I Danger for the Many-Worlds Interpretation?

[PeterDonis]

Apparently decoherence just makes this very, very unlikely that branches come back together.

Not just very, very unlikely: impossible given the expected time for such an event to happen.

For example, say you observe the result of a measurement which has two possible outcomes. Under the MWI, the interaction between you and the observed system + apparatus creates two copies of you, which decohere. But part of that decoherence is, for example, you radiating infrared photons because of your body temperature, and some of those infrared photons escape into outer space and fly away at the speed of light. In order to "un-decohere" the two branches, somehow all of those infrared photons would need to be reflected back towards Earth in just the right way to all come together in just the right way to restore the interference terms between the decohered branches. That's not just "very, very unlikely". It's so improbable that the time for the universe to expand to the point that there is only one atom per Hubble volume is still a huge number of orders of magnitude smaller than the time for such a thing to happen--and once the universe has expanded to that point, the "un-decoherence" can no longer happen anyway because even if those photons were all reflected after that time, they wouldn't be able to come back to Earth because they would be outside the Earth's cosmological event horizon.

 

[WWGD]

Not just very, very unlikely: impossible given the expected time for such an event to happen.

For example, say you observe the result of a measurement which has two possible outcomes. Under the MWI, the interaction between you and the observed system + apparatus creates two copies of you, which decohere. But part of that decoherence is, for example, you radiating infrared photons because of your body temperature, and some of those infrared photons escape into outer space and fly away at the speed of light. In order to "un-decohere" the two branches, somehow all of those infrared photons would need to be reflected back towards Earth in just the right way to all come together in just the right way to restore the interference terms between the decohered branches. That's not just "very, very unlikely". It's so improbable that the time for the universe to expand to the point that there is only one atom per Hubble volume is still a huge number of orders of magnitude smaller than the time for such a thing to happen--and once the universe has expanded to that point, the "un-decoherence" can no longer happen anyway because even if those photons were all reflected after that time, they wouldn't be able to come back to Earth because they would be outside the Earth's cosmological event horizon.

How do you test if two objects X, Y are copies of each other?

 

[PeterDonis]

How do you test if two objects X, Y are copies of each other?

What does "copies of each other" mean?

Also, I don't understand how this is relevant.

 

[WWGD]

What does "copies of each other" mean?

Also, I don't understand how this is relevant.

How do you recognize a cooy of yourself that lives in another world, one that expetienced an outcome different from the one you experienced in a different world? Just trying to figure basic definitions.

 

[PeterDonis]

How do you recognize a cooy of yourself that lives in another world, one that expetienced an outcome different from the one you experienced in a different world?

You don't. As I've already said, each copy has no way of knowing if other copies even exist. If there were some way of detecting experimentally whether other copies existed, the MWI would not be an alternative interpretation of QM; it would be a different theory that makes different experimental predictions from standard QM, and we could run the experiment and see which one was right.

 

[Minnesota Joe]

Because your copy in one of the branch worlds has no way of knowing that the other branches even exist, let alone the relative weight of their branch as compared with all of the other branches.

Sure. But my statement wasn't made in that context.

 

[PeterDonis]

my statement wasn't made in that context

I don't understand what you mean. You asked a question, I answered it. What "context" are you talking about?

 

[Minnesota Joe]

Not just very, very unlikely: impossible given the expected time for such an event to happen.

Yikes! So I don't feel so bad for saying never. Very interesting.

 

[Minnesota Joe]

I don't understand what you mean. You asked a question, I answered it. What "context" are you talking about?

The context of the scenario I had in mind is one where you know many worlds is true and I was arguing that it explains that the collapse is only apparent. Both outcomes still occur and if you are a person in a branch world, knowing MWI is true, you aren't going to think "collapse" is incompatible with the Schrodinger equation.

ETA: Typo

 

[PeterDonis]

The context of the scenario I had in mind is one where you know many worlds is true and I was arguing that it explains that the collapse is only apparent.

But that's just an assumption; you have no way of testing it by experiment. As I said in response to @WWGD, if there were a way of testing it by experiment, MWI would not be an interpretation of QM; it would be a different theory from standard QM and we could test experimentally which one was right.

Both outcomes still occur and if you are person in a branch world, knowing MWI is true, you aren't going to think "collapse" is incompatible with the Schrodinger equation.

But you are still going to have to "collapse" the wave function you use to predict your own probabilities for future measurement results. You might say the other branches of the wave function exist, but you won't actually calculate as if they exist; you will calculate as if your own branch is the only one that exists, because that's the only one you will use in your calculations. And doing that is not compatible with the Schrodinger Equation, because there is no "collapse" in the SE. The SE does not tell you to calculate using only one branch. It tells you to calculate using all the branches.

In other words, when the MWI says "collapse is only apparent and there is no contradiction with the Schrodinger Equation", it doesn't really mean that literally; it can't, since taking it literally leads you to calculate things that are contrary to what you actually observe.

 

[zonde]

I think Sabine is really talking about the problem of the Born rule in the many-worlds interpretation (MWI). If that interpretation is true, then where do probabilities come from and why are they given by the Born rule? This is indeed one of the main unsolved problems in MWI.

I think that very similar argument can be said about probabilities. Instead of wondering why particular outcome happens with predicted probability, in MWI we can wonder why my consciousness ended up in particular branch (say there are two branches with unequal weight) with predicted probability.
Only questions about consciousness are not really in domain of physics so we shift the question outside physics domain and we can pretend that physics part is just fine. Sounds a bit like cheating.

 

[PeterDonis]

in MWI we can wonder why my consciousness ended up in particular branch

If consciousness is a matter of the physical state of the brain, then this is not an issue: your consciousness is different in each branch in the MWI because the physical state of your brain is different in each branch.

If consciousness is not a matter of the physical state of your brain, then that is a separate issue that has nothing to do with which interpretation of QM you adopt.

 

[PeroK]

I think that very similar argument can be said about probabilities. Instead of wondering why particular outcome happens with predicted probability, in MWI we can wonder why my consciousness ended up in particular branch (say there are two branches with unequal weight) with predicted probability.
Only questions about consciousness are not really in domain of physics so we shift the question outside physics domain and we can pretend that physics part is just fine. Sounds a bit like cheating.

In classical physics, with the roll of a die for example, there is no mystery about why you can one outcome. Nor is there any real sense of the die being in a superposition before it is thrown.

 

[timmdeeg]

The wave-function collapse, I have to emphasize, is not optional. It is an observational requirement. We never observe a particle that is 50% here and 50% there. That’s just not a thing. If we observe it at all, it’s either here or it isn’t. Speaking of 50% probabilities really makes sense only as long as you are talking about a prediction.

If Hossenfelder's statement is true then I think the MWI fans have a problem, because they can't deny that outcomes are observed which then requires the wave-function collapse.

But is this statement true?

Any comments appreciated.

 

[zonde]

If consciousness is not a matter of the physical state of your brain, then that is a separate issue that has nothing to do with which interpretation of QM you adopt.

If in this context consciousness is not a matter of the physical state of your brain, then MWI is not an interpretation of QM but rather philosophical theory that just as well could be incompatible with philosophical basis of science, don't you agree?

 

[zonde]

I think the problem is this:

The wave-function collapse, I have to emphasize, is not optional. It is an observational requirement. We never observe a particle that is 50% here and 50% there. That’s just not a thing. If we observe it at all, it’s either here or it isn’t. Speaking of 50% probabilities really makes sense only as long as you are talking about a prediction.

If Hossenfelder's statement is true then I think the MWI fans have a problem, because they can't deny that outcomes are observed which then requires the wave-function collapse.

But is this statement true?

Yes, it's true. In science we have to have physical facts against which we compare our models and decide which models are discarded. If some observations are questioned then they have to be replaced with other observations that we could consider more reliable.

 

[entropy1]

What does it even mean to have fractions of different real outcomes?

Then again, what does 'real' mean anyway.

The way I see it is that a measurement outcome doesn't represent the wave function that was measured (unless the corresponding amplitude of the wavefunction is 1), and I guess I consider the wave function the whole story/whole reality. The amplitudes span the wavefunction as vectors. Outcomes don't because they get amplitude 1. So measurement outcomes don't represent the entire reality (the wavefunction ). One should be able to reconstruct the wavefunction from outcomes, but that is not so, except in ensembles where the examined wavefunction is reproduced many times.

 

[timmdeeg]

Yes, it's true.

Thanks.

So how would you comment this reasoning:

"If Hossenfelder's statement is true then I think the MWI fans have a problem, because they can't deny that outcomes are observed which then requires the wave-function collapse."

 

[zonde]

So how would you comment this reasoning:

"If Hossenfelder's statement is true then I think the MWI fans have a problem, because they can't deny that outcomes are observed which then requires the wave-function collapse."

MWI fans deny that objective outcomes are observed. Outcomes are subjective experience of "collapsed consciousness".
You can argue against MWI only using philosophical arguments that, while it might be true, it's a dead end in the aspect of gaining new knowledge about reality, like solipsism and superdeterminism. If MWI fan denies relevance of philosophical arguments you have no other grounds on which you can argue against MWI.

 

[DarMM]

Nicely put, okay, that makes sense to me. I guess that person $B$, before she looks at the detector, reasons that it is rational to assign $p\left(\omega\right) = 1/n$ for number of outcomes $n$ if the amplitudes in the original setup are equal. Otherwise, she adjusts her credence accordingly to the amplitudes $p\left(\omega\right) = |a(\omega)|^2$. Person $A$ I guess has to put himself in his copies' shoes but argues the same way.

ETA: This is Sean Carroll's version anyway, typo.

The difficulty is in justifying why she would adjust her credences to be $|a(\omega)|^2$ in the non-uniform case. That's essentially what all proofs of the Born Rule in MWI attempt to do, but none manage it in a way that is considered generally convincing in the Foundations community.

If you want to look into the attempted proofs fall into three rough classes. Frequency type proofs offered by de Witt and others that build on the work of Everett. Proofs based on a certain type of invariance under swapping of environmental states due to Zurek and the proofs based on decision theoretic arguments by Wallace and Deutsch.

 

[PeterDonis]

If in this context consciousness is not a matter of the physical state of your brain, then MWI is not an interpretation of QM but rather philosophical theory that just as well could be incompatible with philosophical basis of science, don't you agree?

If consciousness is not a matter of the physical state of your brain, then no theory of physics can account for your conscious experience. But this has nothing to do with whether the MWI is an interpretation of QM or a different theory; that depends only on whether the MWI makes different experimental predictions from standard QM. As MWI is currently formulated, it doesn't.

 

[Minnesota Joe]

The difficulty is in justifying why she would adjust her credences to be $|a(\omega)|^2$ in the non-uniform case. That's essentially what all proofs of the Born Rule in MWI attempt to do, but none manage it in a way that is considered generally convincing in the Foundations community.

If you want to look into the attempted proofs fall into three rough classes. Frequency type proofs offered by de Witt and others that build on the work of Everett. Proofs based on a certain type of invariance under swapping of environmental states due to Zurek and the proofs based on decision theoretic arguments by Wallace and Deutsch.

Yeah, I think the frequentist approach has all the problems that frequentism has in probability theory. I don't know anything about the invariance approach, thanks! The last approach is what Hossenfelder complains about at the end of her video and what David Albert addresses in his interview with Sean Carroll.

Meanwhile it seems like Carroll argues that, in the non-uniform case, you are forced to make the $|a(\omega)|^2$ choice to prevent future branching from changing the probabilities on parallel branches.

 

[Minnesota Joe]

But you are still going to have to "collapse" the wave function you use to predict your own probabilities for future measurement results. You might say the other branches of the wave function exist, but you won't actually calculate as if they exist; you will calculate as if your own branch is the only one that exists, because that's the only one you will use in your calculations. And doing that is not compatible with the Schrodinger Equation, because there is no "collapse" in the SE. The SE does not tell you to calculate using only one branch. It tells you to calculate using all the branches.

In other words, when the MWI says "collapse is only apparent and there is no contradiction with the Schrodinger Equation", it doesn't really mean that literally; it can't, since taking it literally leads you to calculate things that are contrary to what you actually observe.

Hmm. It doesn't mean literally? You mean they would agree that really there is Schrodinger equation violating collapse? Because they don't seem to, but I could be misunderstanding them.

It seems to me there is a distinction between the universal wave function collapsing and a branch-bound observer ignoring other current, non-interacting branches in order to describe future branches. But I don't know. And it's making my head hurt. An example would be appreciated.

 

[DarMM]

Yeah, I think the frequentist approach has all the problems that frequentism has in probability theory.

It's problems are separate technical ones related to certain limiting operators not converging.

I don't know anything about the invariance approach, thanks! The last approach is what Hossenfelder complains about at the end of her video and what David Albert addresses in his interview with Sean Carroll.

Carroll's proof is in the same class as the invariance proofs. The decision theoretic proofs are a bit different.

Meanwhile it seems like Carroll argues that, in the non-uniform case, you are forced to make the $|a(\omega)|^2$ choice to prevent future branching from changing the probabilities on parallel branches.

It doesn't work out as even other Many Worlds advocates have noted. See here:
http://philsci-archive.pitt.edu/14389/

 

[PeterDonis]

It seems to me there is a distinction between the universal wave function collapsing and a branch-bound observer ignoring other current, non-interacting branches in order to describe future branches.

There is according to the MWI, yes: the MWI says the universal wave function never collapses, and it also says that an observer who observes a particular measurement result can ignore all other branches except his own. The MWI has to say that because it's the only way to make it consistent with experiment.

The problem Hossenfelder is pointing out, as I understand it, is that admitting the latter claim, that an observer can ignore all branches other than his own, means the former claim, that the universal wave function never collapses, is untestable by definition, which makes it unscientific.

 

[Minnesota Joe]

Carroll's proof is in the same class as the invariance proofs. The decision theoretic proofs are a bit different.

That helps!

It doesn't work out as even other Many Worlds advocates have noted. See here:
http://philsci-archive.pitt.edu/14389/

Ugh, what a mess. I'm not sure what to think about that or what should be granted or challenged in Sean's description.

I'm just trying to understand the outline of the MWI argument and the motivation right now.

The claim seems to be that the Schrodinger equation describes a branching structure that introduces an uncertainty in your epistemic situation so that it makes sense to apply elementary probability theory to the branch structure at some stage so that you can derive the Born Rule which would justify your claim to having derived the Born Rule from only the Schrodinger equation and simpler stuff that everyone agrees with.

Sound right?

 

[timmdeeg]

The problem Hossenfelder is pointing out, as I understand it, is that admitting the latter claim, that an observer can ignore all branches other than his own, means the former claim, that the universal wave function never collapses, is untestable by definition, which makes it unscientific.

Sabine Hossenfelder claims [s. the OP]:

To "evaluate the probability relative to the detector in one specific branch at a time" is "logically entirely equivalent to the measurement postulate."

But isn't this claim not just Kopenhagen view? And if yes, so what? On the other side this reasoning seems too simple, so how would you comment on that?

 

[DarMM]

I'm just trying to understand the outline of the MWI argument and the motivation right now.

The claim seems to be that the Schrodinger equation describes a branching structure that introduces an uncertainty in your epistemic situation so that it makes sense to apply elementary probability theory to the branch structure at some stage so that you can derive the Born Rule which would justify your claim to having derived the Born Rule from only the Schrodinger equation and simpler stuff that everyone agrees with.

That is roughly claim as it was in the late 1990s. Problems include the fact that to derive the branching structure you need the Born rule, so to take a branching structure as given and then use it to derive a Born rule is already quite circular.

Thus most MWI people now try to get the branch structure out without using the Born rule. This has not succeeded yet. Thus it's not yet an interpretation of QM strictly speaking as it is unknown if it can real replicate the predictions.

 

[Minnesota Joe]

That is roughly claim as it was in the late 1990s. Problems include the fact that to derive the branching structure you need the Born rule, so to take a branching structure as given and then use it to derive a Born rule is already quite circular.

Thus most MWI people now try to get the branch structure out without using the Born rule. This has not succeeded yet. Thus it's not yet an interpretation of QM strictly speaking as it is unknown if it can real replicate the predictions.

Perhaps my phrasing was poor, because I don't understand this. The only thing MWI has is the Schrodinger equation. It's just a real wave equation to them. The branching comes through entanglement and decoherence which are features of the Schrodinger equation and matter. It isn't necessary to assume probability at this stage, just interaction. So the branching structure doesn't assume the Born Rule.

 

[DarMM]

Perhaps my phrasing was poor, because I don't understand this. The only thing MWI has is the Schrodinger equation. It's just a real wave equation to them. The branching comes through entanglement and decoherence which are features of the Schrodinger equation and matter. It isn't necessary to assume probability at this stage, just interaction. So the branching structure doesn't assume the Born Rule.

Decoherence requires the Born rule, it's not purely a feature of the Schrodinger equation .