Parallel Universes: Multiverse vs Many Worlds - Splits?

[ChrisPNZ]

Hi, sorry - forgive me for posting (I am not much of a science guy but I am still interested). I am just watching a documentary (I know not a great source for scientific information - but it got me thinking). In the documentary it talks about the many worlds theory (which I believe is different from the multiverse theory? It would seem that other copies of Earth makes more sense in multiverse theory as opposed to many worlds?) In the documentary it says that (with many worlds theory) in every decision we make we split the universe. I.e. If I choose a to order a latte over a mocco, then the universe will split into two parallel universes. I know this theory isn't fully accepted but the problem I see with it is that if everything is identical in the universes before hand (down to the smallest particles in our bodies) - then wouldn't the exact same decision be made every single time? - i.e. we have no free will - only fate due to the way certain atoms behave in our bodies? Meaning there could be no split and therefore no creation of a parallel universe? - I don't know much about science tho - just interested in others thoughts :). Or is it something to do with a particle being able to be in two places at the same time?

 

[Nugatory]

but the problem I see with it is that if everything is identical in the universes before hand (down to the smallest particles in our bodies) - then wouldn't the exact same decision be made every single time?

Not according to quantum mechanics. The randomness is inherent in the theory: if I send an electron with its spin oriented vertically into a measuring device (Stern-Gerlach apparatus, if you want the details) oriented to measure the spin along a horizontal axis, I will get spin-left or spin-right randomly and with 50/50 probability. It's tempting to think that the problem here is that we just don't know enough about the exact state of the electron, that if we just knew more about it we'd have enough information to calculate whether it would be spin-left or spin-right - but that's wrong. There are subtle statistical differences between "random when we measure the spin" and "gotta be spin-left or spin-right, but we don't know the state well enough to calculate which it will be", these differences can be tested for in experiments, and the results of these experiments confirm that the quantum-mechanical picture is right; for more you can google for "Bell's Theorem" and pay particular attention to the website maintained by our own @DrChinese.

This inherent randomness may be the most basic difference between classical and quantum mechanics; your line of thought works just fine in classical mechanics.

 

[tatkhj]

Curious about what's current in multiverse and parallel universe speculation. How's the math on this doing?

 

[bhobba]

Curious about what's current in multiverse and parallel universe speculation. How's the math on this doing?

We have some rather good textbooks in the theorem proof style ie mathematically reasonably rigorous on many worlds these days:
https://www.amazon.com/dp/0198707541/?tag=pfamazon01-20

There is a strong connection between Many Worlds and De-coherent Histories:


De-coherent Histories, without going into the details, was the formulation of QM Feynman was converted to a bit before he died.

The following paper by Gell-Mann and Hartle might also help
https://arxiv.org/abs/1106.0767

But since this is a B level thread the following commentary in ordinary language is likely more helpful:
https://www.sciencenews.org/blog/context/gell-mann-hartle-spin-quantum-narrative-about-reality

If somebody were to ask me personally I would side with Gell-Mann - despite the mathematical Beauty of Many Worlds there is really no reason to introduce it - its nice features can be accommodated in a theory such as as de-coherent histories without the baggage of the idea of many worlds. However it must be emphasized as Gell-Mann does in the video what Everett thought of as a world is not necessarily what popularization's think of it as.

Just for completeness sake we come up against the real fundamental issue in QM. Everything is quantum. But then how do you rigorously define an observation? Some mentors/science advisers here think, like Einstein, it makes QM incomplete (Einstein towards the end, and after many discussions and disagreements with his good friend Bohr, the most famous one being the Einstein Box thought experiment, I will give a link to at the end, believed QM was correct - but to his dying day believed it incomplete - but that is another story best left to one of the many biographies such as Subtle Is The Lord). Others say it's an unsolved problem but think it can be eventually solved. Still others like me think that some key theorems are missing and once they are worked out what's going on will be a lot clearer. I think its more dotting your i's and crossing you t's sort of thing. But until its done, those that believe QM incomplete have a perfectly valid view. And who knows - solving it may lead to a revolution in understanding of QM - we just do not know yet.

Now for a more modern take on the Einstein Box Thought experiment after which Einstein never questioned the actual validity of QM - but always thought it incomplete:
https://arxiv.org/abs/1203.1139

Later of course Einstein was able to state clearly his real objection in the EPR paper. It was not resolved until the work of Bell. Its interesting the above author (who is a science adviser here) thinks the Einstein Box experiment is really an early version of EPR.

Thanks
Bill

 

[Michael Price]

Is it something to do with a particle being in two places at the same time?
Yes, it is exactly this. Or three, or four...
Before the split the particle in question is smeared out across multiple locations. After the split each branch of reality sees the particle in a different position. and hence different things happen in different branches.

 

[DarMM]

De-coherent Histories, without going into the details, was the formulation of QM Feynman was converted to a bit before he died.

Thanks for the video (although the cards at the bottom by the video uploader are a bit odd!) Just since you seem to know this stuff...

I've finished reading Omnès's book. He says (as is typical of decoherent histories) that decoherent histories tells you when you have classical histories/sequences of porpositions and can assign a probability to them (very similar to Richard Healey's views). He says which event actually occurs in a measurement and what is occurring outside of decoherence is basically beyond science, representing a limit beyond which mathematics cannot model the world.

Does Gell-Mann have similar views do you know?

 

[PEGELLA]

Curious about what's current in multiverse and parallel universe speculation. How's the math on this doing?

I'm curious and doubtful about the Many Worlds theory ( first suggested by Hugh Everett, 1957 ) because I don't understand how an entire universe can split to create a second universe which claims to be identical to it's mother universe, but not violating the law of conservation of energy?
How do you get a second universe ( call it the baby universe ) from it's "mother" universe with exactly the same amount of matter/energy in BOTH universes as the original mother universe?? It would require energy to be created - a violation!

 

[DarMM]

How do you get a second universe ( call it the baby universe ) from it's "mother" universe with exactly the same amount of matter/energy in BOTH universes as the original mother universe?? It would require energy to be created - a violation!

It doesn't violate conservation of energy as they're just two different states of the same material as such. Another way to think about it is that both worlds were already there and then became different (this would be more in line with the modern view on Many-Worlds).

 

[PeterDonis]

I don't understand how an entire universe can split to create a second universe which claims to be identical to it's mother universe, but not violating the law of conservation of energy?

There is no "splitting" in the many worlds interpretation. The entire universe is in a pure state that evolves in time according to a unitary transformation. Unitary transformations preserve all conservation laws.

The "worlds" in the MWI are names that we give to particular parts of the pure state of the universe. They don't affect the above at all.

 

[PEGELLA]

But if they "became different" even ever so slightly, then there would be at least one molecule, atom, sub-atomic,string, SLIGHTLY different that would be part of describing the slight difference and so, necessarily an imbalance in matter and therefore, a loss or gain in matter/energy and thus a violation.

 

[DarMM]

But if they "became different" even ever so slightly, then there would be at least one molecule, atom, sub-atomic,string, SLIGHTLY different that would be part of describing the slight difference and so, necessarily an imbalance in matter and therefore, a loss or gain in matter/energy and thus a violation.

Systems move into different states that have the same total energy, so no. That is in one world the molecule does X and in another world it does Y, but being in X and Y are associated with the same energy.

A flaw this blatant would have stopped Many Worlds in its tracks years ago if it were true.

 

[PeterDonis]

But if they "became different" even ever so slightly

If what "became different"? Go read my post again, carefully.

 

[PeterDonis]

in one world the molecule does X and in another world it does Y, but being in X and Y are associated with the same energy.

This does not have to be true for subsystems; they can go into superpositions of states with different energies. The conservation law only has to hold for the entire system, i.e., universe as a whole.

 

[DarMM]

This does not have to be true for subsystems; they can go into superpositions of states with different energies. The conservation law only has to hold for the entire system, i.e., universe as a whole.

True, I was just assuming a world where the only thing that changed was an isolated molecule like @PEGELLA 's example, or at least how I read it. Of course if not isolated one has to replace "molecule" with "universe" or whatever is the smallest isolated system containing the molecule.

 

[PEGELLA]

Thanks, I think I understand it better now, though, still a most difficult concept to grasp. Any suggestions as to the best books written on this subject ( including math ) would be appreciated. I read recently that most of the world's greatest Physicists are leaning toward the MW theory over other theories.

 

[DarMM]

Thanks, I think I understand it better now, though, still a most difficult concept to grasp. Any suggestions as to the best books written on this subject ( including math ) would be appreciated. I read recently that most of the world's greatest Physicists are leaning toward the MW theory over other theories.

I don't think most are, some are. Or at least I'm not aware of anything showing most are.

What level of quantum mechanics are you familiar with, e.g. texts you have read?

 

[PEGELLA]

www.quora.com/Why-do-60-of-theoretical-physicists-believe-in-the-many-worlds-theory-Is-this-statistic-accurate
www.quantamagazine.org/why-the-many-worlds-interpretation-of-quantum-mechanics-has-many-problems-20181018

probably there is enough on the internet ( including Everett's published thesis ) that will suffice, for now.

 

[DarMM]

That survey was at a very small conference, other surveys give completely different results.

 

[bhobba]

Does Gell-Mann have similar views do you know?

I think so:
https://arxiv.org/abs/1106.0767

Negative probabilities - I used to discuss that one with my probability professor - its subtle.

Thanks
Bill

 

[Michael Price]

Thanks, I think I understand it better now, though, still a most difficult concept to grasp. Any suggestions as to the best books written on this subject ( including math ) would be appreciated. I read recently that most of the world's greatest Physicists are leaning toward the MW theory over other theories.

It is certainly true that many worlds has steadily grown in popularity. In the 1960s of was it ignored, in the 1970s it attracted the attention of flakes and nutcases, alongside DeWitt and Zeh's more sober assessments. In the 1980s it was treated seriously with the emergence of decoherence into the mainstream. Since then it has steady risen in profile. A mainstream physicist can now declare him/herself a fan without it ending their career. Progress indeed.

 

[zonde]

www.quantamagazine.org/why-the-many-worlds-interpretation-of-quantum-mechanics-has-many-problems-20181018

I think the author of this article identifies the most severe problem of MWI:
It [MWI] says that our unique experience as individuals is not simply a bit imperfect, a bit unreliable and fuzzy, but is a complete illusion.

I would say that MWI is worse than solipsism. In solipsism you at least assume that your own consciousness is real. In MWI you deny even that.

 

[Michael Price]

You can see my response on quora in the link. In brief, the article claims MWI is incomprehensible, but is itself incomprehensible about why. In MWI reality forms a branching tree and our consciousness branches along with reality. There is nothing in incomprehensible about that, although it takes some adjusting too. But then so does relativity - we long ago got over the idea that the universe was intuitive.

 

[DarMM]

I think the author of this article identifies the most severe problem of MWI:

I would have said the most severe problem of MWI is the derivation of probabilities, of which there is no noncircular justification.

 

[zonde]

You can see my response on quora in the link.

You mean in comments section? I don't know how to identify it.

In brief, the article claims MWI is incomprehensible, but is itself incomprehensible about why. In MWI reality forms a branching tree and our consciousness branches along with reality. There is nothing in incomprehensible about that, although it takes some adjusting too. But then so does relativity - we long ago got over the idea that the universe was intuitive.

Well, if you take the "split" literally, these objections might seem incomprehensible. So we can say that there are two versions of MWI. One with literal "split" and the other one with "separation" of two superposition components within the same global wavefunction.
So we can say that arguments in the article apply to MWI version with the "separation" concept.

 

[zonde]

I would have said the most severe problem of MWI is the derivation of probabilities, of which there is no noncircular justification.

Justification is not a strict requirement for scientific theory. It's enough that predictions are unequivocally derivable.
But if a theory is built on philosophical basis that is inconsistent with philosophical basis of scientific approach, there is nothing much you can do about it.

 

[Michael Price]

I would have said the most severe problem of MWI is the derivation of probabilities, of which there is no noncircular justification.

Zurek has a non-circular derivation, see
https://www.quora.com/How-does-the-...ding-to-the-Born-rule/answer/Michael-Price-29

 

[Michael Price]

You mean in comments section? I don't know how to identify it.

Well, if you take the "split" literally, these objections might seem incomprehensible. So we can say that there are two versions of MWI. One with literal "split" and the other one with "separation" of two superposition components within the same global wavefunction.
So we can say that arguments in the article apply to MWI version with the "separation" concept.

Only if you take the idea of splitting as "incomprehensible" . Splitting is comprehensible to me, is all I can say.

 

[Michael Price]

You mean in comments section? I don't know how to identify it.

Good point. Here it is.
https://www.quora.com/How-do-Everet...chanics-is-incoherent/answer/Michael-Price-29

 

[DarMM]

Justification is not a strict requirement for scientific theory. It's enough that predictions are unequivocally derivable.
But if a theory is built on philosophical basis that is inconsistent with philosophical basis of scientific approach, there is nothing much you can do about it.

I'm not sure what you take "justify" to mean such that it's not important, but to be clearer Many Worlds can't derive the probabilities and since the probabilities are the empirical core of QM then the predictions are not unequivocally derivable.

 

[DarMM]

Zurek has a non-circular derivation, see
https://www.quora.com/How-does-the-...ding-to-the-Born-rule/answer/Michael-Price-29

I have read Zurek's paper and related work, the derivation is circular still. There is a huge amount of assumptions on the environment and the coefficients are assumed to already contain probabilties within them. His proof is essentially:
"If the coefficients are assumed to relate to probabilities of the system and its subsystems and those probabilities obey one of three assumptions and the system is embedded in an environment that has a certain structure, then under continuity arguments the probabilities are those given by the Born rule".

Since this is a beginner thread I won't go into more detail, I'll discuss in a more advanced thread, but even Zurek himself does not view it as a non-circular derivation of the probabilities.

 

[Michael Price]

I have read Zurek's paper and related work, the derivation is circular still. There is a huge amount of assumptions on the environment and the coefficients are assumed to already contain probabilties within them. His proof is essentially:
"If the coefficients are assumed to relate to probabilities of the system and its subsystems and those probabilities obey one of three assumptions and the system is embedded in an environment that has a certain structure, then under continuity arguments the probabilities are those given by the Born rule".

Since this is a beginner thread I won't go into more detail, I'll discuss in a more advanced thread, but even Zurek himself does not view it as a non-circular derivation of the probabilities.

We shall have to disagree then. Feel free to discuss it further, elsewhere.

 

[DarMM]

We shall have to disagree then. Feel free to discuss it further, elsewhere.

It might be worth reading why Zurek himself says it is circular in this essay:
Zurek, W. (2010). Quantum Jumps, Born’s Rule, and Objective Reality. In: S. Saunders et al, ed., Many Worlds? Everett, Quantum Theory, and Reality, 1st ed. Oxford University Press, pp. 409-432.

We have already noted the problem with this strategy: it courts circularity.

 

[zonde]

Only if you take the idea of splitting as "incomprehensible" . Splitting is comprehensible to me, is all I can say.

Sorry, but I don't understand how it is a reply to my post. I said that if you consider version of MWI with literal "split" I'm not arguing with you, at least not with the argument I used here.

 

[zonde]

but to be clearer Many Worlds can't derive the probabilities and since the probabilities are the empirical core of QM then the predictions are not unequivocally derivable.

Of course, if predictions are not unequivocally derivable, there is not much of a theory to talk about.

 

[Michael Price]

It might be worth reading why Zurek himself says it is circular in this essay:
Zurek, W. (2010). Quantum Jumps, Born’s Rule, and Objective Reality. In: S. Saunders et al, ed., Many Worlds? Everett, Quantum Theory, and Reality, 1st ed. Oxford University Press, pp. 409-432.

Thanks, I shall dig out my copy and judge context, but in the meantime, saying it courts circulatory is not the same as saying it is circular.

 

[DarMM]

Thanks, I shall dig out my copy and judge context, but in the meantime, saying it courts circulatory is not the same as saying it is circular.

It's just a poetic way of saying it. Here is the full context, he clearly rejects his old proof, see the last line:

Decoherence done ‘in the usual way’ (which, by the way, is a step in the right
direction, in the understanding of the practical and even many of the fundamental
aspects of the quantum–classical transition!) is not a good starting point for
addressing the more fundamental aspects of the origins of the classical. In particular,
decoherence is not a good starting point for the derivation of Born’s rule. As
the saying goes, there is no preacher like a reformed sinner. I previously proposed
a derivation of Born’s rule based on the symmetries—invariance of a state of the
system under permutations of pointer states, ‘events’ obtained in the usual way
from decoherence (Zurek [1998]). We have already noted the problem with this
strategy: it courts circularity. It employs Born’s rule to arrive at the pointer states
by using reduced density matrix which is obtained through trace—i.e., averaging,
which is where Born’s rule is implicitly invoked (see e.g. Nielsen and Chuang
[2000]). Therefore, using decoherence to derive Born’s rule is at best a consistency
check. While the above is a mea culpa, this circularity would also afflict other
approaches, including proposals based on decision theory (Deutsch [1999], Wallace
[2003], Saunders [2004]), as noted also by Forrester [2007] among others.
So one has to start the task from a different end.

 

[Michael Price]

It's just a poetic way of saying it. Here is the full context, he clearly rejects his old proof, see the last line:

Okay, well that seems clear, thanks. When I locate my copy I'll see just what Zurek is proposing, if anything.

 

[caspiansea3]

Another thing to consider for deriving the Born Rule in MWI is the work of Hsu and Hanson. Hsu postulates a discreteness of the quantum state space and Hanson postulates a cutoff where small amplitude worlds are 'mangled' by larger ones.

https://arxiv.org/abs/quant-ph/0108070 When Worlds Collide: Quantum Probability From Observer Selection?
In Everett's many worlds interpretation, quantum measurements are considered to be decoherence events. If so, then inexact decoherence may allow large worlds to mangle the memory of observers in small worlds, creating a cutoff in observable world size. Smaller world are mangled and so not observed. If this cutoff is much closer to the median measure size than to the median world size, the distribution of outcomes seen in unmangled worlds follows the Born rule. Thus deviations from exact decoherence may allow the Born rule to be derived via world counting, with a finite number of worlds and no new fundamental physics.

https://arxiv.org/abs/hep-th/0606062 Discreteness and the origin of probability in quantum mechanics
Attempts to derive the Born rule, either in the Many Worlds or Copenhagen interpretation, are unsatisfactory for systems with only a finite number of degrees of freedom. In the case of Many Worlds this is a serious problem, since its goal is to account for apparent collapse phenomena, including the Born rule for probabilities, assuming only unitary evolution of the wavefunction. For finite number of degrees of freedom, observers on the vast majority of branches would not deduce the Born rule. However, discreteness of the quantum state space, even if extremely tiny, may restore the validity of the usual arguments.

 

[Michael Price]

Okay, well that seems clear, thanks. When I locate my copy I'll see just what Zurek is proposing, if anything.

Okay, well that seems clear, thanks. When I locate my copy I'll see just what Zurek is proposing, if anything.

Since there is another 18 pages after the "last line" it is a bit misleading to imply Zurek hasn't got a solution. He is criticizing earlier "solutions" (including his own), which start from decoherence, and then proceeds to present an alternative approach he is happier with. Please note that my recapitulation (link previously posted) of Zurek's derivation did not involve decoherence.
On page 417 he states Here we present the ideas behind a circularity-free approach.

 

[DarMM]

Since there is another 18 pages after the "last line" it is a bit misleading to imply Zurek hasn't got a solution

My point was that the derivation in your Quora post is circular. No implications about other work by Zurek. Your quora post doesn't deal with his later Quantum Darwinism work and it's a very different proof. From a mathematical perspective it would be equally valid to discuss Wallace's proof. I've no need to discuss it as you weren't talking about it. I know that he presents what he claims is a "circularity-free approach", but the essay was quoted to show you Zurek (and the rest of the community) thinks his 2005 work (the one you use in your quora post) is circular (and thus you shouldn't be using it as a definitive proof), not to say that Zurek has had no further alternative ideas.
The "last line" simply meant it was the last line of my quote of the article, not the last line of the article.

Please note that my recapitulation (link previously posted) of Zurek's derivation did not involve decoherence.

You are claiming to have modified Zurek's 2005 argument, but removed the dependence on decoherence? The envariance argument requires decoherence (his later Quantum Darwinism argument does not, but it is a separate argument with other weaknesses), as Zurek himself says in the essay I referenced.

 

[akvadrako]

Here we present the ideas behind a circularity-free approach.

It's fair to say all these derivations of Born's rule are not completely circular, but they do require some additional assumptions. One of the most innocuous ones I know of is: if you have a perfectly symmetrical state you can assume it represents two worlds of equal Born weights. That's what your quora derivation assumes and it's the most controversial part.

Using only the axioms of unitary QM, can you show why a symmetric state should give rise to equal probabilities?

To steel man the arguments of critics: if you are given two separate worlds it seems like their weights are completely irrelevant and can be scaled arbitrarily with no effect (except to zero). For the occupants of worlds with unequal weights, a safe assumption is they will both have equally real experiences.

 

[DarMM]

if you have a perfectly symmetrical state you can assume it represents two worlds of equal Born weights...
Using only the axioms of unitary QM, can you show why a symmetric state should give rise to equal probabilities?

This is often a question posed to MWI, but the circularity goes deeper. One is starting here with a symmetric superposition of two worlds. There is the problem of showing that with just unitary QM one can demonstrate equal probabilities, as you said.

More fundamental however is using unitary QM to demonstrate that such stable non-interfering superpositions of classics worlds exist, even ignoring managing to get probabilities out of them.

Trying to answer this question is where the circularity originates. The worlds can only be shown to be stable and separated via decoherence, but decoherence requires the Born rule. For this reason the derivations are all somewhat circular, or so it seems to me.

 

[akvadrako]

More fundamental however is using unitary QM to demonstrate that such stable non-interfering superpositions of classics worlds exist, even ignoring managing to get probabilities out of them.

Why? Is it because there are always some interference terms (or maverick worlds) and you need the Born rule to neglect them? I have seen a few attempts to show exact decoherence, with no terms that need to be neglected. In the context of those models, would you say this circularity is avoided?

PS — In most cases exact classical worlds don't exist in unitary QM, but that's probably nitpicking.

 

[DarMM]

PS — In most cases exact classical worlds don't exist in unitary QM, but that's probably nitpicking.

You're right, I should be saying quasi-classical. I believe Wallace uses that phrase, i.e. classical down to a certain scale.

Why? Is it because there are always some interference terms (or maverick worlds) and you need the Born rule to neglect them?

That's one part, also the stability part. Without decoherence you don't have stable pointer states, so you don't have a stable basis defining the worlds.

I have seen a few attempts to show exact decoherence, with no terms that need to be neglected. In the context of those models, would you say this circularity is avoided?

Taking for example the paper referenced here earlier https://arxiv.org/abs/hep-th/0606062, something like that would seem to avoid circularity if it worked out. However it's not so much exact decoherence, but some kind of decoherence without the Born rule, i.e. being able to neglect certain terms in some Born independent way, even if it isn't exact.

Do you know of some references to non-Born or exact approaches to decoherence, I only know of Zurek's Quantum Darwinism in any depth.

 

[akvadrako]

Do you know of some references to non-Born or exact approaches to decoherence, I only know of Zurek's Quantum Darwinism in any depth.

I don't know it in any depth and I doubt it's general enough; my question was mostly trying to flesh out what would be needed for a satisfying derivation. For reference the paper I though of is: Exact Decoherence Brought by One Internal Degree of Freedom (https://arxiv.org/pdf/1808.02645.pdf). The main idea was published in 2016.

 

[Michael Price]

My point was that the derivation in your Quora post is circular. No implications about other work by Zurek. Your quora post doesn't deal with his later Quantum Darwinism work and it's a very different proof. From a mathematical perspective it would be equally valid to discuss Wallace's proof. I've no need to discuss it as you weren't talking about it. I know that he presents what he claims is a "circularity-free approach", but the essay was quoted to show you Zurek (and the rest of the community) thinks his 2005 work (the one you use in your quora post) is circular (and thus you shouldn't be using it as a definitive proof), not to say that Zurek has had no further alternative ideas.
The "last line" simply meant it was the last line of my quote of the article, not the last line of the article.You are claiming to have modified Zurek's 2005 argument, but removed the dependence on decoherence? The envariance argument requires decoherence (his later Quantum Darwinism argument does not, but it is a separate argument with other weaknesses), as Zurek himself says in the essay I referenced.

Okay, having studied Zurek's informal review in Many Worlds?, and the references therein, it is clear that all I need do to tidy up my quora answer is to reference Zureks's 2007 paper, rather than the 2005 paper. Zurek gives a detailed and non-circular explanation of why probabilities will be equal when the coefficients are all equal, and then proceeds to generalize this to the more interesting non-equal-coefficients case.

 

[Michael Price]

That's one part, also the stability part. Without decoherence you don't have stable pointer states, so you don't have a stable basis defining the worlds.

I think Zurek's quantum Darwinism is trying to show that you have stable pointer states even without decoherence. His point is that information about the stable pointers can be copied endlessly, without restriction, throughout the environment.

 

[DarMM]

it is clear that all I need do to tidy up my quora answer is to reference Zureks's 2007 paper, rather than the 2005 paper. Zurek gives a detailed and non-circular explanation of why probabilities will be equal when the coefficients are all equal

I think Zurek's quantum Darwinism is trying to show that you have stable pointer states even without decoherence. His point is that information about the stable pointers can be copied endlessly, without restriction, throughout the environment.

I'm well aware what Zurek's Quantum Darwinism tries to show, I've read all his papers. Why do you think it is noncircular or unproblematic?

It assumes that there is a universal preferred division of the universe into system+environment and that all the different components of the environment have relatively thermalised phases. What in a pure unitary approach justifies this?

 

[Michael Price]

I can't see where Zurek assumes thermalisation. The copying of information from the object system to the measuring apparatus and/or environment is an explicitly unitary process (as is all time evolution). I don't think Zurek writes very clearly, in places, so I may have missed something, but I don't see the circularity. The quantum Darwinism comes at the end, after he's constructed pointer states and derived the Born rule.