# Why does nothing happen in MWI?

#### bhobba

Mentor
I thought this was elementary. Splits that support decoherence describe worlds such as the world opf Schrodinger Cats in which decoherence operates and a preferred basis emerges. Splits which do not support decoherence describe worlds in which decoherence does not operate such as polarization experiments. Clearly it would be silly to insist that decoherence is fundamental, when, in fact it depends on the factorization. This is why I say that decoherence is not fundamental to MWI, it is a welcome refinement.
Its not elementary - its in fact quite deep.

Conceptually its simple. Consider the particle detector in Schrodinger Cat. I have posted a very elementary argument showing if you just observe the detector then because its entangled with the emitted particle its in a mixed state and MW, or any other decoherence based explanation for that matter, handles it easy peasy. But now lets decompose it into half the particle detector and the other half + the particle; do you still get the same result? Its difficult to analyse it that way and you need rocks in your head to do it, but we are speaking of matters of principle. We do not have theorems that show you will get the same result ie the same mixed state.

Thanks
Bill

#### bhobba

Mentor
Cats decohere rapidly Decoherence therefore applies.
You are missing the point. What if you factor it differently?

Thanks
Bill

#### Demystifier

2018 Award
You are missing the point. What if you factor it differently?

Thanks
Bill
Yes, that's exactly my point (or my question).

#### Derek Potter

Its not elementary - its in fact quite deep.
We do not have theorems that show you will get the same result ie the same mixed state.
You are missing the point. What if you factor it differently?
Thanks
Bill
I probably am missing the point. As I see it, MWI doesn't have proper mixed states. Factorization doesn't alter the pure state nor the improper mixed state. I can't make sense of "the same mixed state" when we are considering different factorizations which by definition means different bases and different observables.

#### bhobba

Mentor
I probably am missing the point. As I see it, MWI doesn't have proper mixed states. Factorization doesn't alter the pure state nor the improper mixed state.
It's simple. If you factor it differently do you still get the same result ie if you do the half detector thing and the bit that flashes or clicks is in one of these halves does it still have the same state?

Thanks
Bill

#### tzimie

We have already discussed this paper in
To define separate worlds of MWI, one needs a preferred basis, which is an old well-known problem of MWI. In modern literature, one often finds the claim that the basis problem is solved by decoherence. What J-M Schwindt points out is that decoherence is not enough. Namely, decoherence solves the basis problem only if it is already known how to split the system into subsystems (typically, the measured system and the environment). But if the state in the Hilbert space is all what exists, then such a split is not unique. Therefore, MWI claiming that state in the Hilbert space is all what exists cannot resolve the basis problem, and thus cannot define separate worlds. Period! One needs some additional structure not present in the states of the Hilbert space themselves.
This problem had been solved by Max Tegmark
http://arxiv.org/abs/1401.1219

#### bhobba

Mentor
This problem had been solved by Max Tegmark
That's his view.

I suspect most are like me and think its a load of the proverbial like a lot of Tegmarks stuff.

Thanks
Bill

#### PeterDonis

Mentor
As I see it, MWI doesn't have proper mixed states.
The problem, according to Schwindt, is that the MWI doesn't have any states other than "the pure state of the entire universe". For example, when you say "cats decohere", you are assuming that there are "cats" picked out somewhere as identifiable quantum states. But if all we have is the pure state of the universe, there are no "cats"--or humans, or anything else. So you don't even have the structure needed to talk about "decoherence" at all.

#### tzimie

The problem, according to Schwindt, is that the MWI doesn't have any states other than "the pure state of the entire universe". For example, when you say "cats decohere", you are assuming that there are "cats" picked out somewhere as identifiable quantum states. But if all we have is the pure state of the universe, there are no "cats"--or humans, or anything else. So you don't even have the structure needed to talk about "decoherence" at all.
Again, please check the Max Tegmark's solution.
Please ignore the provocative title of his article.

#### Demystifier

2018 Award
This problem had been solved by Max Tegmark
http://arxiv.org/abs/1401.1219
Essentially, Tegmark proposes that the preferred basis is determined by consciousness. It could be true, but he certainly does not derive consciousness from MWI. Instead, one has to postulate consciousness as an independent assumption. I am fine with it, but this only confirms what I already said that pure MWI (i.e. MWI without any additional assumptions) cannot solve the problem. One has to assume something additional, be it Copehangen observers, Tegmark cosciousness, Bohmian trajectories, or something else.

#### Demystifier

2018 Award
The problem, according to Schwindt, is that the MWI doesn't have any states other than "the pure state of the entire universe". For example, when you say "cats decohere", you are assuming that there are "cats" picked out somewhere as identifiable quantum states. But if all we have is the pure state of the universe, there are no "cats"--or humans, or anything else. So you don't even have the structure needed to talk about "decoherence" at all.
Exactly!

#### tzimie

one has to postulate consciousness as an independent assumption. ... One has to assume something additional, be it Copehangen observers, Tegmark cosciousness, Bohmian trajectories, or something else.
There is a difference: Bohmian stuff is "built in" on an axiomatic level while consciousness emerges in the Universe at some point, and early Universe existed even without it (as well as life-unfriendly universes). However, I agree that the issue is not completely settled (even I see it as a step int the right direction) - you can use Mark's own weapon (MUH) against his own theory: as consciousness is, as he calls it, just "wordy baggage" it can't be fundamental in TOE, as based on MUH, TOE must be just "pure math". I hope Mark would address this question in the future...

#### Derek Potter

It's simple. If you factor it differently do you still get the same result ie if you do the half detector thing and the bit that flashes or clicks is in one of these halves does it still have the same state?

Thanks
Bill
I think I'd need to see an example of how mere factorization could possibly result in its not being the same state. Perhaps the problem is whether the mixed states are the same rather than the pure states? But in that case surely there is no need for them to be the same, in fact they had better not be - we want outcomes that are different under different observations.

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#### Derek Potter

The problem, according to Schwindt, is that the MWI doesn't have any states other than "the pure state of the entire universe". For example, when you say "cats decohere", you are assuming that there are "cats" picked out somewhere as identifiable quantum states. But if all we have is the pure state of the universe, there are no "cats"--or humans, or anything else. So you don't even have the structure needed to talk about "decoherence" at all.
I do not see that. Assuming that you can define a cat in classical terms by its attributes, a Schrodinger cat is not merely in a mixture of alive and dead, it is also in a mixture of colour states derived from quantum events in its genetic past. This means that we can define a cat state space as a subspace of the universe's state space. Decoherence occurs way down the line, it's a physical process that affects how much of the cat space is visible to observers.

#### PeterDonis

Mentor
Assuming that you can define a cat in classical terms by its attributes
How do you do this if all you have is the pure state vector of the entire universe? You can't just help yourself to the assumption that you can do it, since that's precisely the point at issue.

we can define a cat state space as a subspace of the universe's state space.
How? If all you have is the pure state vector of the entire universe, how do you pick out the "cat" subspace? If your answer is, "well, I pick some particular basis...", then what justifies picking out that particular basis? If your answer to that is "well, that's the basis in which we have cats that are either dead or alive, instead of a superposition of dead and alive", then you're arguing in a circle.

#### Derek Potter

How do you do this if all you have is the pure state vector of the entire universe?
The assumption is epistemological, not physical, it is simply the definition of a cat and a cat space.
You can't just help yourself to this assumption, since it's precisely the point at issue...
How? If all you have is the pure state vector of the entire universe, how do you pick out the "cat" subspace? If your answer is, "well, I pick some particular basis...", then what justifies picking out that particular basis?
Why should MWI have to justify the mathematical axiom of choice? See https://en.wikipedia.org/wiki/Axiom_of_choice

The point is either trivial or lost on me entirely. I'll try the trivial case. The definition of a cat is up to the observer in the same factorization. If a given factorization creates dog observations then the observer will call the other system a dog. Whether a subsystem is a cat or a dog depends whether the observer sees a cat or a dog. Perhaps you can supply the "lost on me" version?
.

#### atyy

The assumption is epistemological, not physical, it is simply the definition of a cat and a cat space.

Why should MWI have to justify the mathematical axiom of choice? See https://en.wikipedia.org/wiki/Axiom_of_choice

The point is either trivial or lost on me entirely. I'll try the trivial case. The definition of a cat is up to the observer in the same factorization. If a given factorization creates dog observations then the observer will call the other system a dog. Whether a subsystem is a cat or a dog depends whether the observer sees a cat or a dog. Perhaps you can supply the "lost on me" version?
.
So your solution is that everything happens (in contrast to nothings happens). Then we just pick what we like.

#### tzimie

There is nothing to do with the axiom of choice. We have an ***extreme*** case of unfair sampling of reality. Just a reminder, from 10^500 of universes, only 10^100 have life-friendly combination of parameters. Inside such life-friendly universe, observations are bound to particular basis, where "observers" could be defined.

#### bhobba

Mentor
I think I'd need to see an example of how mere factorization could possibly result in its not being the same state.
That's the point - we don't know one way or the other. I think its highly unlikely - and the dearth of concerns about it in the professional literature suggests that's most peoples view - but as a matter of principle that means diddlely squat.

Thanks
Bill

#### PeterDonis

Mentor
The definition of a cat is up to the observer in the same factorization.
This would help if you could show that "observers" only appear in certain factorizations--the ones with cats that are either dead or alive, but not the ones with cats in a superposition of dead and alive. But if you can't show that--if there are "observers" in every factorization--then this "definition" argument doesn't help, because there's nothing in the state vector that tells us what definition we should adopt.

Bear in mind, I am not arguing that the things you are suggesting can't be done; of course they can. Of course we define "objects" according to our own arbitrary criteria all the time. But the question is, if the only structure that physics gives you is the pure state vector of the entire universe, how can this thing we do all the time be explained? All your "explanations" amount to adding additional structure (picking a factorization), but that just concedes the point: the state vector itself doesn't contain the necessary structure, you have to add it in by hand.

#### Derek Potter

So your solution is that everything happens (in contrast to nothings happens). Then we just pick what we like.
Yes. That's why it's Many Worlds, not Just a Few Selected Worlds.

#### PeterDonis

Mentor
But we don't observe everything happening. We only observe a few selected things happening. How can this be, if everything happens?

#### atyy

Yes. That's why it's Many Worlds, not Just a Few Selected Worlds.
Good. Just making sure I understood!

The criticism is that a theory that says we have all worlds, all laws of physics is just as good, since we can just pick what we like.

The counterargument is that MWI is not all laws pf physics - only those that evolve unitarily.

However, even with the counter - can MWI be said to solve the measurement problem? It doesn't say who "we" are, except that we are those for whom observations are classical. Unless we can show that a classical world is needed for consciousness, MWI seems to leave the problem open. The advantage is that it merges the measurement problem with the hard problem of consciouness. So it is a unified theory of problems.

#### Derek Potter

This would help if you could show that "observers" only appear in certain factorizations--the ones with cats that are either dead or alive, but not the ones with cats in a superposition of dead and alive. But if you can't show that--if there are "observers" in every factorization--then this "definition" argument doesn't help, because there's nothing in the state vector that tells us what definition we should adopt.

Bear in mind, I am not arguing that the things you are suggesting can't be done; of course they can. Of course we define "objects" according to our own arbitrary criteria all the time. But the question is, if the only structure that physics gives you is the pure state vector of the entire universe, how can this thing we do all the time be explained? All your "explanations" amount to adding additional structure (picking a factorization), but that just concedes the point: the state vector itself doesn't contain the necessary structure, you have to add it in by hand.
That logic completely eludes me. Observers can be anything at all but if you want MWI to account for the emergence of cats, you need some criterion for saying that cats have arisen. That's all. Once again it is not a physical process it is epistemological. The superposition argument is specious since decoherence does the job.

#### Derek Potter

But we don't observe everything happening. We only observe a few selected things happening. How can this be, if everything happens?
In Many Worlds we do observe everything happening. That's what it means.

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