I Danger for the Many-Worlds Interpretation?

[Tendex]

I never said that, at all. I said it applies to field theories which obey the appropriate subset of the Haag-Kastler axioms. There's nothing forbidding renormalized theories from obeying the Haag-Kastler axioms. And for most field theories constructed their perturbation theory matches that of conventional "physicist" field theory.

You said "this concerns infinite volume QFTs with no cutoffs". There maybe nothing forbidding renormalized theories from obeying it but its proof doesn't include those. Or as you said it has nothing to do with the regularized theory.

 

[DarMM]

You said "this concerns infinite volume QFTs with no cutoffs". There maybe nothing forbidding renormalized theories from obeying it but its proof doesn't include those. Or as you said it has nothing to do with the regularized theory.

When you renormalize you impose the regularization, perform the subtractions and then remove the regularization. Thus it can apply to QFT after renormalization. There's nothing in its proof that excludes renormalized theories. In fact we know of renormalized theories that obey it.

 

[Tendex]

When you renormalize you impose the regularization, perform the subtractions and then remove the regularization. Thus it can apply to QFT after renormalization. There's nothing in its proof that excludes renormalized theories. In fact we know of renormalized theories that obey it.

Sure, it may but there is no proof for the relevant dimensions, and this is a physics forum where we should be talking about proofs with physical relevance, not just about proofs in nice theoretical constructions that seem to be most dear to you but that just might or might not have anything to do with something physical, and even less presenting them as if they definitely did.

 

[DarMM]

Sure, it may but there is no proof for the relevant dimensions

I already told you above there was, that Balaban has established Yang-Mills has the properties required for the proof to go through.
Perhaps you could say which Haag-Kastler axioms you think will fail for 4D QFTs and prevent the establishment of the theorem. Given that we already know Yang-Mills has Type III factors, what exact form are you expecting this failure to take? That on coupling with fermions a Type I structure is restored or something? (that would be provably impossible)

Since the fact that renormalized Yang-Mills has the structures required isn't enough for you, what is? What has to be true for the theorem to be relevant.

 

[Elias1960]

Assuming I understand what you mean by "additional structure" (macroscopic and microscopic subsystems) I would think that MWI could help themselves to it, just like everyone else. Do you disagree? Is this more profound a problem for MWI specifically?

Of course, there may be possibilities to solve these issues. But this would require to define what is necessary to start decoherence.

And these additional structures, subdivisions of the universe into different systems, have to be defined on the fundamental level. They cannot simply use laboratories, measurement devices, and so on, because these are not defined at the fundamental level. It could be something like, say, fermions are one system, bosons the other system. Given that the real observable particles are "dressed", thus, complex combinations of the fundamental, undressed, fermions and bosons, I doubt that one can start a reasonable theory of decoherence with this toy idea.

Whatever, they have to define the additional structures they need to use decoherence.

And this puts them into a different position. They can no longer argue against dBB theory that it needs a split of the phase space into configuration space and momentum space, while MWI can do everything with the wave function alone. They cannot.

 

[Quanundrum]

And this puts them into a different position. They can no longer argue against dBB theory that it needs a split of the phase space into configuration space and momentum space, while MWI can do everything with the wave function alone. They cannot.

This is what baffles me about MWI proponents. They tend to be a clever bunch, but somehow they have allowed the elegance of a naïve toy model MWI to persuade them to insist that the Born Rule, additional structure etc. are just "details". It reminds me a whole lot about the infatuation people had with the early versions of supersummetry pre-LHC. They were elegant, the rest were details, then reality said: "No, I'm not that simple"

 

[Minnesota Joe]

Of course, there may be possibilities to solve these issues. But this would require to define what is necessary to start decoherence.

And these additional structures, subdivisions of the universe into different systems, have to be defined on the fundamental level. They cannot simply use laboratories, measurement devices, and so on, because these are not defined at the fundamental level. It could be something like, say, fermions are one system, bosons the other system. Given that the real observable particles are "dressed", thus, complex combinations of the fundamental, undressed, fermions and bosons, I doubt that one can start a reasonable theory of decoherence with this toy idea.

Whatever, they have to define the additional structures they need to use decoherence.

I don't know enough about MWI or foundations to speak to whether or not they need to do this. I'm not sure I understand what you are saying they need to do, specifically.

And this puts them into a different position. They can no longer argue against dBB theory that it needs a split of the phase space into configuration space and momentum space, while MWI can do everything with the wave function alone. They cannot.

That isn't their main objection surely? From what I've read the main objection is the extra guidance equation and all the trouble it entails. (Whereas others would see that guidance equation as fixing a lot of problems, mind.)

Still, I'm also pretty skeptical that the MWI party line leans so heavily on the simplicity claim. Because it sure seems like they are making the interpretation a good deal more complex if nothing else. That's just moving propositions from one side of the balance sheet to the other and in a pretty angels-dancing-on-pinheads sort of way.

 

[bhobba]

It's not that there are no proposed solutions, but neither of the proposed solutions is generally accepted in the MWI community, let alone outside of the community.

Yes that is one of its big issues. I personally think it's just a simple matter of applying Gleason, and Wallace, in his book the Emergent Multiverse, although he gives the betting strategy argument, also proves a no contextuality theorem that makes Gleason harder to refute.

I actually do not like MW as its usually interpreted - that the different worlds are actually in some sense real - rather I like Gell-Mann's view - which is exactly what does real mean - he would prefer, as do I, treated on equal footing rather than real:


When you do that then MW is basically the same as decoherent histories. I actually learned a lot about Decoherent Histories from reading Wallace's book.

Thanks
Bill

 

[Quanundrum]

Yes that is one of its big issues. I personally think it's just a simple matter of applying Gleason, and Wallace, in his book the Emergent Multiverse, although he gives the betting strategy argument, also proves a no contextuality theorem that makes Gleason harder to refute.

I actually do not like MW as its usually interpreted - that the different worlds are actually in some sense real - rather I like Gell-Mann's view - which is exactly what does real mean - he would prefer, as do I, treated on equal footing rather than real:


When you do that then MW is basically the same as decoherent histories. I actually learned a lot about Decoherent Histories from reading Wallace's book.

Thanks
Bill

How is this not "MWI in permanent denial" though? You add an extra postulate of one world somehow magically being real, without any explanation of why that history is real while the others magically vanish?

 

[DarMM]

How is this not "MWI in permanent denial" though? You add an extra postulate of one world somehow magically being real, without any explanation of why that history is real while the others magically vanish?

It's just taking the probability assignments as simply that, the probability for that history to occur. Rather than some measure of the number of such worlds. The worlds no more vanish than the other dice outcomes "vanish" in a given roll.

 

[DarMM]

I personally think it's just a simple matter of applying Gleason, and Wallace, in his book the Emergent Multiverse, although he gives the betting strategy argument, also proves a no contextuality theorem that makes Gleason harder to refute

Having read Wallace his theorem is a very different beast to Gleason's theorem as far as I can tell. Can you expand on this a bit?

 

[bhobba]

Having read Wallace his theorem is a very different beast to Gleason's theorem as far as I can tell. Can you expand on this a bit?

See page 474 on the Everettian Inference Theorem. It uses as well as proves the POVM version of Gleason. But on reacquaintance with it, it may still rely on some of his decision theory arguments in previews chapters - it makes use of the Born Rule Theorem that I seem to remember uses it for example.

Thanks
Bill

 

[DarMM]

See page 474 on the Everettian Inference Theorem. It uses as well as proves the POVM version of Gleason. But on reacquaintance with it, it may still rely on some of his decision theory arguments in previews chapters - it makes use of the Born Rule Theorem that I seem to remember uses it for example.

Thanks
Bill

I've read it before and worked through it, but the assumptions are quite different as far as I can see. He uses a combination of assumptions about a Savage style decision theory, as well as complex dynamical and partitioning assumptions about macroscopic sectors of the Hilbert space to develop something close enough to contextuality to use a Gleason style proof.

However the whole orientation of the proof is quite different from Gleason. In Gleason we take the observable algebra as fundamental and derive the state as a probability assignment on it. In Wallace the state is fundamental and we develop a Savage decision theory for occupation in a macroscopic coarse graining of it. Mathematically the later part of Wallace's theorem simply looks somewhat like Gleason's theorem, but the motivation, background and the majority of the legwork of the proof are quite different.

At least as far as I can see.

 

[akvadrako]

It's just taking the probability assignments as simply that, the probability for that history to occur. Rather than some measure of the number of such worlds. The worlds no more vanish than the other dice outcomes "vanish" in a given roll.

That doesn't explain anything. In many worlds all the dice outcomes are real too; they don't vanish. At least that's a common assumption to work under when talking about a large multiverse.

 

[DarMM]

That doesn't explain anything. In many worlds all the dice outcomes are real too; they don't vanish. At least that's a common assumption to work under when talking about a large multiverse.

I'm not sure exactly what you mean, but @Quanundrum was asking about non-MWI views. What part doesn't explain anything, or what is it failing to explain?

In those views the other outcomes weren't present and then suddenly vanish, the quantum state is simply a type of probability assignment, so you don't have to explain how the unobserved outcomes vanished since you aren't saying that the state giving them a non-vanishing assignment implies they are present in some sense.

Unless I have you wrong. Are you saying when you roll a dice there are worlds out there where you rolled the other outcomes, i.e. there are worlds even for classical probability.

 

[Quanundrum]

It's just taking the probability assignments as simply that, the probability for that history to occur. Rather than some measure of the number of such worlds. The worlds no more vanish than the other dice outcomes "vanish" in a given roll.

The problem with that is that you don't explain how this world manifests or emerges from something ontological, not to mention that it is wildly indeterminist. If you are ok with antirealism and randomness being at the heart of reality that is of course fair, but... yeah, it's the two things most people into MWI disagree with the most.

 

[Demystifier]

I actually do not like MW as its usually interpreted - that the different worlds are actually in some sense real - rather I like Gell-Mann's view - which is exactly what does real mean - he would prefer, as do I, treated on equal footing rather than real:

Yes, I like that view. In that sense, Bohmian mechanics is MW theory too. A version of MW with a natural solution of the Born-rule problem.

 

[DarMM]

The problem with that is that you don't explain how this world manifests or emerges from something ontological, not to mention that it is wildly indeterminist. If you are ok with antirealism and randomness being at the heart of reality that is of course fair, but... yeah, it's the two things most people into MWI disagree with the most.

Yes of course, but we must strictly separate issues in order to properly discuss interpretations. Copenhagen like views don't have explain how the worlds vanish since they aren't saying there are all these worlds in the first place. They don't claim the wave-function is a real wave or something so they don't have to explain where parts of it went. They claim it's just a collection of expectations.

Now of course there is non-representationalism and indeterminism in Copenhagen and one might have a problem with them, but they are separate issues.

I should also say that most Copenhagen people acknowledge that a non-representational theory isn't what you would like fundamentally. However they just don't agree that QM is giving you a representational account, i.e. they'd also prefer a representational view but they simply think the mathematical structure of QM implies it isn't such an account.

 

[akvadrako]

I'm not sure exactly what you mean, but @Quanundrum was asking about non-MWI views. What part doesn't explain anything, or what is it failing to explain?

In those views the other outcomes weren't present and then suddenly vanish, the quantum state is simply a type of probability assignment, so you don't have to explain how the unobserved outcomes vanished since you aren't saying that the state giving them a non-vanishing assignment implies they are present in some sense.

Unless I have you wrong. Are you saying when you roll a dice there are worlds out there where you rolled the other outcomes, i.e. there are worlds even for classical probability.

I know it's not about MWI; it's the single-outcome case that contains an element of arbitrariness. Saying it's just like dice roles doesn't explain it because it doesn't tell you why one outcome is seen and not the others. You might say it depends on initial conditions, but then you have to explain why one initial condition is chosen over the others. The vanishing might be happening earlier, but it still needs to happen. The mystery is only removed if all initial conditions lead to the same outcome.

I guess it seems like you are suggesting that it should be taken for granted that classical probability makes sense, so when the results can be explained without resorting to a non-separable state, single outcomes start to make sense. But I would say it's not a big difference; before quantum probability people didn't think about the measurement problem as much because it wasn't as obvious.

 

[DarMM]

Saying it's just like dice roles doesn't explain it because it doesn't tell you why one outcome is seen and not the others

Of course, it's a probabilistic view. Thus there isn't an explanation given to the appearance of a given outcome. Nobody would deny the mystery you're pointing out here. What most people would argue is that you can't pass from wanting such an explanation to the claim that QM actually provides it, which in a sense would be the main difference between non-representational and representational views of QM.

You might say it depends on initial conditions

Nobody with a Copenhagen view would say this as the events are viewed as intrinsically indeterminate. It wouldn't even be compatible with how they view QM, as to them the initial conditions in QM are simply initial values for probabilities for measurement events not representational initial conditions for the system's intrinsic properties.

I guess it seems like you are suggesting that it should be taken for granted that classical probability makes sense

Classical probability is mathematically well-founded and well defined as a subset of measure theory. I think that is without question. So you must mean something else here by "makes sense".

Do you think classical probability must also be taken in a Many Worlds sense?

 

[zonde]

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.

The point is that MWI does not make experimental predictions. MWI predicts experiences of experimenter. But experiences of any person are subjective until he compares them with experiences of other people and extracts objective content out of his subjective experience.
In science we have a model and physical reality. We compare the two. And we assume that that the model is independent from physical reality i.e. the part of physical reality that does the modeling and compares it with observations is sort of separated from the part of physical reality that we are modeling and observing. Otherwise it becomes hopelessly circular.
What is prediction in MWI? Observation? Does not seem so. Conscious experience of of experimenter? Who then is registering this conscious experience of experimenter? Is it experimenter #2? So is then observations of experimenter #2 what is predicted? Or again it is conscious experience of of experimenter #2?

 

[akvadrako]

Do you think classical probability must also be taken in a Many Worlds sense?

I wouldn't say it has to, but I think it provides an explanation. If you have two worlds, an up world and a down world, or measures of infinite worlds with equal proportions, then assuming the situation is completely symmetrical, one can say what it means to give 50% probability of experiencing each. The probability is real; no vanishing is required.

But saying that some situation is the same as a dice role doesn't provide an explanation in the same sense. It's only helpful if you find the randomness in a die role unproblematic.

 

[Minnesota Joe]

I actually do not like MW as its usually interpreted - that the different worlds are actually in some sense real - rather I like Gell-Mann's view - which is exactly what does real mean - he would prefer, as do I, treated on equal footing rather than real:

How do one of the branches treated on equal footing end up being actual on Gell-Mann's view?

 

[Minnesota Joe]

But saying that some situation is the same as a dice role doesn't provide an explanation in the same sense. It's only helpful if you find the randomness in a die role unproblematic.

I don't understand why anyone would ever think it unproblematic: given identical initial conditions there were 6 possible outcomes and nothing explains why you rolled a 1 instead of 2-6. It's just a brute fact. That's a very strong claim.

 

[DarMM]

How do one of the branches treated on equal footing end up being actual on Gell-Mann's view?

According to this view it isn't explained/ lies outside QM. Roland Omnès's book "The Interpretation of Quantum Mechanics" discusses it more particularly in the final chapter.

 

[PeterDonis]

The point is that MWI does not make experimental predictions. MWI predicts experiences of experimenter.

So does every other interpretation of QM, if you want to take this viewpoint. MWI is no different in this respect from other interpretations.

What is prediction in MWI? Observation? Does not seem so.

Why not?

 

[Quanundrum]

According to this view it isn't explained/ lies outside QM. Roland Omnès's book "The Interpretation of Quantum Mechanics" discusses it more particularly in the final chapter.

How is this then different from "Shut up and calculate" ?

 

[DarMM]

How is this then different from "Shut up and calculate" ?

You need to expand on exactly what you mean. "Shut up and calculate" is a short pithy phrase that David Mermin used and later retracted.

Saying that the quantum formalism doesn't make a deterministic statement about which outcome will occur is not the same as making no comment about nature or the formalism at all. So I wouldn't say they are the same.

 

[bhobba]

How do one of the branches treated on equal footing end up being actual on Gell-Mann's view?

What was it Newton said - Hypotheses non fingo (Latin for "I feign no hypotheses", "I frame no hypotheses", or "I contrive no hypotheses"). It was used by Isaac Newton in an essay, "General Scholium", which was appended to the second (1713) edition of the Principia.

This is the interpretation game. You pick the thing that worries you and that decides the interpretation you prefer. The question to ask is why does it worry you? Einstein initially worried about it, but eventually he put his finger on what really worried him - lack of objective reality as suggested by implications of entanglement. I have recently read a paper on the history of science that suggests complementarity was really entanglement, but obscurely expressed (Bohr was well known for being obscure and he also was a famous mumbler). So one could say that the real issue between Einstein and Bohr was how to view entanglement. Bohr thought it was a general feature of nature and was simply the way it is, Einstein was deeply worried to the point he thought QM incomplete.

Thanks
Bill

 

[bhobba]

How is this then different from "Shut up and calculate" ?

Shut up and calculate does not worry about the issue of how to define observation, measurement or whatever you want to call it, in a purely quantum way.

Thanks
Bill

 

[Minnesota Joe]

According to this view it isn't explained/ lies outside QM. Roland Omnès's book "The Interpretation of Quantum Mechanics" discusses it more particularly in the final chapter.

What was it Newton said - Hypotheses non fingo (Latin for "I feign no hypotheses", "I frame no hypotheses", or "I contrive no hypotheses"). It was used by Isaac Newton in an essay, "General Scholium", which was appended to the second (1713) edition of the Principia.

In the video Gell-Mann dismisses as meaningless the idea that two branches are "equally real". Unfortunately he doesn't justify this with anything except incredulity there. So he seems to have at least that much of a hypothesis: before branching things are on the same footing and after branching, there is one actuality.

Do you know the missing argument? Why is it meaningless to talk about two real branches?

This is the interpretation game. You pick the thing that worries you and that decides the interpretation you prefer. The question to ask is why does it worry you? Einstein initially worried about it, but eventually he put his finger on what really worried him - lack of objective reality as suggested by implications of entanglement. I have recently read a paper on the history of science that suggests complementarity was really entanglement, but obscurely expressed (Bohr was well known for being obscure and he also was a famous mumbler). So one could say that the real issue between Einstein and Bohr was how to view entanglement. Bohr thought it was a general feature of nature and was simply the way it is, Einstein was deeply worried to the point he thought QM incomplete.

I have at least two authors telling me how charismatic Bohr was, so it cracked me up to learn he was a mumbler. And Einstein didn't work well with others. Geniuses, what are you going to do?

Anyway, I know the EPR paper stresses those things, but apparently Einstein thought that the non-local aspect of entanglement was the most important issue. He thought Podolsky, who was responsible for the writing of the paper, made a mess of it and they reportedly had a falling out after.

 

[bhobba]

In the video Gell-Mann dismisses as meaningless the idea that two branches are "equally real".

I just had a look at the video again. What he says is what does equally real mean because real is a rather vague term. He prefers on equal footing except for probabilities. Real is one of those terms that is very difficult to pin down exactly - philosophers have been arguing what it is for centuries with no progress as far as I can see. Gell-Mann was just trying to remove that vagueness. While I can't think off hand of another way to do it, a lot of interpretation stuff is just personal preference - Gell-Mann does not like real so replaces it with on equal footing which he prefers - as do I.

Yes the mannerisms of some famous scientists is an interesting topic. I hadn't heard Einstein didn't work well with others though. But gleaning things from recollection, especially others, is fraught with danger as I was reminded again only today. I answered a question on another forum on a topic I have no direct experience with. It was a story someone told me years ago at work - of course I made that clear in the post. One reply was - and today on things that never happened. From his profile he knew a lot more about the subject than me so I would say he is likely right. Human recollection can be quite flawed, especially if its second or third hand, so its best to try and avoid doing what I did, but sometimes its hard to resist. I am sure most have done the exercise on you tell someone in a class something, and they pass it on to the next person and so on. Then the last person repeats what they were told - and it often bears no relation to what the first person was told. For Einstein my reference is Pais - Subtle is the Lord - but even that is fraught with the recollection issue.

Thanks
Bill

 

[DarMM]

In the video Gell-Mann dismisses as meaningless the idea that two branches are "equally real". Unfortunately he doesn't justify this with anything except incredulity there. So he seems to have at least that much of a hypothesis: before branching things are on the same footing and after branching, there is one actuality.

Do you know the missing argument? Why is it meaningless to talk about two real branches?

If you look at his papers and those of others who've worked on similar problems like Omnès or people who work in quantum information it's just basically that QM is probabilistic. It treats them all equally in the sense that each history is assigned a probability of occurrence, but only one history actually occurs. This is no different in a sense to how brownian motion or any other stochastic process treats all paths on equal footing, but only one actually occurs.

He thought Podolsky, who was responsible for the writing of the paper, made a mess of it and they reportedly had a falling out after.

This is because Podolsky leaked details of the paper to the popular press.

 

[DarMM]

But saying that some situation is the same as a dice role doesn't provide an explanation in the same sense.

Again Copenhagen people wouldn't claim it does, they say QM doesn't provide a mechanism for how the history is selected. It's not that they're saying this is an explanation or satisfying in some way, they're saying the quantum formalism doesn't give such an explanation.

 

[zonde]

The point is that MWI does not make experimental predictions. MWI predicts experiences of experimenter.

So does every other interpretation of QM, if you want to take this viewpoint. MWI is no different in this respect from other interpretations.

In other interpretations outcome of experiment is objective physical fact independent from experiences of experimenter. This is no so in MWI, in MWI outcome is subjective. That's the difference.

What is prediction in MWI? Observation? Does not seem so.

Why not?

MWI predicts that every possible outcome happens. In order to mach what is observed it predicts that experimenter observes only one of the possible outcomes. So outcome of experiment does not stand on it's own, you have to include experimenter into model and certain relations between outcomes and experimenter. Only then you get what's observed.

 

[Minnesota Joe]

If you look at his papers and those of others who've worked on similar problems like Omnès or people who work in quantum information it's just basically that QM is probabilistic. It treats them all equally in the sense that each history is assigned a probability of occurrence, but only one history actually occurs. This is no different in a sense to how brownian motion or any other stochastic process treats all paths on equal footing, but only one actually occurs.

That just sounds like a Copenhagen Interpretation. Now with more decoherence.

 

[Minnesota Joe]

In other interpretations outcome of experiment is objective physical fact independent from experiences of experimenter. This is no so in MWI, in MWI outcome is subjective. That's the difference.

MWI predicts that every possible outcome happens. In order to mach what is observed it predicts that experimenter observes only one of the possible outcomes. So outcome of experiment does not stand on it's own, you have to include experimenter into model and certain relations between outcomes and experimenter. Only then you get what's observed.

This seems to be what Sabine Hossenfelder is complaining about in the OP. She is saying that the measurement problem is reintroduced by the prescriptive, non-mathematical part of the theory--the part that attempts to account for our experiences.

 

[DarMM]

That just sounds like a Copenhagen Interpretation. Now with more decoherence.

It is basically the Copenhagen interpretation, but expanded to include sequences of events, i.e. histories, rather than singular experiments alone, and a more precise notion of "classicality". I'm not sure what you mean by "now with more decoherence", decoherence is involved but it's only an aspect of the expansion of Copenhagen. One also has the phase cell analysis for the demonstration that classical logic is a subset of QM. This demonstration is independent of decoherence and there is much more.

I didn't mention decoherence above, so I could be misunderstanding your remark as I'm not sure of the context.

 

[Minnesota Joe]

It is basically the Copenhagen interpretation, but expanded to include sequences of events, i.e. histories, rather than singular experiments alone, and a more precise notion of "classicality". I'm not sure what you mean by "now with more decoherence", decoherence is involved but it's only an aspect of the expansion of Copenhagen. One also has the phase cell analysis for the demonstration that classical logic is a subset of QM. This demonstration is independent of decoherence and there is much more.

I didn't mention decoherence above, so I could be misunderstanding your remark as I'm not sure of the context.

Not your fault, but mine and multiple conversations. Decoherent histories was being compared favorably to MWI earlier in the thread, so I was just trying to determine what it explains.

 

[zonde]

This seems to be what Sabine Hossenfelder is complaining about in the OP. She is saying that the measurement problem is reintroduced by the prescriptive, non-mathematical part of the theory--the part that attempts to account for our experiences.

In her video Sabine replaced experimenter with detector and that way made MWI philosophically more acceptable but less elegant and therefore unacceptable for MWI proponents.
But anyways there have to be collapse at some point where we say: from this point onward we will consider only one outcome. If this point happens to be within MWI model then MWI looses it's main selling point - no collapse (Sabine's argument). If it's outside MWI model then MWI does not produce objective predictions (my argument).

 

[Elias1960]

AFAIU about an interpretation which is name "consistent histories":

It is basically the Copenhagen interpretation, but expanded to include sequences of events, i.e. histories, rather than singular experiments alone, and a more precise notion of "classicality". ... One also has the phase cell analysis for the demonstration that classical logic is a subset of QM.

If it contains statements that are in conflict with classical logic, it should be renamed "inconsistent histories" and thrown away.

If there is something in the mathematical apparatus worth not to be thrown away, one should reformulate it in a logically consistent way. The key would be to follow Bell's paper "Against measurement" and stop to name something a measurement which depends not only on the system itself but also on the context.

 

[PeterDonis]

In other interpretations outcome of experiment is objective physical fact independent from experiences of experimenter. This is no so in MWI, in MWI outcome is subjective. That's the difference.

No, in MWI the outcome is not subjective. In each branch, all observers agree on what the outcome of a particular experiment was.

The difference between MWI and other interpretations is that in the MWI, there is not a single result of a measurement; instead, all possible results happen, each in a different branch.

MWI predicts that every possible outcome happens.

In order to mach what is observed it predicts that experimenter observes only one of the possible outcomes.

No, it doesn't. It predicts that the copy of the experimenter in each branch observes only one outcome. But none of those copies is "the same" as the experimenter before the measurement, so you can't say that any of the outcomes is "the" outcome that "the" experimenter observed. That's the difference between the MWI and other interpretations; in other interpretations, you can say that "the" experimenter observed just one outcome. In MWI, you can't.

outcome of experiment does not stand on it's own, you have to include experimenter into model and certain relations between outcomes and experimenter. Only then you get what's observed.

Including the relations between the experimenter and the system being experimented on has to be done in any interpretation of QM; it's part of the minimal "shut up and calculate" math of QM.

I can see a way of making "prediction is not observation" true in the MWI, but it's not quite the way you are describing. In the MWI, if we look at the experimenter subsystem as a quantum system, not just one particular branch but the degrees of freedom in that subsystem as a whole, then after the experiment, that subsystem does not have a well-defined state at all, because it is entangled with the system that was experimented on. (The system that was experimented on doesn't have a well-defined state either.) That's what MWI predicts. But that means that, considering the system as a whole, not just one particular branch in isolation, there is no "observation"--the subsystem that is supposed to "observe" is not in a well-defined state at all, so it can't have "observed" anything since that requires it to be in a well-defined state.

 

[Minnesota Joe]

I just had a look at the video again. What he says is what does equally real mean because real is a rather vague term. He prefers on equal footing except for probabilities. Real is one of those terms that is very difficult to pin down exactly - philosophers have been arguing what it is for centuries with no progress as far as I can see. Gell-Mann was just trying to remove that vagueness. While I can't think off hand of another way to do it, a lot of interpretation stuff is just personal preference - Gell-Mann does not like real so replaces it with on equal footing which he prefers - as do I.

Except Gell-Mann didn't say vague. He said "equally real" has no useful meaning. A vague term can still have useful meaning. What is meant by a "pile of clothes" is vague, because who knows what a 'pile' exactly is, but I understand it well enough to step over it.

I was just curious what he was on about there, because it sounded rather dismissive.

 

[A. Neumaier]

the copy of the experimenter in each branch observes only one outcome. But none of those copies is "the same" as the experimenter before the measurement, so you can't say that any of the outcomes is "the" outcome that "the" experimenter observed.

This would be strange since then it would be impossible to have identity across measurements for anything, even in the branch that we actually observe...

 

[PeterDonis]

This would be strange since then it would be impossible to have idenitty across measurements for anything, even in the branch that we actually observe...

Your "the branch that we actually observe" is a misuse of language when describing the MWI. There is no single "we" after the measurement, because "we" branch along with everything else. After the measurement there are multiple copies of "we", and each copy observes a different measurement result.

As I have said in previous discussions of the MWI, it's hard even to talk about the MWI using ordinary language, because ordinary language has assumptions built into it that the MWI violates.

 

[bahamagreen]

How short is the period of time for a world between MWI branching?

Is it perhaps universally short enough (sub Planck time) that the individual worlds would never have a history, just dt, like an initial condition?

 

[zonde]

I can see a way of making "prediction is not observation" true in the MWI, but it's not quite the way you are describing. In the MWI, if we look at the experimenter subsystem as a quantum system, not just one particular branch but the degrees of freedom in that subsystem as a whole, then after the experiment, that subsystem does not have a well-defined state at all, because it is entangled with the system that was experimented on. (The system that was experimented on doesn't have a well-defined state either.) That's what MWI predicts. But that means that, considering the system as a whole, not just one particular branch in isolation, there is no "observation"--the subsystem that is supposed to "observe" is not in a well-defined state at all, so it can't have "observed" anything since that requires it to be in a well-defined state.

Ok, you have doubts that MWI has well defined "observer" at all. But let's assume we can speak about "observers" in MWI.

No, it doesn't. It predicts that the copy of the experimenter in each branch observes only one outcome. But none of those copies is "the same" as the experimenter before the measurement, so you can't say that any of the outcomes is "the" outcome that "the" experimenter observed. That's the difference between the MWI and other interpretations; in other interpretations, you can say that "the" experimenter observed just one outcome. In MWI, you can't.

This seems like terminology issue. I call a "copy of the experimenter" in single branch an "experimenter". And it stands to reason. Experimenter observes single outcome - that's the claim. So the copy of experimenter observing particular outcome is distinct from another copy of experimenter that observes different outcome - these copies should be called distinct "observers".

No, in MWI the outcome is not subjective. In each branch, all observers agree on what the outcome of a particular experiment was.

The difference between MWI and other interpretations is that in the MWI, there is not a single result of a measurement; instead, all possible results happen, each in a different branch.

So we have different groups of observers that observe the same outcome withing the group. Observers from one group can't communicate with observers from different group. And we call these groups of observers "branches". That way for all observers it appears that their observations are objective.

But then what is prediction? It's not that particular outcome happens objectively. Prediction is that group of observers observe the same outcome. So we have prediction and we can go on to the next step - test this prediction.
Who is going to be the experimenter and what measurements he is going to perform to observe this "group of observers observing the same outcome"? And there is the problem - we don't have any superobservers that are not already included into prediction. We can't compare this prediction with reality in a theory neutral way. MWI is modeling it's own confirmation. So it's not scientific theory, it is philosophical theory (we don't test philosophical models against anything - we just embrace them if we consider them useful or ignore them if we consider them useless).

Including the relations between the experimenter and the system being experimented on has to be done in any interpretation of QM; it's part of the minimal "shut up and calculate" math of QM.

I don't follow you there. Where in the math is that relation?

 

[PeterDonis]

you have doubts that MWI has well defined "observer" at all

No, I'm just pointing out that you can't use the word "observer" the way it is usually used when talking about the MWI. The same goes for many other ordinary language words, as I have already pointed out.

let's assume we can speak about "observers" in MWI.

No, you can't just "assume" this if you mean by "observer" what is usually meant, because the usual meaning of "observer" makes assumptions that the MWI violates. So "assuming" that you can just speak normally about observers in the MWI means you are being inconsistent.

I call a "copy of the experimenter" in single branch an "experimenter".

Yes, but you can't call the copy "the" experimenter because there isn't just one of them after the measurement. Nor can you say anything about "experimenters" that assumes that there is only one of them after the measurement, or that one particular copy is "the same" as the experimenter before the measurement and the other ones aren't. Doing any of those things means you're being inconsistent.

the copy of experimenter observing particular outcome is distinct from another copy of experimenter that observes different outcome - these copies should be called distinct "observers".

Ok, this is fine. But then you actually have to do it and accept the implications. See below.

what is prediction? It's not that particular outcome happens objectively. Prediction is that group of observers observe the same outcome.

With the above meaning of "observers", so that "group of observers" means "the copies of a group of observers that are all in the same branch", yes.

Who is going to be the experimenter and what measurements he is going to perform to observe this "group of observers observing the same outcome"?

It doesn't matter; all of the observers in the group (in a particular branch, since that's what we're using "observers" and "group" to mean) can communicate with each other (since they're all in the same branch) and verify that they all observed the same outcome. This communication is a classical process and doesn't require any quantum branch points anywhere, so it doesn't raise any quantum measurement issue.

we don't have any superobservers that are not already included into prediction.

We don't need any. See above.

We can't compare this prediction with reality in a theory neutral way.

Yes, we can, for this particular prediction. But that's because you restricted this particular prediction to one branch. The kinds of predictions of the MWI we can't compare with reality in a theory neutral way are predictions about the relative weights of multiple branches (or, for that matter, the existence of multiple branches).

Where in the math is that relation?

In your choice of the measurement operator.

 

[Morbert]

Decoherent histories was being compared favorably to MWI earlier in the thread, so I was just trying to determine what it explains.

You might find this additional video by Gell-Mann useful for seeing what DH potentially brings to the table

https://www.webofstories.com/people/murray.gell-mann/163?o=SH
DH is very closely related to Copenhagen, but it ventures where Copenhagen dares not.

 

[Minnesota Joe]

You might find this additional video by Gell-Mann useful for seeing what DH potentially brings to the table

https://www.webofstories.com/people/murray.gell-mann/163?o=SH
DH is very closely related to Copenhagen, but it ventures where Copenhagen dares not.

Definitely interesting. Thanks! Ha! Copenhagen "sort of strains credulity...it's not a convincing fundamental presentation". That's putting it mildly.