepless nights thinking about this all). Zurek mentions that:

"Repeatability leads to branch-like states, Eq. (13),
suggesting Everettian `relative states' [19]. There is no
need to attribute reality to all the branches. Quantum
states are part information. As we have seen, objective
reality is an emergent property. Unobserved branches
can be regarded as events potentially consistent with the
initially available information that did not happen. Information
we gather can be used to advantage - it can lead
to actions conditioned on measurement outcomes"

My inquiry is, if other branches are just information and not really there.. are they all still called Unitary?? And can you still reverse all branches (that includes the real one and even those Unobserved branches that "can be regarded as events potentially consistent with the initially available information that did not happen"?
 
  • #58
PeterDonis
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if other branches are just information and not really there
Then we are not talking about unitary evolution. Unitary evolution requires that all of the branches are really there. Any interpretation where they are not all really there requires unitary evolution to be violated at some point in the process.
 

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