Entanglement swapping and Bohmian mechanics

[Matterwave]

I'm not criticizing Daumer et al - it's a strong group of scientists - but there is a bit of humor to be found in the paper. Obviously, they push the Bohmian "deterministic" perspective. Exactly what I'm interested in, especially vis a vis the Zeilinger experiments.

I'm not sure if humor was what they were going for, but it did seem humorous to me how directly they called out Zeilinger.

i) "Thus, the experimental facts of quantum mechanics do not establish indeterminism." Actually, essentially every quantum experiment ever run confirms the root nature of randomness in QM.* No experiment has ever been run that correctly predicts the outcome of a 50:50 Beam splitter using Bohmian theory. (oQM says that there is no more complete specification of the system.)

This sentence probably needs you to view it through the lens of a Bohmian (or, interestingly, perhaps a MWI person). What they mean with "establish" here is more like "proved beyond a reasonable doubt". I.e. that all fundamentally deterministic alternatives have been ruled out. The experimental results does not quite rise to this standard (since it does not rule out BM or MWI).

I feel that this paper, as written, kind of needs you to already be pretty amenable to a Bohmian view to "get" it and I don't think it's changing the mind of Zeilinger or other physicists in other camps.

The non-sequitar about evolution vs intelligent design at the end was also rather unusual.

ii) To be fair, MWI proponents miss the ball on the same point. In case no one noticed: Only a single outcome of every quantum experiment ever run has been observed.*

Yeah, that is a standard remark against MWI. But to be fair to MWI, it does try to address this objection by saying the observations of different outcomes happen in different, disconnected, worlds. As an interpretation, it wasn't trying to make testably different predictions from standard QM anyways.

Neither i) or ii) make the concepts of BM or MWI worthless. But I would not say experimental results haven't yet done either of them a favor. Their stronger points are on the conceptual side. Experiments show no hints of underlying determinism, or of other worlds. With both MWI and BM, the hope is that a future theoretical breakthrough might lead to a confirming experiment. That would be great.

That would make both no longer interpretations of QM but rather new theories. There are some mumblings about whether these interpretations "make new predictions" but none of the mumblings have, AFAIK, gained widespread acceptance even within their respective subcommunities.

Please keep in mind that I don't pay much attention to Zeilinger's views on QM outside of his experimental papers themselves. He is very careful to limit the scope of his comments to what is generally accepted science in those.

They seem to be mostly targeting this paper: https://pubmed.ncbi.nlm.nih.gov/16340995/ But I have not read it so I haven't seen both sides.

 

[Morbert]

2. I might call (2) a superposition. Again, the authors' usage of language does not really specify your point of view - or mine exactly. However, at this point it's clearly semantics and not science.

What is unambiguous is (1) and (2) are the same state, just written differently. With this solid base, we are free reproduce the statistics of the experiment however we wish. We can for example have the BSM on 2 and 3 project 1 and 4 onto a Bell state, or we can have the polarization measurements of 1 and 4 project 2 and 3 onto a product state. Or we can add apparatus degrees of freedom and compute an Everettian wavefunction with branches yielding probabilities. Or we can use that Everettian wavefunction as a Bohmian pilot wave and compute Bohmian trajectories. Or we can use (1)/(2) to build a decoherence functional and a consistent histories framework.

All of these are readily possible with the right understanding of the initial state and its implications.

 

[Sambuco]

The non-sequitar about evolution vs intelligent design at the end was also rather unusual.

Yes, the end Daumer's paper surprised me too:

"And it is perhaps also worth asking the editors of Nature how, at a time when, rightly, papers on Intelligent Design are consistently rejected by peer-reviewed journals, an essay like Zeilinger’s is not."

They seem to be mostly targeting this paper: https://pubmed.ncbi.nlm.nih.gov/16340995/ But I have not read it so I haven't seen both sides.

I have read Zeillinger's paper, and it doesn't strike me as all that controversial. In a way, it reflects his information-based stance he always held. It is true that some of his assertions may be somewhat open to debate, such as the issue of determinism or the assumptions behind Bell's theorem, but after all, it is an essay, and he is simply sharing his view.

It is often said that "operationalists" tend to be unable to grasp the motivations behind someone accepting an interpretation like Bohmian mechanics, yet it is equally true that many strong "realists" struggle to understand the reasoning behind a position like Zeilinger's.

Lucas.

 

[DrChinese]

1. This sentence probably needs you to view it through the lens of a Bohmian (or, interestingly, perhaps a MWI person). What they mean with "establish" here is more like "proved beyond a reasonable doubt". I.e. that all fundamentally deterministic alternatives have been ruled out. The experimental results does not quite rise to this standard (since it does not rule out BM or MWI). I feel that this paper, as written, kind of needs you to already be pretty amenable to a Bohmian view to "get" it and I don't think it's changing the mind of Zeilinger or other physicists in other camps.

2. Yeah, that is a standard remark against MWI. But to be fair to MWI, it does try to address this objection by saying the observations of different outcomes happen in different, disconnected, worlds. As an interpretation, it wasn't trying to make testably different predictions from standard QM anyways.

3. That would make both no longer interpretations of QM but rather new theories. There are some mumblings about whether these interpretations "make new predictions" but none of the mumblings have, AFAIK, gained widespread acceptance even within their respective subcommunities.

4. They seem to be mostly targeting this paper: https://pubmed.ncbi.nlm.nih.gov/16340995/ But I have not read it so I haven't seen both sides.

1. Agreed.

2. Also agreed.

3. Also agreed. My viewpoint is that there "might" be a hidden testable assumption built into an Interpretation that is susceptible to being teased out. Not sure what that might be, but these newer experiments - DCES being my target - might be a fertile area. BM and MWI appeared in the 1950's. Of course orthodox QM is even older.

4. I haven't read that paper, but generally the explanations provided by many of the "greats" (Peres, Weinberg, etc.) are not that satisfying. That's not a critique at all. Everyone gets tongue tied at some point while discussing the quantum world. Zeilinger coauthored a paper on the Interpretational views of a group of 33 scientists, and they were all over the place.

https://arxiv.org/abs/1301.1069

 

[DrChinese]

What is unambiguous is (1) and (2) are the same state, just written differently. With this solid base, we are free reproduce the statistics of the experiment however we wish. We can for example have the BSM on 2 and 3 project 1 and 4 onto a Bell state, or we can have the polarization measurements of 1 and 4 project 2 and 3 onto a product state. Or we can add apparatus degrees of freedom and compute an Everettian wavefunction with branches yielding probabilities. Or we can use that Everettian wavefunction as a Bohmian pilot wave and compute Bohmian trajectories. Or we can use (1)/(2) to build a decoherence functional and a consistent histories framework.

All of these are readily possible with the right understanding of the initial state and its implications.

And yet: Not one of the things you mentioned require me to accept your viewpoint. Which hopefully doesn't hold us back.

We agree (I believe): Starting with (1), a physical overlap during a BSM is required to remotely produce one of the 4 Bell States listed in (2) for photons 1 and 4.

 

[martinbn]

And yet: Not one of the things you mentioned require me to accept your viewpoint. Which hopefully doesn't hold us back.

What you fail to understand is that this is not his point of view. It is quantum mechanics. If you don't accept quntum mechanics say so.

 

[martinbn]

Yes, the end Daumer's paper surprised me too:

"And it is perhaps also worth asking the editors of Nature how, at a time when, rightly, papers on Intelligent Design are consistently rejected by peer-reviewed journals, an essay like Zeilinger’s is not."


I have read Zeillinger's paper, and it doesn't strike me as all that controversial. In a way, it reflects his information-based stance he always held. It is true that some of his assertions may be somewhat open to debate, such as the issue of determinism or the assumptions behind Bell's theorem, but after all, it is an essay, and he is simply sharing his view.

It is often said that "operationalists" tend to be unable to grasp the motivations behind someone accepting an interpretation like Bohmian mechanics, yet it is equally true that many strong "realists" struggle to understand the reasoning behind a position like Zeilinger's.

Lucas.

The paper is short, but i'd say it can be made even shorter "We like BM, we don't like Zeilinger's interpretation."

 

[Demystifier]

And if it is more comfortable to you to specify that the post-BSM state is not (2) but rather one of the 4 terms of (2); then that's fine, no I have no specific disagreement with this distinction. What you say is true. I think everyone understands that we end up with one randomly selected Bell state,

Yes, we all agree on that.

and that at some point prior the state was (2); and at some point prior to that it was (1).

Not really, because (1) and (2) are the same state, at the same time.

And yet again, I find myself asking about the relationship of these nuances to Bohmian Mechanics. I get the feeling that there is a relevant point lurking around that is going to appear soon.

One cannot understand BM before one understands the standard textbook QM. It's a matter of textbook QM that (1) and (2) are the same. Even BM says that (1) and (2) are the same. And just to remind you, you said that you will accept anything I tell you.

 

[Demystifier]

The paper is short, but i'd say it can be made even shorter "We like BM, we don't like Zeilinger's interpretation."

What irritates them is not so much that someone likes a different interpretation than they do, but the fact that the vagueness of the Copenhagenish interpretations is a norm in the community of physicists, who are supposed to be careful and precise thinkers.

It was beautifully expressed by Tumulka in his book on quantum foundations:
"Narratives, But No Serious Ones
When calculating predictions that can be compared to experimental data, adherents of CI [Copenhagen Interpretation] often tell a story about the physical meaning of the mathematical elements of the calculation. This story may involve particles or waves, may be imprecise, may conflict with other stories told on another occasion, or may contain several parts conflicting with each other. But this story is not intended to describe what actually happens. On the contrary, CI insists that such narratives should not be taken seriously. They are just metaphor, or allegory, or analogy; they just serve as a mnemonic for the calculation, as a help for remembering the correct formulas or for setting up the corresponding formulas in similar calculations. In contrast, theories such as Bohmian mechanics or GRW or many-worlds are hypothesizing about what actually happens in nature and correspondingly aim at providing a single, coherent story that fits all experiments and situations."

 

[Morbert]

And yet: Not one of the things you mentioned require me to accept your viewpoint. Which hopefully doesn't hold us back.

You are absolutely required to accept that (1) and (2) are unambiguously the same state. Ma says it explicitly and it follows immediately from basic algebra in quantum mechanics. If you don't accept this you will just compound misunderstanding on misunderstanding.

 

[Demystifier]

You are absolutely required to accept that (1) and (2) are unambiguously the same state. Ma says it explicitly and it follows immediately from basic algebra in quantum mechanics. If you don't accept this you will just compound misunderstanding on misunderstanding.

Perhaps @DrChinese intuitively understands "+" in $|a\rangle+|b\rangle$ as something similar to a logical "or". When others write $$|a\rangle+|b\rangle$$
he sees
$$|a\rangle \;{\rm or}\; |b\rangle$$
In some measurement contexts such an interpretation may lead to correct results, so maybe he imagines that it always does. But in general, it doesn't. In particular, I don't see how could anyone understand quantum interference (e.g., the 2-slit experiment) with such a way of thinking. Nevertheless, he does not seem bothered by the 2-slit experiment, he cares much more about spin-like observables that can only take a few possible values.

 

[Demystifier]

Obviously, they push the Bohmian "deterministic" perspective.

Not really. They push the ontological perspective, including the GRW theory which is not deterministic. As I said billion times, the point of theories such as Bohmian mechanics and GRW is an explicit ontology (realism), not determinism. I never understood why so many physicists think that realism=determinism.

 

[Lord Jestocost]

This corresponds to the information interpretation of QM, typical of Zeilinger. A Bohmian realist, on the other hand, is not satisfied with the notion of information living in some Platonic world of ideas, but seeks a physical object carrying this information. And since the relevant information is available in the future, the carrier of information cannot be the photon in the past. That's why, for a Bohmian, it's perfectly natural to accept that information is carried by the computer memory and its environment. To invert the quoted sentence above, that causes no conceptual problems if one accepts that real properties of a system are a more basic feature than information about quantum system that might be available to a particular observer.

For a more general critique of the Zeilinger's informational point of view, as seen by Bohmian realists, see https://arxiv.org/abs/quant-ph/0604173 .

@Demystifier

In the “The Message of the Quantum?” by Martin Daumer, Detlef Dürr, Sheldon Goldstein, Tim Maudlin, Roderich Tumulka and Nino Zanghì (https://arxiv.org/abs/quant-ph/0604173) one reads:

“Perhaps Zeilinger merely means that the predictions of quantum theory—or at least the experimental facts on which quantum mechanics is based—strongly support a nondeterministic formulation. But this view is easily refuted by the counter-example provided by Bohmian mechanics [4, 5], a theory describing the deterministic evolution of particles that accounts for all of Zeilinger’s examples and indeed all of the phenomena of nonrelativistic quantum mechanics, from spectral lines to the two-slit experiment and random decay times.”

As a physicist, I agree with meaning of the first sentence of this quote. I have, however, my doubts whether the meaning of the second sentence can be upheld in such a simplifying way. As Klaas Landsman remarks in his paper "Bohmian Mechanics is Not Deterministic” (Foundations of Physics (2022) 52:73 (https://link.springer.com/content/pdf/10.1007/s10701-022-00591-9.pdf):

“My aim is to show that the alleged determinism of Bohmian mechanics is parasitical on some external random sampling mechanism (“oracle”) the theory has to invoke in order to state the specific value of the hidden variable (i.e. position) in each experiment. Short of the above oracle, Bohmian mechanics by itself is not only unable to predict the outcome of individual experiments, but cannot even reproduce their provable (algorithmic) randomness properties (which follow from the Born rule). Furthermore, Bohmian mechanics cannot be extended by any deterministic theory so as to replace the random oracle, even if it is deemed acceptable that a deterministic theory fails to specify its initial conditions. Thus the performance of Bohmian mechanics and especially its level of determinism are similar to that of minimal versions of the Copenhagen interpretation, which also leaves the outcomes of experiments to a black box (arguably a more obscure one).”