Do macro objects get entangled?

[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.

 

[Lord Jestocost]

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

From an operational point of view, the formalism of quantum mechanics amounts to nothing but a calculational recipe, set up in the last resort to predict the probabilities of various directly observed macroscopic outcomes. Born's rule is the link and the only link that connects our perceptions - what we think to be “observations of a physical reality” - with the mathematics of quantum mechanics. If you don't have this link, the mathematical formalism of quantum mechanics has nothing to do with “reality”, viz. the symbols occurring in the quantum mechanical formalism, such as the probability amplitudes, correspond to nothing in the ”real world”. The wave function is merely an intellectual tool and there is nothing which has to “collapse” physically.

 

[entropy1]

Not only does unitary evolution "come with" the MWI, but that is a big piece of its appeal - MWI uses just unitary evolution and ...

I think that decoherence involves unitary time evolution, right? So does decoherence also come with MWI?

 

[entropy1]

How can MWI accommodate a split of the worldthread in the case of entangled particles, if the probabilities of the threads are unknown? For example: a thread split could be: 0.25|A> + 0.75|B>. What if the coefficients of the threads cannot be known as in the case of entanglement?

 

[PeterDonis]

How can MWI accommodate a split of the worldthread in the case of entangled particles, if the probabilities of the threads are unknown?

Nature always knows the amplitudes. It's only us humans who often don't know them.

 

[entropy1]

I mistaked probabilities and amplitudes: should be amplitudes indeed. (really?? )