A MWI and the entangled photon experiment

[kered rettop]

[Moderator's note: Thread spun off from previous discussion of the experiment due to focusing on the MWI as an interpretation, so discussion belongs in the interpretations subforum. Previous thread is linked to below.]

https://www.physicsforums.com/threa...photons-split-into-2-different-paths.1055786/

To be clear about path length: changing length will never change the quantum statistical prediction. The coincidence probability depends on the relationship between the A and B pairs, but does not depend in any way on time ordering or path length. Consequently, as @DrClaude stated, lengthening one path to make it appear that one photon is measured before the other does not change anything.

The effect you refer to as retrocausal is an artifact of trying to describe what happens as if one element of the setup is the cause of something else in the setup. To do that, you must rely on a particular interpretation of QM. Experimentally, you cannot distinguish cause from effect when asking: does measurement of A cause/change the outcome of B, or does measurement of B cause/change the outcome of A? These scenarios cannot be distinguished.

It is also an artifact of assuming that observations are definite. Under MWI, for instance, every measurement results in an indefinite outcome, with different outcomes occurring in different branches. No need for A and B to influence each other at all, let alone with retrocausality.

 

[DrChinese]

It is also an artifact of assuming that observations are definite. Under MWI, for instance, every measurement results in an indefinite outcome, with different outcomes occurring in different branches. No need for A and B to influence each other at all, let alone with retrocausality.

Not sure I follow your thinking here. Experiments have definite outcomes, even were there MWI branching and there were a different outcome in another branch.

Also: There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.*Local versions, of course. Some see MWI as non-local.

 

[kered rettop]

Not sure I follow your thinking here. Experiments have definite outcomes, even were there MWI branching and there were a different outcome in another branch.

I was saying that the global outcome is a superposition so it's not definite.

Also: There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.

MWI does predict the existence of such worlds. But they are very rare so their frequency is vanishingly small.

*Local versions, of course. Some see MWI as non-local.

Well interactions are certainly local in MWI. The non-locality appears to be what I think you referred to as "contextual" recently - for example, Alice and Bob's settings. The debate would therefore hang on how world-splitting depends on the (quantum) context. I don't know whether your discussion was ever resolved, but I was very glad to see it as it has considerably dampened my enthusiasm for MWI.

 

[PeterDonis]

There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.

This is not correct. We already went over this in great detail in another thread. The MWI does not say this and does not work this way. We ended up agreeing that those who claim that the MWI is "local" are not correct, but we did not end up agreeing that the MWI cannot explain the correlations. Its explanation is not "local", but that doesn't mean it isn't valid.

 

[PeterDonis]

MWI does predict the existence of such worlds.

Only if the alignment between the distant measuring devices is not perfect. In the idealized case where the alignment is perfect, the MWI predicts that there are no worlds in which the perfect correlation is not present. This was discussed in great detail in another thread.

 

[PeterDonis]

interactions are certainly local in MWI.

Yes, but the wave function is not; it explicitly includes entangled degrees of freedom that are spatially separated, and a measurement on one of those degrees of freedom instantaneously updates the entangled wave function, which affects all of the entangled degrees of freedom. This was discussed in great detail in another thread.

 

[PeterDonis]

The previous thread I referred to is this one:

https://www.physicsforums.com/threads/is-the-mwi-local.1057817/

 

[kered rettop]

Only if the alignment between the distant measuring devices is not perfect. In the idealized case where the alignment is perfect, the MWI predicts that there are no worlds in which the perfect correlation is not present. This was discussed in great detail in another thread.

OK. I'd overlooked the perfect alignment. But in that case, I can't make any sense of what DrChinese says next: "Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly." How can an outcome occur at all if it has precisely zero amplitude?

 

[kered rettop]

The previous thread I referred to is this one:

https://www.physicsforums.com/threads/is-the-mwi-local.1057817/

Great, that's what I was looking for. Thanks.

 

[PeterDonis]

in that case, I can't make any sense of what DrChinese says next

Yes, that's why I referred to the previous thread.

 

[PeterDonis]

all outcomes occur randomly

I'm commenting on this separately because I can't remember how much it was discussed in the previous thread. This statement is wrong independently of the other issues I referred to before: there is no randomness of outcomes in the MWI. The MWI is deterministic: all outcomes deterministically occur. MWI proponents have various proposals to make sense of some concept of "probability" in the MWI (I don't think any of them succeed, but that's a different discussion), but none of those concepts of "probability" for the MWI involve any randomness about which outcomes occur. All outcomes always occur in the MWI.

 

[DrChinese]

This is not correct. We already went over this in great detail in another thread. The MWI does not say this and does not work this way. We ended up agreeing that those who claim that the MWI is "local" are not correct, but we did not end up agreeing that the MWI cannot explain the correlations. Its explanation is not "local", but that doesn't mean it isn't valid.

I had a * beside MWI specifically to avoid this comment from you. I failed, obviously...

 

[kered rettop]

Yes, that's why I referred to the previous thread.

<irony>Well, I'll have a good look at it then, and let you know who's right. </irony>

 

[DrChinese]

...there is no randomness of outcomes in the MWI. The MWI is deterministic: all outcomes deterministically occur. MWI proponents have various proposals to make sense of some concept of "probability" in the MWI (I don't think any of them succeed, but that's a different discussion), but none of those concepts of "probability" for the MWI involve any randomness about which outcomes occur. All outcomes always occur in the MWI.

You say tomato, I say tomato. What branch we exist in now is purely random, there really is no denying that point. So what if all branches allegedly occur? I can't begin to call that deterministic. And I say there is no way to wind any quantum system evolution backward to the past or forward to the future from the branch we are in now. Apparently, "something" discontinuous happens to split worlds, whether you call it "collapse" or something else. You obviously cannot see that in the past of a particle, nor can you see it in the future of any particle. How many papers* have been written explaining that it is meaningless to discuss what happens in the quantum world when no one is looking?

There are articles such as Vaidman's 2014 (a great paper, by the way, regardless of your viewpoint) that argue (exactly as you say) that MWI is both local and deterministic, and there is no randomness. I don't find it convincing, but it is almost an encyclopedia of Interpretations and No-Go's.*Such as Vaidman 2013 (different paper than above) who also said this, for example: "Quantum mechanics does not provide a clear answer to the question: What was the past of a photon which went through an interferometer ...we conclude that the past of the photons is not represented by continuous trajectories, although a “common sense” analysis adopted in various welcher weg measurements, delayed-choice which-path experiments and counterfactual communication demonstrations yields a single trajectory."

 

[PeterDonis]

I had a * beside MWI

Which means what? The MWI is what it is. There is no asterisked version of it that works differently.

 

[DrChinese]

<irony>Well, I'll have a good look at it then, and let you know who's right. </irony>

Read the Vaidman 2014 paper also. Although I disagree strongly with his final conclusion(s)*, he covers the pros and cons better than anything I've seen anywhere else. It's 25 pages, and 175 references!

*"The theory of Universal wave function is deterministic, local, free of paradoxes, and fully consistent with
our experience."

 

[PeterDonis]

What branch we exist in now is purely random

Wrong. "We" exist in all branches.

There may be other interpretations that say something along the lines of "all of the branches exist mathematically, but which one becomes real and we end up in is randomly chosen". But the MWI does not say that. If you want to use one of those other interpretations instead of the MWI, that's fine, but then please correctly describe which interpretation you are actually using.

there really is no denying that point.

Yes, there is. You are making repeated wrong claims about what the MWI says.

So what if all branches allegedly occur?

There is no "allegedly". They all occur, because they are all in the wave function.

I can't begin to call that deterministic.

In the MWI, the wave function's time evolution is unitary, always, all the time. Unitary evolution is deterministic. That is just a mathematical fact. If you are using an interpretation that does not say this, whatever it is, it is not the MWI.

There are articles such as Vaidman's 2014 (a great paper, by the way, regardless of your viewpoint) that argue (exactly as you say) that MWI is both local and deterministic, and there is no randomness.

You misdescribe the paper. Vaidman is not arguing about what the MWI is; he is just stating what the MWI is. The only caveat he does not mention is that his usage of the term "local" is limited: it does not mean the wave function is local (it's not), it just means that individual interactions, which only operate on degrees of freedom that are spatially co-located, are local.

 

[PeterDonis]

deterministic, and there is no randomness

And these two things are obvious, simple consequences of the basic premises of the MWI: that the wave function is real and contains all of reality, and that the wave function's time evolution is unitary, all the time. While there are different presentations and formulations of the MWI in the literature, and different proponents might disagree about some particular fine points, all of them agree on those two premises as the fundamental basis of the MWI.

 

[DrChinese]

Which means what? The MWI is what it is. There is no asterisked version of it that works differently.

I don't think MWI can be considered local (the asterisk was to denote claims of locality in MWI). For it to make any sense, IMHO, there must be nonlocality of some kind. Anyone can claim anything about an interpretation, but at some point it needs to be reconciled with experiment. If someone says it is local, there is the issue of how to explain the thousands of experiments on nonlocality - entanglement swapping for starters.

I have referenced Vaidman's strong defense of MWI as local and deterministic with a comprehensive paper he wrote. I will quote the entirety of his discussion on entanglement swapping (actually quantum teleportation):

"Spatially separated entangled particles, through local interaction with macroscopic objects, create worlds with nonlocal correlations. Measurement of one particle of the EPR pair changes nothing for the other particle if it is considered in the Universe, but it creates worlds with definite spin of a remote particle.
"If the EPR pair is used for teleportation, the Bell measurement in one site creates four worlds with the quantum state teleported to the second particle of the pair and rotated in four definite ways. The mixture of these four states corresponds to a completely unpolarized density matrix, the description of the particle of an undisturbed EPR pair. Thus again, from the point of view of the Universe, no change in the second EPR particle took place. "

If that's not hand-waving, I don't know what is. We have previously discussed that an experimenter can freely choose to entangle (i.e. nonlocally change the state of) remote systems. Vaidman apparently acknowledges this, saying: "Entanglement is the essence of the nonlocality of the Universe." So he says this, and then concludes MWI is local at the end of the paper.

My point is that MWI makes some sense as an interpretation when you consider the idea that there is a Universal wave function, but... there must still be nonlocal elements. Just saying its "local" isn't enough in today's world. We've come too far for that.

 

[kered rettop]

I'm commenting on this separately because I can't remember how much it was discussed in the previous thread. This statement is wrong independently of the other issues I referred to before: there is no randomness of outcomes in the MWI. The MWI is deterministic: all outcomes deterministically occur. MWI proponents have various proposals to make sense of some concept of "probability" in the MWI (I don't think any of them succeed, but that's a different discussion), but none of those concepts of "probability" for the MWI involve any randomness about which outcomes occur. All outcomes always occur in the MWI.

Making sense of probability - I still don't understand the problem. It seems to me that there is a rather simple solution which anyone here should be able to follow without my having to find a reputable reference to support it. Is it possible to discuss the matter without incurring the wrath of the mods?

 

[Nugatory]

all outcomes deterministically occur.

Agree about the determinism, but I would be more comfortable saying something along the lines of “all outcomes are deterministically in the wave function“ instead because that’s as far as the math (deterministic evolution of the state) will take us. "Occurs" carries much more ontological baggage - there's a big gap between a wave function with post-decoherence amplitude peaks for each outcome and what has occurred in the operational sense of the word.

 

[PeterDonis]

I don't think MWI can be considered local (the asterisk was to denote claims of locality in MWI).

Ok. Yes, we agreed in the previous thread that neither of us could see how the MWI could be completely local. Interactions are, but the wave function is not.

 

[PeterDonis]

It seems to me that there is a rather simple solution which anyone here should be able to follow without my having to find a reputable reference to support it.

If there is such a simple solution, it should already be in the literature. People have been discussing this topic for decades. There is plenty of literature on how to make sense of probabilities in the MWI. Please take the time to work through it.

 

[PeterDonis]

I would be more comfortable saying something along the lines of “all outcomes are deterministically in the wave function“ instead because that’s as far as the math (deterministic evolution of the state) will take us. "Occurs" carries much more ontological baggage

Sure, but the baggage is the same whether you say that all outcomes occur (which is the MWI) or that only one occurs and which one it is is random (other interpretations that are not the MWI). So I don't see this as a reason not to say "all outcomes occur" in the MWI. You have the same problem saying that just one outcome occurs in, say, the Copenhagen interpretation, and nobody shies away from saying that an outcome occurs in the Copenhagen interpretation and insists on saying "there is an outcome in the wave function" instead.

 

[DrChinese]

1. "We" exist in all branches.

2. You mis-describe the paper. Vaidman is not arguing about what the MWI is; he is just stating what the MWI is. The only caveat he does not mention is that his usage of the term "local" is limited: it does not mean the wave function is local (it's not), ...

3. ...it just means that individual interactions, which only operate on degrees of freedom that are spatially co-located, are local.

1. That's the hypothesis.

But there are no other "we's" to confirm that hypothesis, nor is there the slightest evidence to indicate anything other than we live in a random branch - if there are other branches.

Every single experiment, every day of every week, says the outcomes of all quantum experiments are random. There is no experimental evidence to the contrary. To describe that as "deterministic" is a gross mischaracterization of the word. Even considering that the universal wave function evolves deterministically, I don't see how you describe the quantum world as deterministic. Obviously, there is supposed to be branching - and how can that be called pre-determined?2. Of course he is arguing for MWI. And I think it is a pretty good defense at that. He deftly compares it to some of the most important alternative ideas, and he goes through a pretty long list of quantum phenomena. Very comprehensive, and fairly up to date.3. I'm not sure I agree that "individual interactions, which only operate on degrees of freedom that are spatially co-located, are local." I mean, I guess that's technically true if you parse it a certain way. But there is certainly no strict requirement that individual quantum interactions only operate on properties (degrees of freedom) that are spatially co-located. Elements of a general quantum interaction need no specific locale, and may or may not be co-located.

One of Vaidman's many specialties is systems where there are quantum interactions that are NOT spatially co-located. That's indicated in his prolific work with Mach-Zehnder systems, specifically his work on weak measurements - and of course there's his bomb tester.

I certainly wouldn't call an individual interaction "here" (such as Bell State Measurement) that changes a system far distant elsewhere "local" anyway. Where would you even begin to localize such interaction? Let's start with the Bell State Measurement apparatus itself. A Bell State Measurement occurs in a system of 3 optical splitters and 4 detectors (let's ignore the experimenter for now). Those 7 components can be located anywhere, miles apart even. So when and where is there a "local interaction"? Yet again, the relevant quantum context is completely nonlocal. None of the component events need to occur simultaneously (except for a small area in one splitter where there is overlap to induce indistinguishability). But the selection of the Bell State (1 of the possible 4) is done by the remaining components - which can be done anywhere at any relative time.

---

@PeterDonis : I don't feel our back and forth is helping those who might otherwise be interested in this thread. So I will note your objections to my descriptions of MWI, and try to stay away from further comments that I know you will disagree with.

Apologies offered to the extent I may have led the thread astray. I will again encourage anyone reading this to look at the reference on Vaidman's MWI work.

 

[kered rettop]

If there is such a simple solution, it should already be in the literature. People have been discussing this topic for decades. There is plenty of literature on how to make sense of probabilities in the MWI. Please take the time to work through it.

I'll take that as a "no", then.

 

[PeterDonis]

That's the hypothesis.

It's part of the interpretation. Calling it a "hypothesis" seems odd, because you can't test interpretations; they all make the same predictions for experimental results. They're interpretations. There is no way to test an interpretation experimentally. You can be skeptical about an interpretation because of claims it makes that seem extravagant to you--for example, the MWI claims that all branches of the wave function really exist even though we, in whatever branch we are in, have no way of ever testing whether that claim is true.

Of course he is arguing for MWI.

In the sense that he appears to be a proponent of it, yes. But his descriptions of what it says are not arguments, as if he needed to prove or establish somehow that that's what the MWI says. He's just describing what it says. And then he makes arguments for why he favors it as an intepretation.

One of Vaidman's many specialties is systems where there are quantum interactions that are NOT spatially co-located.

I would not describe those scenarios that way. I would describe them as scenarios in which quantum nonlocality is brought out: a local interaction on one degree of freedom affects some other non-co-located degree of freedom. Or, in some cases (like the bomb tester), the lack of an interaction on one degree of freedom affects another non-co-located degree of freedom.

To an extent these are just debates about words, but if we're going to look at particular references we should try to be clear about how they use words, even if we ourselves might not agree with their choices of words.

 

[PeterDonis]

Every single experiment, every day of every week, says the outcomes of all quantum experiments are random. There is no experimental evidence to the contrary. To describe that as "deterministic" is a gross mischaracterization of the word.

I'm sorry, but all this is completely irrelevant to the actual point I made. I gave a perfectly valid reason for why the MWI is deterministic. Nothing you say makes that reason invalid. You are not saying anything that actually rules out the MWI as an interpretation. Nor are you saying anything that changes what the MWI says as an interpretation. And that is what my posts are about: what the MWI says as an interpretation. I am not trying to argue that the MWI is true. I am only trying to correct your mistaken claims about what it says.

 

[PeterDonis]

I don't feel our back and forth is helping those who might otherwise be interested in this thread. So I will note your objections to my descriptions of MWI, and try to stay away from further comments that I know you will disagree with.

Please bear in mind that, as I said in my previous post just now, I am not trying to argue that the MWI is true. I don't actually think it is; I am not an MWI proponent as a matter of personal opinion. I am only trying to make sure it is clear what the MWI says. That is the only reason I have objected to your posts: that they are incorrect as descriptions of what the MWI says. I am not at all claiming that your posts are incorrect as a description of how things actually are, or as a description of what we actually observe in experiments. Those are separate questions from the question of what the MWI, or any interpretation, says.

 

[PeterDonis]

I don't feel our back and forth is helping those who might otherwise be interested in this thread...

Apologies offered to the extent I may have led the thread astray.

I have addressed this by spinning this discussion off into a new thread in the interpretations subforum.

 

[gentzen]

How many papers* have been written explaining that it is meaningless to discuss what happens in the quantum world when no one is looking?
...
*Such as Vaidman 2013 (different paper than above) who also said this, for example: "Quantum mechanics does not provide a clear answer to the question: What was the past of a photon which went through an interferometer ...we conclude that the past of the photons is not represented by continuous trajectories, although a “common sense” analysis adopted in various welcher weg measurements, delayed-choice which-path experiments and counterfactual communication demonstrations yields a single trajectory."

Looks like this reference is more about the two-state vector formalism and the related theory of weak values than about it being "meaningless to discuss what happens in the quantum world when no one is looking". What is true is that the past of the photons is not represented by continuous trajectories of point particles for the experiments described in the paper. Instead, there is that "two state vector" object with better spatial localization and a better "claim for existence" than the wavefunction (which only exists in the form of probabilies for outcomes of potential measurements). The point of the weak values is that you can look without disturbing things too much, and hence it is meaningful to discuss them even when no one is looking.

(I guess you found that refence while collecting TSVF references, but then decided to include it here instead, because even so it is an extremely nice paper, it didn't follow the pattern of your other references. Namely, it didn't include Aharonov as coauthor, and gave measurement data from "real" experiments instead of focusing on theory.)

 

[kith]

Sure, but the baggage is the same whether you say that all outcomes occur (which is the MWI) or that only one occurs and which one it is is random (other interpretations that are not the MWI). So I don't see this as a reason not to say "all outcomes occur" in the MWI.

I don't disagree but I want to highlight that it isn't the most natural thing to say from a MWI perspective.

When we perform a measurement, our fundamental notion from a Copenhagen perspective is that a single outcome occurs. Our fundamental notion from a MWI perspective is that a branching of worlds occurs. This leads to multiple instances of ourself experienceing different single outcomes.

I wonder if both @PeterDonis and @DrChinese (or neither of them ;-)) would agree with the MWI saying "the worlds of all instances are equally real" and "every instance experiences only a single outcome".

 

[vanhees71]

Not sure I follow your thinking here. Experiments have definite outcomes, even were there MWI branching and there were a different outcome in another branch.

That's precisely what I don't understand about the MWI interpretation. On the one hand they say there is this branching, but on the other we have definite outcomes when we are doing the experiments. So what does this branching then mean from an experimental/observational point of view?

Also: There is no concept in MWI* to explain "perfect" correlations between distant measurements of entangled pairs. You never see them violate the quantum expectation value. Were MWI correct, there should be worlds in which they don't act in unison - since all outcomes occur randomly.

I don't think so, because the perfect correlations are properties of the entangled state. E.g., if you have a polarization-singlet state of two photons, when finding photon 1 H-polarized then the other photon will be found V-polarized with probability 1 and vice versa. Which of these to possible outcomes of the measurements on the two photons is of course completely random, but the 100% correlation is always (with 100% probability) occuring. That means according to MWI there are two branches when making this measurement: photon 1 H and photon 2 V and another with photon 1 V and photon 2 H polarized.

 

[DrChinese]

I don't disagree but I want to highlight that it isn't the most natural thing to say from a MWI perspective.

...

I wonder if both @PeterDonis and @DrChinese (or neither of them ;-)) would agree with the MWI saying "the worlds of all instances are equally real" and "every instance experiences only a single outcome".

First, thanks to @PeterDonis for splitting this off the main thread.

Sure. I'd agree with that @kith. And I certainly am not trying to mischaracterize MWI by my choice of words. I will attempt to use the descriptive phrases that are more in keeping with the usual MWI terms, even if I don't think they really apply that well.

I am going to reply to @vanhees71 's post #33 to discuss some of the issues. My reply should not be considered any disagreement with him.

 

[gentzen]

Making sense of probability - I still don't understand the problem. It seems to me that there is a rather simple solution which anyone here should be able to follow without my having to find a reputable reference to support it. Is it possible to discuss the matter without incurring the wrath of the mods?

Starting an answer in an existing thread with "I still don't understand ..." risks to annoy the other participants in that thread. If there is something which you don't understand, why don't you ask that as a separate question in a new thread? On the other hand, if you really want to give an answer in an existing thread, then requesting you to find a reputable reference in case your answer causes confusion or disagreement seems reasonable.

However, sometimes even slight hints that somebody might be trying to promote personal research seem to get him banned. Jarek Duda is certainly not a crank, and his research is often creative, novel and impactful. I guess he had received warnings before about which parts of his behavior won't be tolerated. And I guess that quoting poor references was not part of it.

That said, I would be curious about your simple solution. But not in this or any other existing thread. I also would prefer if you do some research for references first (and tell us if you found nothing related), so that I as reader am at least spared the effort to research whether your solution is completely new, or not.

 

[kered rettop]

That's precisely what I don't understand about the MWI interpretation. On the one hand they say there is this branching, but on the other we have definite outcomes when we are doing the experiments. So what does this branching then mean from an experimental/observational point of view?

And there lies the problem. We don't have definite outcomes when we do the experiments. We have definite apparent outcomes. They do not reflect the global superposition: the definiteness is an artifact of the branching.

 

[DrChinese]

1. That's precisely what I don't understand about the MWI interpretation. On the one hand they say there is this branching, but on the other we have definite outcomes when we are doing the experiments. So what does this branching then mean from an experimental/observational point of view?

2. I don't think so, because the perfect correlations are properties of the entangled state. E.g., if you have a polarization-singlet state of two photons, when finding photon 1 H-polarized then the other photon will be found V-polarized with probability 1 and vice versa. Which of these to possible outcomes of the measurements on the two photons is of course completely random, but the 100% correlation is always (with 100% probability) occuring.

3. That means according to MWI there are two branches when making this measurement: photon 1 H and photon 2 V and another with photon 1 V and photon 2 H polarized.

1. None of my comments in this post should be taken as disagreement with you. Every time I examine what MWI says about the details, I get squirmy answers (in sources supporting MWI) to the obvious tough questions. So let's examine your 2. and 3. in a very specific MWI example.

---

We have a straight Type I PDC setup outputting a pair of entangled photons in the |HH> + |VV> Bell basis. The inputs are single photons oriented diagonal at 45 degrees (which could also be considered an equal superposition of |H> + |V>). Each entangled output pair is sent to linear polarization detector setups far distant from each other, and are measured at the same angle - but at an angle randomly selected mid-flight and outside the light cone of the photons at time of selection. Here are the questions I have:

i. The output pairs must not yet have a specific definite polarization, correct? Because we need them to match at whatever angle they are to be detected at, and that has not been selected yet. So they must still be in a superposition (due to their "preparation" as you call it).

ii. The selected angle by some RNG is 120 degrees. Alice measures first (say), and gets result V. When exactly does that branching occur? We know in some other MWI branch the outcome was definitely H, right? The polarization detection setup itself consists of 3 components: the polarizing beam splitter (PBS) and the 2 avalanche detectors (one H, and one V). The branching occurs at one or more of these spots: a) the PBS; b) the V detector; and/or c) the H detector (which didn't fire in our branch). And in fact, the relative time of fire of the V and H detectors can be adjusted (by distance of placement after the PBS) so that they are clearly separated. Where/when does the branching occur? a)? Of course, this is a point at which the action is still reversible. b)? Of course, there has certainly been branching by this point in our particular branch, because we measured the V outcome. c)? The H detector did not fire in our branch, but we are certain it did in the other branch. But that outcome presumably came later in that branch, right?

iii. Here's the hard part: how does the branching from ii. above affect the photon Bob is getting ready to detect? That photon is far away. How does the branching action over by Alice affect Bob? Because we presumably determined Bob's photon was still in superposition as a result of i. above, right? Some of us here suspect that something "nonlocal" might be occurring. Even Vaidman seems to acknowledge something along this line. To quote, and note that there were no answers to any of my questions in his paper (and certainly no answers in his "next" section):

"But there are connections between different parts of the Universe, the wave function of the Universe is entangled. Entanglement is the essence of the nonlocality of the Universe. “Worlds” correspond to sets of well localized objects all over in space, so, in this sense, worlds are nonlocal entities. Quantum measurements performed on entangled particles lead to splitting of worlds with different local descriptions. Frequently such measurements lead to quantum paradoxes which will be discussed in the next section."

But in his parlance, whatever "nonlocal" occurs cannot quality as "action at a distance". I don't have a particular objection to this characterization, but I would not call it "spot on" either.

iv. And finally, this little gem of a question which is often overlooked with Type I PDC. We may say MWI is deterministic, but this leads to something of a paradox. Type I PDC consists of 2 thin orthogonal crystals placed face to face. One has an input of H and produces |VV>, while the other takes an input of V and produces output of |HH>. Neither of those are entangled outputs! So how does the entanglement occur? The answer is that the diagonal input to the pair of crystals takes an indistinguishable path, and the particular spot where down conversion occurs is indeterminate. So for the MWI explanation to make sense, we need to assert that NO branching occurs as the input photon splits into 2 entangled photons. What? So branching occurs everywhere else BUT the very spot where/when there's a choice of paths through the PDC setup. Huh?

We must have the entangled pair exit in a superposition for the rest of the MWI magic to occur. And yet, we need there to be branching by the time Alice and Bob read and record their respective results. But aren't we capable of establishing a consistent rule as to when branching occurs that doesn't appear ad hoc? Because I say that according to the MWI concept of definite deterministic outcomes: the diagonal input photon split at either the H PDC crystal (in our branch) or the V PDC crystal (in the other branch, or vice versa) - and would NOT have led to an entangled state if either of those things occurred. They would instead exit as VV or HH, and there would not be perfect correlations when later measured at 120 degrees (as selected by the RNG).

---

Making sense of this kind of setup causes me all kinds of confusion, and yet this is precisely the kind of experiment that a viable interpretation should explain today. I am *not* trying to support or reject MWI by any of my comments, I am just trying to understand the rules MWI plays by. Every interpretation seems to have some consistency issues at some level, and I believe MWI does too.

Cheers,

-DrC

 

[kered rettop]

Starting an answer in an existing thread with "I still don't understand ..." risks to annoy the other participants in that thread. If there is something which you don't understand, why don't you ask that as a separate question in a new thread? On the other hand, if you really want to give an answer in an existing thread, then requesting you to find a reputable reference in case your answer causes confusion or disagreement seems reasonable.

I appreciate your explanation of how PF works. I was reluctant to start a new thread because, of course, it loses the context. In any case, Peter Donis has said that I need to work through the literature (which I did several years ago anyway) so I think that starting a new thread would be tempting fate.

However, sometimes even slight hints that somebody might be trying to promote personal research seem to get him banned. Jarek Duda is certainly not a crank, and his research is often creative, novel and impactful. I guess he had received warnings before about which parts of his behavior won't be tolerated. And I guess that quoting poor references was not part of it.

That's very encouraging, thanks.

That said, I would be curious about your simple solution. But not in this or any other existing thread. I also would prefer if you do some research for references first (and tell us if you found nothing related), so that I as reader am at least spared the effort to research whether your solution is completely new, or not.

I am 100% certain there's nothing new about it. But it is not universally accepted and I don't know why not. It's simple enough. If you're happy to look at the matter very briefly, I can PM you.

 

[gentzen]

If you're happy to look at the matter very briefly, I can PM you.

I am certainly also happy if you just PM me your simple solution.

 

[kered rettop]

I am certainly also happy if you just PM me your simple solution.

Will do. Thanks.

 

[PeterDonis]

Our fundamental notion from a MWI perspective is that a branching of worlds occurs. This leads to multiple instances of ourself experienceing different single outcomes.

Yes, and that implies that all outcomes occur, each one in its own branch. I left out the "each one in its own branch" part, yes.

"the worlds of all instances are equally real" and "every instance experiences only a single outcome".

What does "instance" mean here? Introducing a new undefined term isn't likely to help matters.

 

[PeterDonis]

We don't have definite outcomes when we do the experiments. We have definite apparent outcomes. They do not reflect the global superposition: the definiteness is an artifact of the branching.

This is indeed a sticking point for many MWI skeptics (I'm sympathetic to it myself): ordinarily when a quantum system is in an entangled superposition, we say it doesn't have any definite state at all. We don't say each individual term in the superposition is a "branch" or a "world" in which the system has a definite state.

MWI proponents, however, simply refuse to accept this criticism, and maintain that what you call "definite apparent outcomes" are "real" definite outcomes. (Some of them even go so far as to describe, for example, quantum computing experiments as involving multiple "worlds", when what they actually mean is having systems of multiple qubits in entangled superpositions, when no decoherence has taken place anywhere and so even according to the modern version of the MWI, where "branching" happens when decoherence happens, there aren't multiple "worlds".)

Unfortunately, this is the kind of dispute that can't be resolved, at least not while all we have is our current theory of QM and the MWI as an interpretation of it. Someone will have to come up with an actual different theory based on the MWI that makes different predictions in at least some experiment.

 

[PeterDonis]

how does the branching from ii. above affect the photon Bob is getting ready to detect?

Through the wave function. I already showed the math for this in some detail. The wave function is nonlocal, so this effect is also nonlocal.

whatever "nonlocal" occurs cannot quality as "action at a distance"

Yes, because this nonlocality doesn't violate the no signaling theorem and doesn't allow FTL communication. But it's still nonlocality: it still produces Bell inequality violations and still rules out the kinds of models that Bell's theorem and other similar theorems are based on. Which leaves us with no intuitively satisfying model of what is going on "behind the scenes".

for the MWI explanation to make sense, we need to assert that NO branching occurs as the input photon splits into 2 entangled photons.

As far as I know, this process does not involve decoherence, so no, there would not be any branching.

branching occurs everywhere else BUT the very spot where/when there's a choice of paths through the PDC setup.

I don't see where this is coming from. Where is the "everywhere else" where branching is occurring? Branching only occurs when there is decoherence.

aren't we capable of establishing a consistent rule as to when branching occurs that doesn't appear ad hoc?

Yes, I've stated it multiple times above--and indeed multiple times previously in this thread. Branching occurs when there is decoherence.

The original MWI proponents, Everett and DeWitt (among others), came before decoherence theory was developed, so their accounts of when branching happened were unavoidably vague and hand-waving. But now we have decoherence theory and that vagueness and hand-waving is no longer necessary.

 

[kith]

What does "instance" mean here? Introducing a new undefined term isn't likely to help matters.

I don't see how we can do without it.

If I prepare the single particle state $|\uparrow\rangle + |\!\downarrow\rangle$ and perform the corresponding spin measurement, branching occurs. Have I measured spin up or spin down? On the one hand, we have to say "There's a world where I have measured spin up and a world where I have measured spin down. But looking at the needle of my apparatus I see it pointing upwards and conclude "I have measured spin up".

So either we have a contradiction or the "I"s in the previous two sentences don't refer to the same thing. The "I" in the second sentence is what I would call something like an "instance", "version", etc. of the generic "I" of the first sentence.

 

[PeterDonis]

If I prepare the single particle state $|\uparrow\rangle + |\!\downarrow\rangle$ and perform the corresponding spin measurement, branching occurs. Have I measured spin up or spin down?

Both. The "you" in the "measured spin up" branch has measured spin up. The "you" in the "measured spin down" branch has measured spin down.

On the one hand, we have to say "There's a world where I have measured spin up and a world where I have measured spin down.

Yes.

But looking at the needle of my apparatus I see it pointing upwards and conclude "I have measured spin up".

No. You can't state this as a unique fact, because it isn't. It only applies to the "you" in the "measured spin up" branch. There is also a "you" in the "measured spin down" branch who sees the needle pointing down.

The MWI is difficult to discuss because ordinary language has built-in assumptions that the MWI violates, like the assumption that pronouns like "you" have unique referents. In the MWI, they don't; there is one referent in each branch. If you use those pronouns, you have to realize that and take it into account. Otherwise you're simply not talking about the MWI, you're talking about some straw man version you've made up.

So either we have a contradiction or the "I"s in the previous two sentences don't refer to the same thing.

False dichotomy. The "I"'s do refer to the same thing in the sense that they refer to the same quantum degrees of freedom. They only differ in whether those degrees of freedom are entangled. See further comments below.

The "I" in the second sentence is what I would call something like an "instance", "version", etc. of the generic "I" of the first sentence.

You're misdescribing again. The "I" before the measurement is not "generic". "I" refers to the same quantum degrees of freedom before and after the measurement. The difference is that, before the measurement, those degrees of freedom are not entangled with the particle whose spin is being measured or the spin measuring apparatus. After the measurement, they are, and the entangled state decoheres.

I see no need to make up a new term, "instance", to describe this. "Branching" already describes what happens. And "instance" suggest that some kind of "copying" or "instantiation" is going on, when nothing of the sort is going on. As I have pointed out repeatedly, time evolution in the MWI is always unitary, and unitary evolution can't create or destroy anything, or "copy" or "instantiate" anything. All that happens is that quantum degrees of freedom that weren't entangled, become entangled. The number of degrees of freedom remains the same.

 

[kith]

No. You can't state this as a unique fact, because it isn't. It only applies to the "you" in the "measured spin up" branch. There is also a "you" in the "measured spin down" branch who sees the needle pointing down.

Just to get you right: You are saying that when we write down a conditional probability for the next measurement (like $P(\uparrow_x | \uparrow_z)$) and are speaking from a MWI perspective, we shouldn't say that this is the probability to measure spin up in the x-direction given that we measured spin up in z-direction (or given that the electron has spin up in the z-direction). What should we say instead?

The MWI is difficult to discuss because ordinary language has built-in assumptions that the MWI violates, like the assumption that pronouns like "you" have unique referents. In the MWI, they don't; there is one referent in each branch. If you use those pronouns, you have to realize that and take it into account.

This seems to be the crux of the matter. I need to think about it a bit. Do you have a reading recommendation which expands on what you wrote?

Otherwise you're simply not talking about the MWI, you're talking about some straw man version you've made up.

This is uncalled-for. I'm not weakening an argument in order to disprove it, I'm providing one. I'm giving my understanding of the MWI and I've used qualifiers like "I don't see" and "what I would call" to indicate this. If my argument is flawed, I'm happy to learn this and improve my understanding. In the two paragraphs above, you raise relevant points but I'm not convinced yet.

The "I" before the measurement is not "generic". [...] "instance" suggest that some kind of "copying" or "instantiation" is going on, when nothing of the sort is going on. [...]

I agree that the terminology I've used has unfortunate connotations. So thanks for improving it by mentioning degrees of freedom and referents.

 

[PeterDonis]

You are saying that when we write down a conditional probability for the next measurement (like $P(\uparrow_x | \uparrow_z)$) and are speaking from a MWI perspective, we shouldn't say that this is the probability to measure spin up in the x-direction given that we measured spin up in z-direction (or given that the electron has spin up in the z-direction). What should we say instead?

That is an open question for MWI proponents: how to make sense of the concept of "probability" when the interpretation says the dynamics are deterministic. As far as I know, MWI proponents do not have a single response to this question that all of them agree on. There are responses in the literature, but they're different and mutually inconsistent and none of them has the support of all MWI proponents.

This is uncalled-for. I'm not weakening an argument in order to disprove it, I'm providing one. I'm giving my understanding of the MWI and I've used qualifiers like "I don't see" and "what I would call" to indicate this. If my argument is flawed, I'm happy to learn this and improve my understanding. In the two paragraphs above, you raise relevant points but I'm not convinced yet.

Not convinced of what?

The MWI is very simple as far as its basic premises go: the wave function is real and contains all of reality, and the time evolution of the wave function is always unitary. Everything I have said is a simple mathematical consequence of those two basic premises. I understand that it's very counterintuitive and many people are skeptical that any such thing could possibly be true. I am one of those people. But being skeptical about whether it's true doesn't mean one can't describe what it says.

 

[PeterDonis]

Do you have a reading recommendation which expands on what you wrote?

Unfortunately, MWI proponents do not like to talk about the issues I raised in what you quoted. I suspect that this is because doing so would make the MWI look less plausible. It would also make it harder for proponents to talk about the MWI because they wouldn't be able to help themselves to ordinary language with ordinary language connotations in order to make it seem like the MWI is just describing ordinary experience. However, that is just my personal view.

As far as papers that describe what the MWI says, in its modern version, I would suggest reading the paper by Vaidman that @DrChinese referenced in post #14.

 

[Morbert]

@PeterDonis You adopted a specific convention re/pronouns, not insisted upon by the interpretation, and then cast aspersions on @kith re/ building a strawman. @kith Is not building a strawman. They are pointing out that the lab experience of performing measurement and observing a unique outcome is not immediately squared with property instantiations in branching worlds.

 

[kered rettop]

The MWI is very simple as far as its basic premises go: the wave function is real and contains all of reality, and the time evolution of the wave function is always unitary. Everything I have said is a simple mathematical consequence of those two basic premises. I understand that it's very counterintuitive and many people are skeptical that any such thing could possibly be true. I am one of those people. But being skeptical about whether it's true doesn't mean one can't describe what it says.

Surely MWI requires decoherence theory to complete it? In which case there needs to be a third premiss, namely that there is an environment with certain properties, such as 1) a large number of degrees of freedom, 2) a high degree of interactivity with the system, and 3) the ability to propagate (disseminate or "amplify") information about an outcome. In fact you recently mentioned a case where there is no environmental interaction, namely the isolated entangled qubits in a quantum computer. They are, erroneously in your view and also in mine, referred to as worlds by some authors. Whatever they may be, they are not worlds in the MWI sense.

Of course I am not suggesting that your skepticism should imply that you would question the fact that there is an environment!