B Cat's point of view

I would like to elaborate on the problem in which "branch" of the Many Worlds I am living as a conscious subject.

If we assume that I consist of 100 elementary particles in a box, then there is a continuum of their configurations. Furthermore, these configurations are entangled with other subsystems in the box. Speaking about a "branch" is not very intuitive in such a setup.

David Bohm's model assigns a marker to each elementary particle in the box. These markers then sail on the wave function in a very complex way, not affecting anything in the wave function. They are just markers.

In Bohm's model, I, as a subject, am in a "branch" determined by the markers. It is a very clear model but hard to generalize to relativistic quantum mechanics.

Bohm's model is a Many Worlds model. The physical thing is the wave function and the markers just point out one individual configuration.

However, the fuzziness of the continuum does creep into the Bohm model, too. The branch which it picks interacts very strongly with nearby branches. We may ask again if I, as a subject, am some collection of nearby branches. If so, what branches we should include in the collection?
 
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I would like to elaborate on the problem in which "branch" of the Many Worlds I am living as a conscious subject
There is no such problem. According to the MWI, there is a "you" in all the branches, and none of them have any more claim than the others to be the "real" you. They are all "real" yous.

In other words, if the MWI is true, words like "you" have to be interpreted very carefully, since the actual reality the MWI describes (the wave function) is nothing like what we ordinarily believe we experience. So you can't just help yourself to the usual meaning of "you" if the MWI is true.

If we assume that I consist of 100 elementary particles in a box, then there is a continuum of their configurations. Furthermore, these configurations are entangled with other subsystems in the box. Speaking about a "branch" is not very intuitive in such a setup.
In such a setup there are no "branches" according to the MWI because no measurement is made and no decoherence occurs.

David Bohm's model
Is not the MWI. It's a different interpretation of QM. Which interpretation do you want to talk about? You can't mix them up.

assigns a marker to each elementary particle in the box
No, it assigns a real position to each particle. See below.

Bohm's model is a Many Worlds model
No, it is not. Bohm's model has only a single "world", i.e., measurements only have one outcome. The wave function is real in Bohm's model, but it is not the only thing that is real. The positions of the particles are also real, and since each particle has only one unique position, every measurement has only one unique outcome. The MWI is not like that because particle positions are not real in the MWI; only the wave function is. That makes a big difference.

The branch which it picks
There are no "branches" in Bohm's model. See above.
 
Bohm's model is an interpretation of the Schrödinger equation and the wave function in it. The full wave function exists in Bohm's model and guides as the "pilot wave" where the markers of the particles move. The markers are the hidden variables of the model. It is not a local hidden variable theory because the wave function explores every corner in the experiment.

If an observer lives inside Bohm's model, there is no wave function collapse at a measurement. All branches exist, but the markers pick the branch which is the "real" one. Thus, it is a Many Worlds model, but the markers elevate one branch over the others. Conscious subjects live in this special branch.

The "real" state of Schrödinger's cat at any moment is the branch picked by the markers. But the other branches exist, too, and affect the final outcome.

In newtonian mechanics, the state of the markers (= particles) determines the future development of the system uniquely. In Bohm's model, the full wave function is required to calculate the development. The state of the markers does not tell much.

https://arxiv.org/abs/0811.0810

David Deutsch calls it a Many Worlds theory "in denial".

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An example of how Bohm's theory works: the double slit experiment. The marker for the particle goes only through one slit. But the marker is guided by the wave function which goes through both slits.

We can say that in this case we have two main branches: the particle goes through the slit 1 or the slit 2. The marker picks one of these branches. It is what "really" happened.
 
The situation as presented reminds me of the Wigner's friend thought experiment.
Hugh Everett, author of the MWI, learnt his QM from Wigner's course at Princeton. Wigner's friend turns up in Everett's PhD thesis, before Wigner formally published the idea.
 

.Scott

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Your looking at the wrong cat.

But first, let's consider a photon in an interferometer.
The easiest interpretation is that that photon travels two paths. If you block one path, it either hits your block or fails to contribute to an interference pattern - potentially with counterfactual results. If you don't block either path, it forms an interference pattern - interacting with its alter-erg. These are not the earmarks of MWI or simply not having enough information about where the photon travels. These are the earmarks of completely non-local photon.

So let me introduce you to my cat. After cleaning up Schroedingers box, I will reuse it - after all, quantum computing research as it is, boxes capable of completely isolating their content from the outside world are in high demand. Once again, atomic decay as it may be, will kill and not kill the cat - and upon opening the box, I will find my cat in one of several states. The most interesting of these possibilities is a cat which has interfered with itself - so perhaps an alive and recently well-fed cat.

So what does this look like from the cats point of view. Well, of course, something like a cat isn't subject to these QM rules - at least not within a cat's lifetime. Ignoring that critical piece of information, my cat should have a memory of events consistent to the evidence I observe. So if I find a well-fed cat, it should report feasting on itself. If this sounds self-contradicting, it's because getting a cat to interfere with itself would require a very long period of isolation during which time the cat would not only die, but become very much uncatlike.
 
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Yes, because the ontology of the world according to the MWI is so different from our intuitive one that it becomes almost impossible to use ordinary words with their ordinary meanings.
And they say us Bohmians are "spooky".
 
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getting a cat to interfere with itself would require a very long period of isolation during which time the cat would not only die, but become very much uncatlike
I don't think it's possible to "isolate" a cat in this sense. Even if you put the cat inside a magic box that prevented all interactions with the external world, the cat has a huge number of internal degrees of freedom that cannot be precisely tracked. So it basically acts as its own "environment". From the decoherence viewpoint, the cat is constantly decohering itself, so it's not possible for a "live" branch and a "dead" branch of the cat to interfere.
 

.Scott

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I don't think it's possible to "isolate" a cat in this sense. Even if you put the cat inside a magic box that prevented all interactions with the external world, the cat has a huge number of internal degrees of freedom that cannot be precisely tracked. So it basically acts as its own "environment". From the decoherence viewpoint, the cat is constantly decohering itself, so it's not possible for a "live" branch and a "dead" branch of the cat to interfere.
There's an obvious issue with being unable to scale something up at all.
Are we sure that it is not possible?

What's the most massive thing that can interfere with itself - and how long would it take for that to happen if it was isolated? Is there a formula for this. Is it degrees of freedom that matter? Can degrees of freedom become part of a single quantum state?
 
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What's the most massive thing that can interfere with itself
They have done the double slit experiment with buckyballs, but that's on the order of a few hundred degrees of freedom. Bose-Einstein condensates have been produced with larger numbers of atoms in them, and those exhibit quantum coherence properties; but that's still many, many orders of magnitude smaller than a cat.

What's more, those objects are highly homogeneous. A buckyball is 60 carbon atoms, all identical, held together by simple chemical bonds; a Bose-Einstein condensate has to be composed of bosons all of the same species, all condensed into a single simple quantum state. A cat is a huge heterogeneous blob of all kinds of different atoms and molecules, grouped together in a complex hierarchical structure. I'm not sure how you would even be able to define a "coherent" quantum state for something like that.
 

.Scott

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I'm not sure how you would even be able to define a "coherent" quantum state for something like that.
Okay then, if it is difficult to imagine scaling up, let me scale down.
But I will warn all that this may answer the OPs question at the cost of putting us all in the box.

Let's say that the device that administers the poison to my cat has a time limit of 1 minute. So after a minute, my lucky cat now believes it has survived my diabolical experiment. Certainly, it has at least as much evidence of this as we have for the Big Bang. And, of course, this cat will never see a dead replica of itself - so it will have to survive on whatever rations are provided in the box.

Similarly, we live in a universe that, that by all accounts, was kicked off by some kind of Big Bang. And I do not question the research or analysis that went into this conclusion. I do not doubt that there really was a Big Bang. But for how long will this be factual? As some point, there will be no more Phys 101 courses, no cosmic background radiation (from the early universe), black holes will have grown and fizzled, and there will be no evidence left behind of a Big Bang - at least not any practical evidence.

Then entropy will continue its forward march and slowly, slowly, any means of time keeping will be lost - even in principle. Time and history will be meaningless, and there will be no evidence of the Big Bang - even in principle.

At that point, should someone "open our box", they would be unable to conclude whether there was or was not a Big Bang.
 
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At that point, should someone "open our box", they would be unable to conclude whether there was or was not a Big Bang.
None of this has anything to do with quantum experiments, decoherence, or anything being discussed in this thread. It's just a straight classical inference from the fact that, in the far future, to the best of our knowledge, our universe will be indistinguishable in practical terms from a de Sitter universe, which has no "Big Bang" in it.
 
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for how long will this be factual?
Saying that in the far future it will no longer be possible to find evidence of a Big Bang is not the same as saying the Big Bang is not factual. Our universe will still have begun in a Big Bang even if the evidence at some point becomes undetectable.
 

.Scott

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None of this has anything to do with quantum experiments, decoherence, or anything being discussed in this thread. It's just a straight classical inference from the fact that, in the far future, to the best of our knowledge, our universe will be indistinguishable in practical terms from a de Sitter universe, which has no "Big Bang" in it.
To address the OP: Is it possible for something that can record and retain its history (such as a cat) ever be in a superposition of states? Obviously not. So long as the cat retains a memory of its history, it will be either dead or alive.

Like the particle in a double slit experiment, so long as there is which-way information - either in the particle, the apparatus, or the environment - no evidence of super-positioning will exists. There is a really nifty experiment by Scully, Englert, Walther (which seems to lie behind paywalls) where the which-way information for an atomic double slit experiment is captured and then deliberately obscured. So long as the information is kept available, there is no super-positioning. Once the information is deliberately obscured, the atom can interfere with itself.

So what is the problem with things more complicated than a Buckeyball? Is it specifically their homogeneous nature, or is it just that retaining their which-way information is so difficult to avoid?

Once all information is lost about the specifics of a historical event, will those specifics persist? To steer clear of that becoming a philosophical statement, let me restate it in experimental terms:
Can we create a repeatable experiment where all "which way" information is lost, but where the outcome of those experiments retain classical statistics to the exclusion of super-positioning?

Since the "Big Bang" is hardly a repeatable experiment - not to mention an impractical one for data collection, it was a bad example.
 
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Is it possible for something that can record and retain its history (such as a cat) ever be in a superposition of states?
What does "record and retain its history" mean?

Also, "superposition" is the wrong word to use here. See below.

Like the particle in a double slit experiment, so long as there is which-way information - either in the particle, the apparatus, or the environment - no evidence of super-positioning will exists.
What happens in the double slit experiment without which-way information is interference, not superposition. But it's interference between alternatives that are not macroscopically distinguishable. It's not properly called superposition because superposition is basis dependent, but interference is not--it's an observable result of the experiment.

In the case of the cat, if the "alive" and "dead" alternatives interfered with each other, that would be interference between alternatives that were macroscopically distinguishable. It would be like getting an interference pattern in the double slit experiment with which-way information, which of course does not occur.

There is a really nifty experiment by Scully, Englert, Walther (which seems to lie behind paywalls) where the which-way information for an atomic double slit experiment is captured and then deliberately obscured.
This is just the double slit version of a quantum eraser experiment, of which there are many. But in all of these experiments, the erasing occurs before any macroscopically distinguishable result is observed. So all of these are just examples of manipulating the internal structure of the experiment to affect whether or not interference can occur.

what is the problem with things more complicated than a Buckeyball?
The more degrees of freedom there are, the more degrees of freedom need to be kept coherent for interference to occur. For example, in the double slit, the presence of interference when there is no which-way information depends on the waves through each slit being coherent--i.e., having a definite phase relationship. That's why the sources in these experiments have to be carefully designed and controlled, and that gets more and more difficult as the number of degrees of freedom in the source goes up, since coherence needs to be maintained among all of the degrees of freedom. It also gets more and more difficult to eliminate all interactions with the external environment that can destroy coherence.
 
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Can we create a repeatable experiment where all "which way" information is lost, but where the outcome of those experiments retain classical statistics to the exclusion of super-positioning?
It depends on what you mean by "lost". If there is enough loss of coherence, there is which-way information even if it is practically impossible to measure it.

For example, if you throw a baseball towards a pair of holes in a wall, and it ends up hitting a detector on the other side, there is no way to avoid having which-way information about which hole the baseball went through, even if you carefully avoid observing it in any way. Heuristically, the internal interactions among the atoms in the baseball, and the interactions between the baseball and its environment, will unavoidably create which-way information about which hole the baseball went through. Which means that if you run this experiment many, many times, the pattern of baseball impacts on the detector will not show interference, even if you carefully avoid ever observing which hole any baseball went through. Which is another way of saying that throwing a baseball in the usual way does not give the baseball enough quantum coherence to create interference at the holes. And nobody knows how to make a baseball source that does; it might well be impossible in any practical sense, even if (certain interpretations of) QM would indicate that it should be possible in principle.
 

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What does "record and retain its history" mean?
I am simply referring to the fact that the cat itself is evidence of being dead or alive. It's like the atom that is carrying the microwave photon - it can't interfere because it is holding the which-way information itself.

It's of particular importance to the OP. Since the fact that the cat knows that it is alive is enough to exclude death as a possibility. Paraphrasing your words, it's macroscopically self-distinguishable. So "from the Cat's point of view", nothing odd can happen - if for no other reason than it has a point of view.

On the other hand, if the "cat" was just a negatively charged lithium atom, it might have to rely on an external record of its travel to avoid self-interference.
 
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But the whole point of Schrodinger's cat was that physicists described a single atom that was initially in an excited state after being unobserved for a short time was now in a combination of excited and ground state.If an atom can be in both states, why not a cat?
 

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It depends on what you mean by "lost". If there is enough loss of coherence, there is which-way information even if it is practically impossible to measure it.

For example, if you throw a baseball towards a pair of holes in a wall, and it ends up hitting a detector on the other side, there is no way to avoid having which-way information about which hole the baseball went through, even if you carefully avoid observing it in any way. Heuristically, the internal interactions among the atoms in the baseball, and the interactions between the baseball and its environment, will unavoidably create which-way information about which hole the baseball went through. ...
Right. So that is why that would not be a qualifying experiment. Throwing a baseball leaves a huge impression on the environment that could not be easily erased.

But my point was that perhaps "distinguishable", macroscopically or otherwise, is the only thing that keeps the ball to a specific path. And should that information become truly lost, the ball would no longer have a specific path.

So, the way to contradict this hypothesis, would be to devise an experiment where the "distinguishing" information could be truly lost and where the result would be something statistically measurable. Interference is just one possibility - although the only one I know of. Any form of potential self-interaction would do. If such an experiment could be devised, and the results indicated that their was no interaction, then "reality" would prevail. Otherwise, upon losing all evidence of the specifics of an event, we cannot say with certainty that those specifics still exist. In fact, results from the double slit experiment could be viewed as suggesting they do not.

At present, I do not know of such an experiment - but there may be one. It could be that this kind of "reality" has already been demonstrated ... and my hypothesis is already macroscopically distinguishable as dead.
 

.Scott

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But the whole point of Schrodinger's cat was that physicists described a single atom that was initially in an excited state after being unobserved for a short time was now in a combination of excited and ground state. If an atom can be in both states, why not a cat?
If the atom is rigged to kill the cat, as soon as it returns to its ground state, it is being "observed".

Clearly, individual atoms within the cat could be in combination states. But the well-being of the cat as a whole leaves a big paw print on the cat and its environment. I have been arguing that eventually, it will be impossible to determine whether the cat had died from the poison or not - not just for practical reasons, but because there is no latent evidence at all. But (assuming I was right) that would take an unimaginable amount of time. And even if I am right, the next question would be whether there was a fundamental similarity between what allows the atom to be in two states and the unknown fate of the cat.
 
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But the whole point of Schrodinger's cat was that physicists described a single atom that was initially in an excited state after being unobserved for a short time was now in a combination of excited and ground state.If an atom can be in both states, why not a cat?
Before you post in a thread, it's a good idea to actually read the other posts in the thread. Your question has already been answered.
 
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I am simply referring to the fact that the cat itself is evidence of being dead or alive.
In the sense that it has enough degrees of freedom to effectively always carry its own "which way" information, yes.

if the "cat" was just a negatively charged lithium atom, it might have to rely on an external record of its travel to avoid self-interference
Meaning, a lithium atom does not have enough degrees of freedom to effectively always carry its own "which way" information.

perhaps "distinguishable", macroscopically or otherwise, is the only thing that keeps the ball to a specific path. And should that information become truly lost, the ball would no longer have a specific path.
The information is "lost" in any practical sense. Unless special arrangements are made, it is not stored anywhere from which it can be recovered. The air molecules, photons, etc. that have interacted with the ball don't store the information about where the ball went in any way that is recoverable. But that doesn't matter. The fact that the "which way" information is there is enough, even if it is not practically recoverable. In "quantum eraser" experiments, the "which way" information is not just made practically unrecoverable by letting it be stored in environmental degrees of freedom from which it can't be practically recovered; it is literally erased by a precisely controlled manipulation of the quantum state.

an experiment where the "distinguishing" information could be truly lost
See above. It's not enough to "lose" the information in the sense of it not being practically recoverrable. It has to be explicitly erased by a precise manipulation of the quantum state. And that can only be done if the information is not "lost" in the sense of not being practically recoverable; the exact degrees of freedom in which the information is stored have to be known and controlled so they can be manipulated appropriately.
 

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See above. It's not enough to "lose" the information in the sense of it not being practically recoverable. It has to be explicitly erased by a precise manipulation of the quantum state. And that can only be done if the information is not "lost" in the sense of not being practically recoverable; the exact degrees of freedom in which the information is stored have to be known and controlled so they can be manipulated appropriately.
Yes, I agree. That's what I have meant by "truly lost" and "in principle".
 

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