I Do macro objects get entangled?

[entropy1]

$|Me\rangle (|Dead\rangle + |Live\rangle)$

and

$|Me_{dead}\rangle |Dead\rangle + |Me_{live}\rangle |Live\rangle$

So is this possibly an epistemological matter? The environment doesn't know whether the cat is dead or alive, and so the formalism keeps both possibilities open, until the data comes in and then "both possibilities open" becomes "one of the two true", but the formalism doesn't tell us which one?

If the $|Me\rangle (|Dead\rangle + |Live\rangle)$ part is a matter of lack of knowledge, then why wouldn't the $|Me_{dead}\rangle |Dead\rangle + |Me_{live}\rangle |Live\rangle$ part also be a matter of lack of knowledge, since they are both pure states that represent a superposition?

Or conversely: why wouldn't the $|Dead\rangle + |Live\rangle$ part be an uncertain state, wherein it is possible that the cat is in fact neither dead nor alive?

This paradox could in my eyes possibly be resolved if retrocausality is brought into the picture, where that what is measured (the cat) and the outcome of the measurement are the result of each other, like the measurement outcomes of entangled particles are the result of each other, at least as far as what we know allows!

Compare it to how the measurement itself creates the measured value, for instance in an SG app. Only now it creates it retrocausaly.

 

[stevendaryl]

So is this possibly an epistemological matter? The environment doesn't know whether the cat is dead or alive, and so the formalism keeps both possibilities open, until the data comes in and then "both possibilities open" becomes "one of the two true", but the formalism doesn't tell us which one?

If the $|Me\rangle (|Dead\rangle + |Live\rangle)$ part is a matter of lack of knowledge,

No, it isn't. If an electron is in a superposition of spin-up in the z-direction and spin-down in the z-direction, it isn't just that I don't know whether it's spin-up or spin-down. A superposition is neither spin-up nor spin-down.

...then why wouldn't the $|Me_{dead}\rangle |Dead\rangle + |Me_{live}\rangle |Live\rangle$ part also be a matter of lack of knowledge, since they are both pure states that represent a superposition?

That's the whole point of distinguishing $|Me_{dead}\rangle$ from $|Me_{alive}\rangle$. They are different versions of me. So the cat's state is determined by my state---if you were smart enough, you'd be able in theory to figure out whether the cat was alive or dead by examining me.

Or conversely: why wouldn't the $|Dead\rangle + |Live\rangle$ part be an uncertain state, wherein it is possible that the cat is in fact neither dead nor alive?

That is the case. If the cat is in a superposition, then it is neither alive nor dead. But the point of this whole thread is that a cat CAN'T be in a superposition of alive and dead for more than a tiny fraction of second before the rest of the universe is "infected" by the cat's state.

 

[entropy1]

No, it isn't. If an electron is in a superposition of spin-up in the z-direction and spin-down in the z-direction, it isn't just that I don't know whether it's spin-up or spin-down. A superposition is neither spin-up nor spin-down.

That is the case. If the cat is in a superposition, then it is neither alive nor dead. But the point of this whole thread is that a cat CAN'T be in a superposition of alive and dead for more than a tiny fraction of second before the rest of the universe is "infected" by the cat's state.

What I thought is that the cat is either dead nor alive, or it is one of the two, but we don't know which (as long as it is in the box). Isn't that mutually exclusive?

So, is the solution that the cat decoheres, so that it no longer is in a superposition?

But doesn't that mean it was in a superposition?

That's the whole point of distinguishing $|Me_{dead}\rangle$ from $|Me_{alive}\rangle$. They are different versions of me. So the cat's state is determined by my state---if you were smart enough, you'd be able in theory to figure out whether the cat was alive or dead by examining me.

That part is crisp clear to me.

 

[stevendaryl]

What I thought is that the cat is either dead nor alive, or it is one of the two, but we don't know which (as long as it is in the box). Isn't that mutually exclusive?

I'm sorry that every other thing you say my response is that I'm not sure what you're asking. But I'm not sure what you're asking. What are the two things that are mutually exclusive?

So, is the solution that the cat decoheres, so that it no longer is in a superposition?

But doesn't that mean it was in a superposition?

Well, no. I would say that there is never a point where the cat is in a macroscopic superposition.

A cat is made of many, many particles. For the entire cat to be in a superposition, there would have to be basically no interactions between those particles during the time it takes for a cat to die from whatever it is that Schrodinger is using to kill it.

 

[entropy1]

A cat is made of many, many particles. For the entire cat to be in a superposition, there would have to be basically no interactions between those particles during the time it takes for a cat to die from whatever it is that Schrodinger is using to kill it.

So then, $|Dead\rangle + |Live\rangle$ (from your $|Me\rangle (|Dead\rangle + |Live\rangle)$), is representing a lack of knowledge of the people outside the box?

I'm sorry that every other thing you say my response is that I'm not sure what you're asking. But I'm not sure what you're asking. What are the two things that are mutually exclusive?

Doesn't matter And also, my English is a bit poor. These two things appear mutually exclusive to me:

• "the cat is neither dead nor alive"
  
• "or it is one of the two, but we don't know which"

(as long as it is in the box)

I should have said "either: neither dead nor alive, or: one of the two but we don't know which."

 

[stevendaryl]

So then, $|Dead\rangle + |Live\rangle$ (from your $|Me\rangle (|Dead\rangle + |Live\rangle)$), is representing a lack of knowledge of the people outside the box?

No. It seems that you keep getting implications from what I say which I certainly don't intend. I thought I had said exactly the opposite of that: If you write something as a ket, that definitely does NOT represent a person's lack of knowledge. As I'm pretty sure I already said, being in a superposition of states $A$ and $B$ is definitely not a case of "The system is in state $A$, or it's in state $B$, I just don't know which".

By bringing up the fact that a cat is composed of many, many particles, I'm just saying that it's a huge oversimplification to say that there is a $|Dead\rangle$ state for the cat.

Doesn't matter And also, my English is a bit poor. These two things appear mutually exclusive to me:

• "the cat is either dead nor alive"
  
• "or it is one of the two, but we don't know which"

(as long as it is in the box)

Do you mean "neither" rather than "either" in the first bullet?

I'm assuming that what you mean was

1. The cat is not alive AND the cat is not dead.
2. The cat is either alive or it is dead, but we don't know which.

Those two distinctions make sense. If a cat is in a superposition of a "live" state and a "dead" state, then 1. would be true. Otherwise, 2. would be true.

The claim at the basis of this thread is that 1. is never the case (or at least never for more than a tiny fraction of a second). The cat is almost never in a superposition of alive or dead. 2. is closer to the truth, regardless of whether you open the box, or not.

 

[entropy1]

If you write something as a ket, that definitely does NOT represent a person's lack of knowledge.

Ok, that is clear. A ket is not a mixed state. So my suggestion was wrong. But it is not clear to me what signifies the superposition then, especially in this context.

2. is closer to the truth,

regardless of whether you open the box, or not.

Ok, got it. (I hope )

 

[stevendaryl]

Ok, that is clear. A ket is not a mixed state. So my suggestion was wrong. But it is not clear to me what signifies the superposition then, especially in this context.

I thought I had answered that question many times.

1. If the situation is described by $|observer\rangle (|Dead\rangle + |Alive\rangle)$, then you would say that, from the point of view of the observer, the cat is in a superposition of states.
2. If the situation is described by $(|observer_d\rangle |Dead\rangle) + (|observer_a\rangle |Alive\rangle)$, then you would not say that the cat is in a superposition (from a many-worlds perspective, you could say that the entire world is in a superposition, but not the cat alone)

Situation #1 is never true. The cat is never in a superposition of alive and dead. (Or at least never for more than a fraction of a second.) So situation #2 is a better description of the situation.

 

[Nugatory]

Ok, that is clear. A ket is not a mixed state. So my suggestion was wrong. But it is not clear to me what signifies the superposition then, especially in this context.

If $|A\rangle=|B\rangle+|C\rangle$ then we say that the state A is a superposition of B and C.

 

[entropy1]

I understand that the situation goes quickly to $(|observer_d\rangle |Dead\rangle) + (|observer_a\rangle |Alive\rangle)$ ¹, in which the cat is not in superposition for the observer. Also, nonwithstanding that the cat is isolated in the box, inside the box it is not in superposition but rather already decohered. However, when the observer has not looked in the box, his assessment of the situation is $|observer\rangle (|Dead\rangle + |Alive\rangle)$ ², right?

So I wonder, if the cat is almost never in superposition, why the observer nevertheless is in state ²? Also, when does state ¹ occur? Already in the box, or only when the observer takes a look? It seems to me that the formulations ¹ and ² depend on whether the observer actually observes the cat.

So, the cat is not in superposition in the box because of (quick) decoherence, but it is in superposition at that time, because of state ²? That seems contradictory!

 

[stevendaryl]

I understand that the situation goes quickly to $(|observer_d\rangle |Dead\rangle) + (|observer_a\rangle |Alive\rangle)$ ¹, in which the cat is not in superposition for the observer. Also, nonwithstanding that the cat is isolated in the box, inside the box it is not in superposition but rather already decohered. However, when the observer has not looked in the box, his assessment of the situation is $|observer\rangle (|Dead\rangle + |Alive\rangle)$ ², right?

No, after decoherence, it's no longer appropriate to use kets to describe the cat. He can either go the route of Many-Worlds, and describe the state of the entire universe as a ket, or he can describe the state of the cat alone as a mixed-state. In a mixed state, the cat has a certain probability of being alive, and a certain probability of being dead. But that is not the same as the cat being in a superposition. Kets are only appropriate for pure states.

So I wonder, if the cat is almost never in superposition, why the observer nevertheless is in state ²?

That's the point---that description is never appropriate for more than a tiny fraction of a second.

Also, when does state ¹ occur? Already in the box, or only when the observer takes a look?

Almost immediately. Cats have influences on the rest of the universe even if nobody looks at them.

It seems to me that the formulations ¹ and ² depend on whether the observer actually observes the cat.

 

[entropy1]

Ok, so I'm wondering, are there two stages of decoherence the case here? First, the cat in the box on its own, and then again, when the box is opened and the observer takes a look? Which one are we talking about in this situation? I guess it matters if the box is open or closed?

 

[stevendaryl]

Ok, so I'm wondering, are there two stages of decoherence the case here? First, the cat in the box on its own, and then again, when the box is opened and the observer takes a look? Which one are we talking about in this situation? I guess it matters if the box is open or closed?

I don't think anything very mysterious happens when you open the box, other than the observer learns what state the cat is in. That's not quantum mechanical, it's just light from the cat reaching the observer's eyes and causing changes to the state of his brain.

 

[entropy1]

I don't think anything very mysterious happens when you open the box, other than the observer learns what state the cat is in. That's not quantum mechanical, it's just light from the cat reaching the observer's eyes and causing changes to the state of his brain.

Ok. So what is it about the box open or closed in Schrödingers Cat?

Then it would mean that the cat in MWI terms is, in the closed box, already in two different branches $|Dead\rangle + |Live\rangle$, right?

 

[stevendaryl]

Ok. So what is it about the box open or closed in Schrödingers Cat?

The original discussion of Schrodinger's cat was from the point of view of the idea that observation "collapses the wave function", and so it was important that the cat was actually observed to be alive or dead. But that ignored decoherence. Decoherence can be thought of (according to one view) as the environment constantly observing macroscopic things, so that they are always in a "collapsed" state. The more sophisticated view is that decoherence is constantly coupling macroscopic objects' states to the rest of the universe, so that no macroscopic object exists for long in a superposition (except possibly the entire universe). But in either interpretation, there is no longer anything special about opening the box and looking inside it, except that afterwards, you know what happened to the cat.

 

[Derek P]

Ok. So what is it about the box open or closed in Schrödingers Cat?

Schroedinger mentioned it, saying that the situation is resolved when the experimenter opens the box and looks in. He did not say that opening the box causes anything to happen, he was referring to the fact that the observer finds out whether the cat is alive or dead. Don't forget, Schroedinger invented the scenario to highlight a problem in the then-current understanding of quantum mechanics. He was quite specific - Heisenberg's (?) "fuzzy reality" idea would mean the cat would be in a fuzzy state. [Pause for fuzzy cat jokes.] But since then we have moved on and Schroedinger's Cat is now a familiar scenario to test various interpretations. There is no significance to opening the box other than the fact that the observer can then look inside.

 

[StevieTNZ]

Remember to keep in mind that decoherence is "FAPP". The probabilities of a mixed state do not represent something that is there in a classical sense (i.e. a cat dead). They are still probabilities of what the outcome will be. See 'Quantum Enigma' by Bruce Rosenblum and Fred Kuttner.

 

[Derek P]

Ok, so I'm wondering, are there two stages of decoherence the case here? First, the cat in the box on its own, and then again, when the box is opened and the observer takes a look? Which one are we talking about in this situation? I guess it matters if the box is open or closed?

Decoherence is inevitable with most systems, especially ones that have charged particles. Even the original particle, which was thought of as being in a superposition of emitted and not emitted, would be decohered because of the recoil etc that it left on the emitting atom. But even if we constructed apparatus that created a clean superposition, the moment the particle interacts with the detector, decoherence begins. And is complete before the electrical signal even leaves the detector!

But yes, if you want to describe exactly what happens, step by step, you can consider a whole cascade of interactions, each one subject to decoherence. Though, as several "A" level threads have done recently, you can actually lump them all together. But saying that the detector, its circuity, the killing mechanism, the cat, the box, the observer, the laboratory and everything else out to Mars (Schroedinger said "after an hour") is in a "dead" state is confusing. and it also obscures the fact that some of the global system (generally referred to as the environment) must be decohering the rest (the detector or observer)..

 

[entropy1]

Remember to keep in mind that decoherence is "FAPP". The probabilities of a mixed state do not represent something that is there in a classical sense (i.e. a cat dead). They are still probabilities of what the outcome will be. See 'Quantum Enigma' by Bruce Rosenblum and Fred Kuttner.

Does that mean the cat being dead or alive could depend on future events (measurements)?

 

[Derek P]

Remember to keep in mind that decoherence is "FAPP". The probabilities of a mixed state do not represent something that is there in a classical sense (i.e. a cat dead). They are still probabilities of what the outcome will be. See 'Quantum Enigma' by Bruce Rosenblum and Fred Kuttner.

Not sure who you're answering, but yes, decoherence is FAPP. But even it it were total, the mixture arising from decoherence would still be improper. There need be no randomness choosing which "possible" state to actualize. The randomness then comes from the fact that the observer doesn't know which state she is in. But I'd say that such a set of states is just as much "there in a classical sense" as a single state that has been selected at random out of it.

 

[Derek P]

Does that mean the cat being dead or alive could depend on future events (measurements)?

FAPP means FAPP. For ALL practical purposes. So when decoherence turns a superposition into a probability distribution FAPP, that means that whatever outcome you have at that point you will get it for ever more. FAPP.

 

[entropy1]

FAPP means FAPP. For ALL practical purposes. So when decoherence turns a superposition into a probability distribution FAPP, that means that whatever outcome you have at that point you will get it for ever more. FAPP.

I am not sure if I understand that, but if a situation is not determined factually at some point (which I understand from @StevieTNZ), it can become determined eventually. When we can find ourselves in a specific branch, the situation will match with the measurement. But that leaves the possibility open that the measurement has a part in determining which branch.

For instance:

• Cat = $|Dead \rangle + |Alive \rangle$
• Wigner's friend = $|friend \rangle(|Dead \rangle + |Alive \rangle)$ -> $|friend_{dead} \rangle|Dead \rangle + |friend_{alive} \rangle|Alive \rangle$
• Wigner = $Wigner(|friend_{dead} \rangle|Dead \rangle + |friend_{alive} \rangle|Alive \rangle)$ -> $|Wigner_{dead} \rangle|friend_{dead} \rangle|Dead \rangle + |Wigner_{alive} \rangle|friend_{alive} \rangle|Alive \rangle$

After his friend, Wigner still has a choice which branch he will take. This would be retrocausal, since Wigner comes after the poisoned jar.

But I think I make a mistake because there never was (except for a very short time) anything in superposition, right? Except for the single particle that was, right?

It seems to me the flow of things is as follows:

• Detector = $|0 \rangle + |1 \rangle$
• Cat = $|Cat \rangle(|0 \rangle + |1 \rangle)$ -> $|1 \rangle|Dead \rangle + |0 \rangle|Alive \rangle$ (a)

So now the cat decoheres. So then (a) is a superposition, but since the cat 'observed' itself, it agrees with itself that it is either dead or alive? So then we are left with:

Cat = $|0 \rangle|Alive \rangle$ (of course )?

 

[Derek P]

• Wigner = $Wigner(|friend_{dead} \rangle|Dead \rangle + |friend_{alive} \rangle|Alive \rangle)$ -> $|Wigner_{dead} \rangle|friend_{dead} \rangle|Dead \rangle + |Wigner_{alive} \rangle|friend_{alive} \rangle|Alive \rangle$

After his friend, Wigner still has a choice which branch he will take. This would be retrocausal, since Wigner comes after the poisoned jar.

You haven't included decoherence. Without decoherence both states persist in superposition so Wigner does not make a choice at all. Wigner simply interacts with |friend_dead⟩|Dead⟩ making |Wigner_dead>|friend_dead⟩|Dead⟩. So Wigner's state in that term is |Wigner_dead> and similarly for the alive state and term, just as you've written. No fancy retro-causal choices.

With decoherence and including the state of the environment you just add the |environment_dead> state to the "dead" term and |environment_alive> state to the "alive" term. So still no choice.

But with decoherence but then omitting or ignoring the state of the environment (perhaps on the grounds that we can't measure it) we have either |friend_dead⟩|Dead⟩ or |friend_alive⟩|Alive⟩ So Wigner interacts with which ever one we have. Again no retrocausality. Whatever the cat was, the friend saw. And whatever the friend saw, Wigner sees.

Note that the environment can be replaced by some of the many degrees of freedom that the cat has which don't have much to do with whether it is alive or dead. So yes, in that sense, the cat can decohere itself. But I wouldn't call it "observing itself" or "agreeing with itself" as that tends to obscure the fact that you're treating the cat as two systems.

 

[StevieTNZ]

Not sure who you're answering

I'm clarifying what decoherence is. It does not produce a definite outcome from, for example, two potentialities (e.g. cat being in a superposition of |alive> + |dead>).

 

[Derek P]

I'm clarifying what decoherence is. It does not produce a definite outcome from, for example, two potentialities (e.g. cat being in a superposition of |alive> + |dead>).

So am I :)

 

[entropy1]

@Derek P So my conjecture would be that Wigner somehow could choose which timeline to be in, if the by Wigner to be observed object ('friend and his lab') is still in isolation! Like he could have a "bias" to the outcome. Same holds for friend and the box of the cat.

The problem with that is, that Wigner's friend could have a bias towards $|Cat_{dead} \rangle$ while Wigner could have a bias towards $|Friend_{alivecat} \rangle$, so they wouldn't be in the same branch. So I think that is the problem with my interpretation.

I have another question: are there only two branches in this story? Is it only the particle that hits the detector in the cat's box that creates branches?

 

[Derek P]

@Derek P So my conjecture would be that Wigner somehow could choose which timeline to be in, if the by Wigner to be observed object ('friend and his lab') is still in isolation! Like he could have a "bias" to the outcome. Same holds for friend and the box of the cat.

The problem with that is, that Wigner's friend could have a bias towards $|Cat_{dead} \rangle$ while Wigner could have a bias towards $|Friend_{alivecat} \rangle$, so they wouldn't be in the same branch. So I think that is the problem with my interpretation.

I have another question: are there only two branches in this story? Is it only the particle that hits the detector in the cat's box that creates branches?

Arguably the emitting atom sets the process off. Where branching actually happens is a matter of semantics, you could define it as the original superposition or you can say decoherence separates them. Either way, there are just two "coarse-grained" branches. But because decoherence involves countless interactions there is a huge number of superposed micro-states to each branch.

I don't understand what you mean by having a bias, the observer can't choose to see a dead cat if it's actually alive in his branch..

 

[Derek P]

Cats have influences on the rest of the universe even if nobody looks at them.

Especially if nobody looks at them.

 

[entropy1]

I don't understand what you mean by having a bias, the observer can't choose to see a dead cat if it's actually alive in his branch..

If there is a superposition of branches for all Wigner knows (when the object is isolated in the box), then my conjecture says it is still not decided which branch he will be in. Of course this requires retrocausality, because the fact in the box is that decoherence already happened. Anyway, we would still have the (huge) problem that Wigner and his friend could end up in different branches, like:

Wigner's friend = $|friend_{deadcat} \rangle|Cat_{dead} \rangle$ and
Wigner = $|Wigner_{alivecat} \rangle|friend_{alivecat} \rangle|Cat_{alive} \rangle$

simultaneously. The friend (and te cat) would still be in superpostion after Wigner looked, because the friend is in the "dead"-branch and Wigner is in the "live"-branch where his friend is also in the "live"-branch. However, if the cat ends up dead, everyone ends up in the "dead"-branch. I think this is the fault in my conjecture. I am just wondering where the choice gets made, I think it is during decoherence or that it is the measurement problem.

 

[Derek P]

If there is a superposition of branches for all Wigner knows (when the object is isolated in the box), then my conjecture says it is still not decided which branch he will be in. Of course this requires retrocausality, because the fact in the box is that decoherence already happened. Anyway, we still have the (huge) problem that Wigner and his friend could end up in different branches.

You don't mean decoherence. Decoherence doesn't remove the superposition. You mean collapse of the wavefunction. If the cat has collapsed and Wigner doesn't know how, his later discovery doesn't change the past. You simply can't mix an epistemological interpretation for Wigner with a real collapse interpretation for the cat. Knowledge of the past may change, events in the past do not.

 

[entropy1]

Knowledge of the past may change, events in the past do not.

That is not what I mean. Events in the past, as well as events in the future, have an origin somewhere. I am suggesting that the events in the past could have been influenced by events in the future. That doesn't mean they change, it just means that their cause is not only in their past.

I thought that choosing a branch was choosing the past. But Wigner could make a differend choice than his friend, which would be inconsistent. If both branches coexist, a choice would be possible, but Wigner's friend would be a zombie in his world if she chose a different past and timeline than him.

 

[Derek P]

I am just wondering where the choice gets made, I think it is during decoherence or that it is the measurement problem.

The BRANCH structure does not involve a CHOICE. If there is a choice it means most of the branches are pruned away. Losing branches is an entirely different matter from Wigner choosing which branch to be in.

 

[entropy1]

The BRANCH structure does not involve a CHOICE. If there is a choice it means most of the branches are pruned away. Losing branches is an entirely different matter from Wigner choosing which branch to be in.

The confusing part to me, then, is that both branches have to be real, but at the same time only one is real.

 

[Derek P]

I thought that choosing a branch was choosing the past. But Wigner could make a differend choice than his friend, which would be inconsistent. If both branches coexist, a choice would be possible, but Wigner's friend would be a zombie in his world if she chose a different past and timeline than him.

Then I have no idea what you are talking about. So I'm going to have to go throuigh your replies point by point until I hit a snag and then ask you to refine your question...

I thought that choosing a branch was choosing the past.

Why did you think that? Nature makes a choice in some interpretations and in those interpretations the branch structuree is pruned back to a single stem. That is the choice and it is only the future that is affected - in the sense that some future possibilities are eliminated.

 

[Derek P]

The confusing part to me, then, is that both branches have to be real, but at the same time only one is real.

I didn't say that. In Many Worlds, all branches are real, in Copenhagen only one is. In either case the branch structure starts the same but in Copenhagen most branches fall off.

 

[entropy1]

Why did you think that?

Because if you could choose a branch, you would choose the past of it along with it.

 

[entropy1]

In Many Worlds, all branches are real

That's what I am talking about.

 

[Derek P]

Because if you could choose a branch, you would choose the past of it along with it.

Well that's the nature of branching. Many futures (whether real or merely possible) but just one past. You wouldn't choose the branch's past, you would be the branch's past. No choice of pasts.

 

[entropy1]

You wouldn't choose the branch's past, you would be the branch's past.

That is where we differ, or should I say differed, for I already found the inconsistence in my view. So you are probably right!

 

[Derek P]

That is where we differ, or should I say differed, because I already found the inconsistence in my view. So you are probably right!

Yes
I think you have been assuming that Wigner can still choose to be in either branch. That's not so. The dead-cat version of Wigner can only stay in the dead-cat world. Under collapse interpretations it's the only world. Under Many Worlds there is a living-cat world as well. But the version of Wigner in the dead-cat world cannot hop across to the living-cat world.

 

[entropy1]

I think you have been assuming that Wigner can still choose to be in either branch. That's not so. The dead-cat version of Wigner can only stay in the dead-cat world. Under collapse interpretations it's the only world. Under Many Worlds there is a living-cat world as well. But the version of Wigner in the dead-cat world cannot hop across to the living-cat world.

I thought that if the observer hadn't looked in the (isolated) box yet, she (outside the box) could still end up in either branch, because both branches (inside the box) are real. However, the histories of a dead cat and a live cat can diverge very quicky, so the observer would have to be in superposition of both possibities too.

Now the thing is I think that the observer might exist in both branches, but that she is only aware of a single history. Alternatively she ends up in only one of the two, and only one of the two is real.

I suspect either branch is possible, but QT doesn't tell us which one will be realized.

• Both branches are real inside the box and there are two observers, two histories, one in each branch, or
• Both branches are real inside the box and the observer collapses this to a single one, or
• Only one of the branches is real inside the box, but QT doesn't tell us which one. Which could also mean that the box is in one of the branches.

I think I get it a littlebit.

 

[Stephen Tashi]

The dead-cat version of Wigner can only stay in the dead-cat world.

Do we assert that this must be a physical property of how "consciousness" or "perception" is implemented? - i.e. do we say that a thing capable of perception can only be "in" one branch at given time?

We could sidestep that issue by saying that all physical process are implemented "in such a manner", but what would "in such a manner" mean? What does it mean to say a particular physical process is "in" a particular branch?

 

[Derek P]

I thought that if the observer hadn't looked in the (isolated) box yet, she (outside the box) could still end up in either branch, because both branches (inside the box) are real.

She does end up in both. That's what branching means.

However, the histories of a dead cat and a live cat can diverge very quicky, so the observer would have to be in superposition of both possibities too.

Part of the same superposition, yes.

Now the thing is I think that the observer might exist in both branches, but that she is only aware of a single history.

In each branch she is only aware of one history.

Alternatively she ends up in only one of the two, and only one of the two is real.

Yes in collapse interpretations.
[/quote]
I suspect either branch is possible, but QT doesn't tell us which one will be realized. [/quote]
Ah.now you're going back to "only one branch".

• Both branches are real inside the box and there are two observers, two histories, one in each branch, or
• Both branches are real inside the box and the observer collapses this to a single one, or
• Only one of the branches is real inside the box, but QT doesn't tell us which one. Which could also mean that the box is in one of the branches.

I think I get it a littlebit.

Respectively no collapse, observation-mediated collapse and pre-observation collapse.

 

[Derek P]

Do we assert that this must be a physical property of how "consciousness" r "perception" is implemented? - i.e. do we say that a thing capable of perception can only be "in" one branch at given time?

Actual conscious experience is subject to Chalmer's Hard Problem and so I guess most people assume it supervenes on a brain state. Which means we can be objective about what the observer experiences. IMO that means the observer experiences different things in different branches. So no, I would not say that a thing capable of perception can only be "in" one branch at given time. I think Many Minds postulates this but that would seem to leave a load of zombie branches.

We could sidestep that issue by saying that all physical process are implemented "in such a manner", but what would "in such a manner" mean? What does it mean to say a particular physical process is "in" a particular branch?

I have no idea. It's your phrase! What did you mean by it?

 

[PeterDonis]

No!

Yes!

Please refrain from posts which are just noise. I have deleted these two.

 

[Stephen Tashi]

I have no idea. It's your phrase! What did you mean by it?

I mean to imply that in the MWI it isn't clear that there are any such things as different branches in the physical sense. They exist in the mathematical sense if we take the role of an omnicient observer and select a particular sequence of outcomes from a sequence of possible outcomes. However, the assertion that such a selection can correspond to the experience of a particular human being or "actual" physical process must be added as an assumption.

If such an assumption is made, it should be made explicitly. Many discussions of the MWI seem to assert:

As human beings, we experience macroscopic events with definite outcomes. Therefore our experience must be implemented by a particular branch in MWI.

I'd agree with "Assume our experience is implemented...". I don't see that "Therefore our experience must..." is correct logic.

 

[Derek P]

I mean to imply that in the MWI it isn't clear that there are any such things as different branches in the physical sense. They exist in the mathematical sense if we take the role of an omnicient observer and select a particular sequence of outcomes from a sequence of possible outcomes. However, the assertion that such a selection can correspond to the experience of a particular human being or "actual" physical process must be added as an assumption.

A human automatically experiences its own state and nothing else so I don't see the problem.

If such an assumption is made, it should be made explicitly. Many discussions of the MWI seem to assert:

As human beings, we experience macroscopic events with definite outcomes. Therefore our experience must be implemented by a particular branch in MWI.

I'd agree with "Assume our experience is implemented...". I don't see that "Therefore our experience must..." is correct logic.

Well if that's what people say then they have missed the point. MWI derives the branches without mentioning experience.

 

[Derek P]

I mean to imply that in the MWI it isn't clear that there are any such things as different branches in the physical sense.

I don't undertand. The whole point of MWI is that it demonstrates exactly that. If MWI were just a matter of saying we can decompose a wavefunction any way we fancy then I would agree - it would be as meaningless as saying that a pot of ink contains all the stories that could ever be written. But MWI doesn't leave it to the imagination of the theorist to see branches. It describes a process of interaction - now understood to involve decoherence - that creates the branches. It's completely physical.

 

[entropy1]

But MWI doesn't leave it to the imagination of the theorist to see branches. It describes a process of interaction - now understood to involve decoherence - that creates the branches. It's completely physical.

Isn't "collapse" physical as well? I think QT just doesn't tell us.

 

[Stephen Tashi]

A human automatically experiences its own state and nothing else so I don't see the problem.

I don't see how to define most of those words in the context of MWI. How can a thing (human or otherwise) have "its own" state? If we are speaking of "state" in the physical sense, what kind of thing is it that "owns" or "belongs to" a physical state? In MWI, there is some physical definition of state for the universe. How do we define "a human" in terms of that state? What part of that state is the human's "own state"?

If we consider "experience" only in terms of a conscious perception or sensation, that phenomena is usually regarded as function of physical state rather than being a physical state - taking a physical state to be a collection of information that completely specifies all physical aspects of a situation. (e.g. Physical events can occur in our bodies without our "experiencing" them.) Taking that viewpoint, is this function 1-to-1 or many-to-1?

Well if that's what people say then they have missed the point. MWI derives the branches without mentioning experience.

A rigorous definition of "branch" may be mathematically possible without mentioning experience. However, much of the discussion about MWI involves reconciling the MWI approach with experience, in the common language sense of "experience" - both subjective and objective. Attempts to do this appeal to branching.