I Do macro objects get entangled?

[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?? )

 

[entropy1]

If a state $|U\rangle(|A\rangle+|B\rangle)$ decoheres into a state $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$ at t₀, and the outcome of the measurement is $|A\rangle$, is the history (before t₀) of this outcome $|A\rangle$ possibly different than the history (before t₀) if the outcome had been $|B\rangle$?

I wonder, because with decoherence the WF of the whole universe is of significance, and if this is the case, then this could include the whole history as well as the whole future of the universe, I figured.

 

[PeterDonis]

If a state $|U\rangle(|A\rangle+|B\rangle)$ decoheres into a state $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$

That's not decoherence, that's a measurement interaction. Decoherence is just the statement that, once you have the state $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$, the two terms in the state will never interfere with each other again.

 

[entropy1]

That's not decoherence, that's a measurement interaction. Decoherence is just the statement that, once you have the state $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$, the two terms in the state will never interfere with each other again.

Ok, but if we make a measurement, we don't measure $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$, but rather either $|U_A\rangle|A\rangle$ or $|U_B\rangle|B\rangle$, right?

So, could the history of $|U_A\rangle$ be different than the history of $|U_B\rangle$?

 

[PeterDonis]

if we make a measurement, we don't measure $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$, but rather either $|U_A\rangle|A\rangle$ or $|U_B\rangle|B\rangle$, right?

Yes.

could the history of $|U_A\rangle$ be different than the history of $|U_B\rangle$?

I don't know what you mean. You already wrote down the "history"--the state started out as $|U\rangle \left(|A\rangle + |B\rangle \right)$. You can't go back and change that based on a measurement result. It is what it is.

Your question is like looking at a tree with a trunk and two branches, and asking if one branch has a different trunk than the other.

 

[entropy1]

Perhaps I would write $(|U_A\rangle+|U_B\rangle)(|A\rangle+|B\rangle)$. And then let this evolve into $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$. Would this be a proper approach?

A yes/no answer would be helpful too.

 

[PeterDonis]

Perhaps I would write $(|U_A\rangle+|U_B\rangle)(|A\rangle+|B\rangle)$. And then let this evolve into $|U_A\rangle|A\rangle+|U_B\rangle|B\rangle$. Would this be a proper approach?

No. That would not be a unitary evolution.

 

[entropy1]

Could you say that $"U_A"$ is a condition for the measurement outcome to be possible to be $A$?

 

[PeterDonis]

Could you say that "$U_A$" is a condition for the measurement outcome to be possible to be $A$?

I don't know what you mean by that. I think you are making this much harder than it needs to be.

You have a measuring device that starts out in state $|U\rangle$. If the measured system is in state $|A\rangle$, the measuring device transitions to state $|U_A\rangle$, which is the state described as "outcome $A$ has been measured". If the measured system is in state $|B\rangle$, the measuring device transitions to state $|U_B\rangle$, which is the state described as "outcome $B$ has been measured". So if the state of the system is a superposition of $|A\rangle$ and $|B\rangle$, the interaction between the measuring device and the system will transition the overall state to $|U_A\rangle |A\rangle + |U_B\rangle |B\rangle$.

The above is what QM says. I don't see the point of trying to ask all these other questions about "conditions" or "history" or anything else.

 

[entropy1]

Ok.

P.S.: I can't tell you why I ask these questions for that reason probably wouldn't be understood by most people on this forum.

BTW, thanks for answering It still is the case that the people here know what they are talking about and I am still searching.

I didn't realize you could and probably should see the measuring apparatus as part of $U$. Further, the kind of distributiveness of $|U\rangle$ puzzles me a little.

 

[PeterDonis]

I didn't realize you could and probably should see the measuring apparatus as part of $U$.

The measuring apparatus is not "part of" $U$, it is $U$, at least for the cases we've been discussing. What else would it be?

the kind of distributiveness of $|U\rangle$ puzzles me a little.

What do you mean by "distributiveness"?

 

[entropy1]

The measuring apparatus is not "part of" $U$, it is $U$, at least for the cases we've been discussing. What else would it be?

I thought: "The entire universe" (of which the measuring apparatus is a part I guess).

What do you mean by "distributiveness"?

Mathematical distributiveness, as in $|U\rangle(|A\rangle+|B\rangle)$ evolves into $|U_1\rangle|A\rangle+|U_2\rangle|B\rangle$.

 

[PeterDonis]

I thought: "The entire universe" (of which the measuring apparatus is a part I guess)

It can't be the entire universe, because you separated out the system which can be in either state $A$ or state $B$, or some superposition of them.

If you mean "the entire universe except for that system", then you're leaving out an awful lot of information, since the rest of the universe contains a lot more than just the measuring device (which of course it must contain). And since (a) you're not interested in all that other information, and (b) in general you don't know what it is anyway, it seems much better to keep your model limited and just consider $U$ to be the measuring device, since its states are the states you're interested in, and admit that the rest of the universe is simply left out of your model. Otherwise you're going to confuse yourself; for example, see below.

Mathematical distributiveness, as in $|U\rangle(|A\rangle+|B\rangle)$ evolves into $|U_1\rangle|A\rangle+|U_2\rangle|B\rangle$.

Ok, got it. This is just the definition of a measurement interaction, assuming that $U$ is the measuring device. What's puzzling about it?

I can see how it might be puzzling if you are thinking of $U$ as the entire rest of the universe, instead of just the measuring device. But anyone else who writes down a state transition like the one above is just thinking of $U$ as the measuring device, for the reasons I gave above. So if you find the transition puzzling, my advice would be to stop thinking of $U$ as the entire rest of the universe and realize that it's only the measuring device.

 

[entropy1]

my advice would be to stop thinking of $U$ as the entire rest of the universe and realize that it's only the measuring device.

Well, I guess it makes a lot of sense like that. But it seems like saying: "If the outcome is A, the measuring device will show outcome A", or: "If the measuring device shows outcome A, the outcome is A", which, if that are the implications, you couldn't be sure of, because you couldn't know if the outcome has been A if the device measured A, right? You could only be sure if one required the other. Just to define it that way seems like a tautology.

Sorry to bother you some further

 

[PeterDonis]

you couldn't know if the outcome has been A if the device measured A, right?

Um, what? I have no idea where you're getting this from. If the device measured A, that means the outcome is A. That's what "measurement" means.

 

[entropy1]

Um, what? I have no idea where you're getting this from. If the device measured A, that means the outcome is A. That's what "measurement" means.

Is that an assumption or does it follow from some? For instance, can you calculate that?