Why does nothing happen in MWI?

[Demystifier]

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.

 

[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]

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]

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]

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]

Yes.

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.

 

[bhobba]

In Many Worlds we do observe everything happening. That's what it means.

No. We only ever observe one world - if you want to use that sort of language.

Thanks
Bill

 

[Derek Potter]

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.

That last phrase deserves a like even though I don't really agree.

 

[Derek Potter]

No. We only ever observe one world - if you want to use that sort of language.

Thanks
Bill

I have to disagree. We are talking about MWI where observer-states co-exist in superposition.

 

[PeterDonis]

In Many Worlds we do observe everything happening.

But in reality, we don't. We only observe dead cats or live cats; we don't observe cats in superpositions of dead and alive. So if MWI predicts that we "do" observe such things, MWI is falsified by experiment.

 

[bhobba]

I have to disagree. We are talking about MWI where observer-states co-exist in superposition.

That is exactly what is NOT going on. The fact you interpret each part of a mixed state as a separate world means it is NOT in superposition. Indeed look at the math I have posted before where when you observe one part of an entangled system it is in a mixed state. It can't be in superposition since its entangled.

Thanks
Bill

 

[PeterDonis]

it is not a physical process it is epistemological

When you observe a cat, photons are reflected off the cat and interact with chemicals in your retina, producing nerve impulses that are processed by your brain. That's not epistemological, that's a physical process. The language I've just used to describe that process is arbitrary, true: nothing requires me to pick out "cat", "photons", "retina", "brain", etc. as the "objects" in this scenario. But however you describe it, there is a physical process going on when you observe something. But if all the information a theory of physics gives me is the pure state vector of the entire universe, the theory does not tell me anything about this physical process; to even identify the process, I have to add additional structure that the theory does not contain.

 

[Derek Potter]

But in reality, we don't. We only observe dead cats or live cats; we don't observe cats in superpositions of dead and alive. So if MWI predicts that we "do" observe such things, MWI is falsified by experiment.

Well that's wrong on two points. Firstly that polarization states are easily observed in superposition, cats are simply subject to decoherence. Secondly MWI does not predict that we see the superposition, it predicts we see both outcomes.

 

[Derek Potter]

That is exactly what is NOT going on. The fact you interpret each part of a mixed state as a separate world means it is NOT in superposition. Indeed look at the math I have posted before where when you observe one part of an entangled system it is in a mixed state. It can't be in superposition since its entangled.

Of course. If you consider one part it is a mixture, if you consider the whole entanglement it is a pure state.

 

[bhobba]

Of course. If you consider one part it is a mixture, if you consider the whole entanglement it is a pure state.

Sure - but you talked about an observer state. The observer, since its entangled with what's being observed, is NOT in superposition - and obviously so.

Thanks
Bill

 

[Derek Potter]

When you observe a cat, photons are reflected off the cat and interact with chemicals in your retina, producing nerve impulses that are processed by your brain. That's not epistemological, that's a physical process. The language I've just used to describe that process is arbitrary, true: nothing requires me to pick out "cat", "photons", "retina", "brain", etc. as the "objects" in this scenario. But however you describe it, there is a physical process going on when you observe something. But if all the information a theory of physics gives me is the pure state vector of the entire universe, the theory does not tell me anything about this physical process; to even identify the process, I have to add additional structure that the theory does not contain.

Obviously the whole of quantum mechanics is based on the idea that a state evolves, the dynamics are typically described by Schrodinger's Equation and the Hamiltonian requires analysis of particular physical laws.

 

[PeterDonis]

Obviously the whole of quantum mechanics is based on the idea that a state evolves

In the Schrodinger picture, yes. Not in the Heisenberg picture: in that picture, states don't change at all; all the time evolution is in the operators.

 

[bhobba]

Obviously the whole of quantum mechanics is based on the idea that a state evolves, the dynamics are typically described by Schrodinger's Equation and the Hamiltonian requires analysis of particular physical laws.

But just as obviously factoring this into cats, people etc etc is not part of those laws. That's the crucial issue. The claim is its this extra structure that's responsible for decoherence.

Thanks
Bill

 

[Derek Potter]

Sure - but you talked about an observer state. The observer, since its entangled with what's being observed, is NOT in superposition - and obviously so.

Thanks
Bill

Well, when fighting on several fronts sometimes a little sloppiness creeps in.

 

[Derek Potter]

But just as obviously factoring this into cats, people etc etc is not part of those laws. That's the crucial issue. The claim is its this extra structure that's responsible for decoherence.

Yes. Without actual physical processes to make the state evolve, there would be no decoherence.

I don't know why such an obvious, if unstated, assumption is crucial. Has someone changed the definition of MWI so that it works without physics?

 

[PeterDonis]

I am beginning to think someone has changed the definition of MWI so that it works without physics.

Well, you're the one that brought up Schwindt's paper , so it's his definition of "MWI" that we're discussing. His definition is basically, that "MWI" means "the universe has a pure state vector that is subject to unitary evolution" and that's it. Anything else (factorization, etc.) is "additional structure", and the "MWI", on his interpretation, doesn't include that additional structure.

That definition of "MWI" probably isn't the one a lot of physicists are implicitly using when they talk about the MWI. But I do think it's a valid question where the "additional structure" comes from; I don't think you can just say "it's in the state vector" and be done with it.

 

[Derek Potter]

Well, you're the one that brought up Schwindt's paper , so it's his definition of "MWI" that we're discussing. His definition is basically, that "MWI" means "the universe has a pure state vector that is subject to unitary evolution" and that's it. Anything else (factorization, etc.) is "additional structure", and the "MWI", on his interpretation, doesn't include that additional structure.
That definition of "MWI" probably isn't the one a lot of physicists are implicitly using when they talk about the MWI. But I do think it's a valid question where the "additional structure" comes from; I don't think you can just say "it's in the state vector" and be done with it.

I didn't exactly" bring up" Schwindt's paper, the thread was started by me asking for help in understanding it.
Now, if you add "unitary evolution" to the state vector postulate, you introduce time dependence and if you introduce time dependence you must say what the dependency is and if you say what it is then you can say how a state evolves. You can then identify which factorizations result in interactions with stable entanglements - a.k.a. observations. The additional structure is the form of the unitary evolution. I cannot believe that Schwindt would have forgotten about that. And from the fact that his claim seems only to apply to the whole universe and not to smaller systems I would think this general discussion about factorization is off the point.

 

[PeterDonis]

if you introduce time dependence you must say what the dependency is and if you say what it is then you can say how a state evolves

Yes, agreed; but "evolves" here just means one state vector for the universe with norm 1 turns into another state vector for the universe with norm 1.

You can then identify which factorizations result in interactions with stable entanglements - a.k.a. observations. The additional structure is the form of the unitary evolution.

This part I'm not sure about. As above, unitary evolution tells you how the state vector of the whole universe evolves, but that in itself contains no information about anything other than one state vector of norm 1 turning into another state vector of norm 1. Or, to put it another way, any such unitary evolution can be written in a basis in which nothing happens; that's the point of the "Nirvana" theorem Schwindt talks about.

his claim seems only to apply to the whole universe and not to smaller systems

I don't think that's true. His claim is that the pure state vector that appears in unitary evolution is a state vector of the whole universe, because, according to the MWI (as he interprets it), that's the only "pure" state that really is pure.

 

[Derek Potter]

Yes, agreed; but "evolves" here just means one state vector for the universe with norm 1 turns into another state vector for the universe with norm 1.

Which now sits askew on whatever basis you previously set it square on. Ergo every damn thing you could ever want to know about it is now different.

This part I'm not sure about. As above, unitary evolution tells you how the state vector of the whole universe evolves, but that in itself contains no information about anything other than one state vector of norm 1 turning into another state vector of norm 1. Or, to put it another way, any such unitary evolution can be written in a basis in which nothing happens; that's the point of the "Nirvana" theorem Schwindt talks about.

Well the exact meaning of the Nirvana theorem is what I was asking about. I'm imagining the state vector pinned down and the basis running all over the place around it, in which case the Nirvana basis would be time-dependent. But states are supposed to be basis-independent so the state would appear to be unchanging in any basis... huh, that's got to be silly! Or could we say that the appearence of activity is purely the result of the Nirvana basis swinging around? Seems like playing with words, but I'm out of my depth.

 

[atyy]

Derek Potter, since there is no branching in your MWI, isn't your MWI really BMW, as we discussed before?

So we just take all variants of BM to be real, and in each variant, all Bohmian worlds are real?

I'm pretty sure there's one world in which BMW does use Vorsprung durch Technik.

 

[Derek Potter]

Derek Potter, since there is no branching in your MWI, isn't your MWI really BMW, as we discussed before?
So we just take all variants of BM to be real, and in each variant, all Bohmian worlds are real?
I'm pretty sure there's one world in which BMW does use Vorsprung durch Technik.

I think it will be simplest if I just agree :)