A Hidden Assumptions in Bell's Theorem?

[martinbn]

I can't quote entire chapters of textbooks. It's not so difficult to just read the book, isn't it?

I am with you on this one. This is an "A" level thread. Participants should have read a standard textbook on the topics involved if they want to comment on them. One cannot quote a paragraph from page 350 when most of those 350 pages are need to understand it.

 

[LittleSchwinger]

Nonlocality in many studies of entanglement means "violating Bell's inequalities".
In most other areas of physics it means "action at a distance".

This obviously causes confusion as seen on this thread, which has lead to the quantum information community increasingly calling the violation of Bell's inequality "Non-classical correlations" as opposed to Non-locality. For example:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.106.220403

It's especially confusing in studies of quantum gravity where one has both violations of Bell's inequality and a form of action at a distance, e.g. String Theoretic treatments of Black Holes.

 

[gentzen]

One cannot quote a paragraph from page 350 when most of those 350 pages are need to understand it.

Of course you can. It is way better than "quoting" 350 pages without any indication of a specific piece that would support your argument.

 

[vanhees71]

To understand the basics of quantum field theory it is not sufficient to read snippets of text. If you are not interested enough in the subject to read some introductory chapters of a textbook, why are you then discussing about it?

 

[gentzen]

To understand the basics of quantum field theory it is not sufficient to read snippets of text. If you are not interested enough in the subject to read some introductory chapters of a textbook, why are you then discussing about it?

Are you saying that you are "quoting" QFT textbooks because you assume that I would not be familiar with some introductory chapters of such textbooks? Why would you assume that? Maybe I even read the introductory chapters of Coleman's lectures, and even compared the arXiv version of them with the published book. Not because I am not familiar with the introductory material, but because I was curious of how he presents the material, and curious of the differences between the arXiv version and the final version? Or maybe not, but is this really relevant for my remark that most of us interpret "quoting" as selecting "a specific piece of text from a longer text to provide a focus point"?

And you know perfectly well that this different attitude towards "quoting" doesn't just arise with respect to QFT textbooks, but already when you reply to somebody in a thread, and you routinely don't find it necessary to select a specific part of a post you reply to, or even indicate which post you are replying to.

 

[Fra]

Here is the arxiv version. The discussion on Boole is not understood clearly by me, at least, because I lack the background knowledge.

This paragraph seems to be illustrative of the idea.
https://arxiv.org/pdf/1108.3583

I might get time to read and get back later but skimming it, I spot that it reflects over things that relates to problems in the foundations of probability theory as applied to measurements that I also acknowledge and thinks needs a solution. I wonder if that its what you meana:

"Boole [2] discussed a mathematical-logical way to deal with statistics and probabilities. He dissected experiments into events that could only assume two values obeying a calculus that resembled the algebra of real numbers, but with the operations of multiplication, addition and subtraction replaced by the respective logical operations of conjunction, disjunction and negation. The experimental results were replaced by these mathematical abstractions, as soon as a valid one-to-one correspondence between the experiments and the abstractions was established."

If one steps back and asks, for how an agent can represent it's "own degree of belief" in the [0,1]range, the real numbers are the typical response. And of course the real numbers do the job and is a good general abstraction, but it has a problem, namely that it suggests that there are not noly infinitely many, but even uncountably many different "states of degree of belief". This is not a problem for mathematics, but if you for a split second mixes up the complexions of the model and the complexions of the actual real physica agent, we are lost in a soup and expected from this are plentiful problems of renormalization. And this not even physics, it's a the foundation of the mathematical model (normally standard probability theory) for representing degrees of belieif. The real numbers seems to me to make things easier so it is well motivate, but it has a con-side, but improve it seems to make it ALOT more complicate as the notion of subjectivity enters at the level of foundations of probability. This alone will reject many.

There are several papers around these issues, but almost none of the papers I've read so far address what disturbs me the most.

To set you off in the direction here is one

Quantifying Rational Belief​

"Some criticisms that have been raised against the Cox approach to probability theory are addressed. Should we use a single real number to measure a degree of rational belief? Can beliefs be compared? Are the Cox axioms obvious? Are there counterexamples to Cox? Rather than justifying Cox's choice of axioms we follow a different path and derive the sum and product rules of probability theory as the unique (up to regraduations) consistent representations of the Boolean AND and OR operations. "
-- https://arxiv.org/abs/0908.3212

/Fredrik

 

[Simple question]

The abstract reads [emphasis mine]

That's mind-boggling. You manage to quote a text that disprove your opinions as it was supporting them
If you believe that QFT is complete and describe everything that can be measured, specifically because space-like events cannot have correlation beyond those of microcausality (strict Einstein causality) following preparation
then
"The necessary spacelike separation of the observations" give you few logical options

1. Those observations are wrong, because your philosophy cannot explain them
2. Those observations are inconsequential because you cannot do FLT signaling with them, so who cares about inconsequential laboratory truth ?
3. Those observations reveal the incompleteness of your mathematical framework (Bell put it in math, to force you picking your axiomatic assumptions, to make them clear). This is more of an inconvenient truth.

BTW 2) is not even inconsequential. Teleportation of state, or information (even if, at the end of the day, is "validated" a speed lower then C) is still a physical guarantee that the information is absolutely protected while "teleporting".

 

[vanhees71]

Are you saying that you are "quoting" QFT textbooks because you assume that I would not be familiar with some introductory chapters of such textbooks? Why would you assume that? Maybe I even read the introductory chapters of Coleman's lectures, and even compared the arXiv version of them with the published book. Not because I am not familiar with the introductory material, but because I was curious of how he presents the material, and curious of the differences between the arXiv version and the final version? Or maybe not, but is this really relevant for my remark that most of us interpret "quoting" as selecting "a specific piece of text from a longer text to provide a focus point"?

And you know perfectly well that this different attitude towards "quoting" doesn't just arise with respect to QFT textbooks, but already when you reply to somebody in a thread, and you routinely don't find it necessary to select a specific part of a post you reply to, or even indicate which post you are replying to.

If I don't quote the posting I'm replying to I'm answering to the posting just before my posting. Otherwise I quote the posting. Since I do this for several years now and nobody has complained before I thought that's self-evident.

Also I haven't had specifically you in mind when quoting these textbooks, but obviously many people here are not familiar with the microcausality condition and its meaning. So I quoted textbooks, where the concept is well explained. What's wrong with that?

I hope we can now come back to discuss physics rather than fighting about formalities of conversations in a forum.

 

[vanhees71]

That's mind-boggling. You manage to quote a text that disprove your opinions as it was supporting them
If you believe that QFT is complete and describe everything that can be measured, specifically because space-like

Of course it supports precisely that locality means that space-like separated events are not causally connected, and this is true in standard relativistic QFT imposing the microcausality constraint on local obsesrvable operators.

events cannot have correlation beyond those of microcausality (strict Einstein causality) following preparation
then
"The necessary spacelike separation of the observations" give you few logical options

1. Those observations are wrong, because your philosophy cannot explain them

I have no philsophy, I have a physical theory called QFT (or specificially in the here discussed experiments with photons) QED, and indeed this theory explains all the observations, including the violation of Bell's inequalities. Why should the observations be wrong? If anything could be wrong is, of course, the theory, assuming that there's no mistake in the experiments, but here obviously both the experiments and the theory agree. So either both are correct or both are wrong.

1. Those observations are inconsequential because you cannot do FLT signaling with them, so who cares about inconsequential laboratory truth ?

Of course you cannot do FLT (I guess you mean FTL) signalling, because space-like separated events are not causally connected within QED. The experiment itself doesn't prove that there's no FTL signalling possible, but the authors of the quoted paper take the fact that the measurement events are space-like separated that there cannot be any causal influence of one measurement on the other and thus that the locality assumption is realized in this experiment, and this is the central point of this very paper.

1. Those observations reveal the incompleteness of your mathematical framework (Bell put it in math, to force you picking your axiomatic assumptions, to make them clear). This is more of an inconvenient truth.

There's no incompleteness of "my mathematical framework" (as if I'd have invented QED ;-)) but to the contrary the experimental results are precisely described by this framework.

BTW 2) is not even inconsequential. Teleportation of state, or information (even if, at the end of the day, is "validated" a speed lower then C) is still a physical guarantee that the information is absolutely protected while "teleporting".

I don't mean what you mean by "the information is absolutely protected".

 

[Demystifier]

Wave functions are not a very good description of relativistic QFT anyway.

I use a common jargon in which "wave function" means a state in the Hilbert space, which may be represented in many ways, e.g. wave functional in the field space, wave function in the momentum space (for fixed number of particles), or a Fourier transform of the latter in the position space.

 

[Simple question]

Of course it supports precisely that locality means that space-like separated events are not causally connected,

But they are connected (and this is indeed non-causal)

and this is true in standard relativistic QFT imposing the microcausality constraint on local obsesrvable operators.

Those "observable" are non-local, remember ? you cannot do FTL signaling with them !

I have no philosophy, I have a physical theory called QFT (or specificially in the here discussed experiments with photons) QED, and indeed this theory explains all the observations, including the violation of Bell's inequalities.

You have a philosophy full of contradiction like: QED predict non-local phenomenon by no being able to compute them. Or hand-waving about "projection" of state between space-like region while your own theory explicitly forbid that.

Why should the observations be wrong?

They are not, you are. So stop saying they are non-local in some other way that breaking "Einstein causality". There is no need to use other language to fit your philosophical need. Breaking Einstein causality do NOT mean FTL signaling.

If anything could be wrong is, of course, the theory, assuming that there's no mistake in the experiments, but here obviously both the experiments and the theory agree. So either both are correct or both are wrong.

Wrong. A theory may be incomplete, but still be useful. Like QM whose "fantastic" precision is to predict 1/2 chance of being spin up. Nature do way better than this, including spooky correlation at a distance.

...that there cannot be any causal influence of one measurement on the other and thus that the locality assumption is realized in this experiment, and this is the central point of this very paper

That you didn't quite grasp, because it means your "projection" could not happened, because it would be non-causal, and you keep denying that a-causal phenomenon are observed, and cannot even be present (in principle, because of micro causality) in your theory solution. And cherry on top you believe that theory to be complete and describing everything.

There's no incompleteness of "my mathematical framework" (as if I'd have invented QED ;-)) but to the contrary the experimental results are precisely described by this framework.

You cannot have your quantum cake and eat it.

I don't mean what you mean by "the information is absolutely protected".

Picture a quantum snail, it can teleport from A to B, even at a snail pace. "While" it is gone from "space-time", while its world line is suspended, no bird will be able to eat it. Absolute protection.

 

[vanhees71]

But they are connected (and this is indeed non-causal)

The two photons are "connected", because they are prepared in an entangled state, but this does not imply that a measurement on one photon has an instantaneous or faster-than-light influence influence on the other photon. The entangled state describes correlations observed for the outcome of such measurements. Einstein called it "inseparability".

Those "observable" are non-local, remember ? you cannot do FTL signaling with them !

I don't understand what you mean by "non-local". The experimentalists measure one photon at one place and the other at another far distant one, i.e., the detectors used to register the photons have a well-defined position and are well separated from each other, i.e., you perform local measurements, and the setup is such that the measurement events ("clicks") are space-like separated. In this sense the observables measured on the photon are "local" in the usual sense of QFT.

You have a philosophy full of contradiction like: QED predict non-local phenomenon by no being able to compute them. Or hand-waving about "projection" of state between space-like region while your own theory explicitly forbid that.

QED cannot predict non-local phenomena, because it's a local QFT by construction. The contradiction is on your side! What's described by an entangled state are correlations of observables referring to parts of the entangled system which are measured at far distant places.

They are not, you are. So stop saying they are non-local in some other way that breaking "Einstein causality". There is no need to use other language to fit your philosophical need. Breaking Einstein causality do NOT mean FTL signaling.

Einstein causality simply means that space-like separated events are not causally connected, and by construction QED cannot violate Einstein causality by construction, i.e., because the micorcausality constraint is fulfilled for local observables.

Wrong. A theory may be incomplete, but still be useful. Like QM whose "fantastic" precision is to predict 1/2 chance of being spin up. Nature do way better than this, including spooky correlation at a distance.

That doesn't make sense. Nature behaves precisely as predicted by QM, including stronger-than-classical correlations between far-distant observables, which however are not spooky in any sense.

That you didn't quite grasp, because it means your "projection" could not happened, because it would be non-causal, and you keep denying that a-causal phenomenon are observed, and cannot even be present (in principle, because of micro causality) in your theory solution. And cherry on top you believe that theory to be complete and describing everything.

The projection is perfectly causal. It's achieved by registering photons 2 and 3 at different detectors and thus ensuring that they are found to be in the polarization-singlet Bell state. There's nothing acausal here. How to you come to this idea?

You cannot have your quantum cake and eat it.Picture a quantum snail, it can teleport from A to B, even at a snail pace. "While" it is gone from "space-time", while its world line is suspended, no bird will be able to eat it. Absolute protection.

I've no idea, what you want to say with ths.

 

[DrChinese]

I can't quote entire chapters of textbooks. It's not so difficult to just read the book, isn't it?

Yesterday by a simple Google research I also found out that the first part of Coleman's brillant QFT book (which emphasizes the importance of locality and microcausality from the very beginning!) is freely available through the arXiv:

https://arxiv.org/abs/1110.5013

Again, where to start? Again we have your gross failure to provide an actual quote that supports a single point you have made to date. Asking someone to read 335 pages and guess which part goes with what point? An insult to our intelligence. Just quote something that sounds like what you claim, then readers can read what leads up to it for themselves.

But that is not the most ridiculous part of the reference. If anyone were to actually READ the material, they would discover these gross deficiencies:

• References to the word "Bell" = 0
• References to the word "Entanglement" = 0
• References to "Monogamy" of Entanglement = 0
• References to Entanglement "Swapping" = 0

Explanation for the above: because all of the material was written pre-1988. The precise issues being discussed in this thread were not even discussed - because most of these ideas were developed post 1988. What a red herring and a waste of our time! Basically, you are just saying the same ol' stuff: QFT is local and microcausal. We get that you think so, and we also get that beyond those words you have yet to add any quotes to this discussion that are on point. Entanglement Swapping experiments are an experimental disproof of ideas of locality and Einsteinian causality. Support your disagreement with a suitable quote.

So I ask once again - since you have evaded the central argument (supported by the Monogamy of Entanglement, which is not disputed by you) I have demonstrated objective action at a distance via the BSM contribution to a distant entanglement swap:

Subensemble Before (Initial Preparation): $$\hat{\rho}=\hat{\rho}_{12} \otimes \hat{\rho}_{34}.$$
Subensemble After (Final Observation): $$\hat{\rho}'=\hat{\rho}_{23} \otimes \hat{\rho}_{14}.$$
Objective Change: $$\hat{\rho}_{12} \otimes \hat{\rho}_{34} ≠ \hat{\rho}_{23} \otimes \hat{\rho}_{14}.$$
If it isn't the BSM "quantum causing" the "quantum nonlocal" swap as I say: What exactly do you think did cause the objective state change that can be called "local and microcausal" in this experiment?

 

[vanhees71]

The objective state change is due to the selection of a subensemble by projecting photons (2&3) to a Bell state (in the discussed paper by Pan et al to the polarization-singlet state).

All you should read is the meaning of the microcausality condition as ruling out causal connections of space-like separated events within relativistic QFT (and that's why it's called a local relativistic QFT at least in the HEP community). For that it's sufficient to read the chapter labelled with "September 30" (pp. 21-28). These are 7, not 355 pages.

https://arxiv.org/abs/1110.5013

I didn't know that it is necessary to explain people who want discuss about entanglement in relativistic QFT, how to use a textbook/manuscript. SCNR.

 

[LittleSchwinger]

Very briefly you can "correlation swap" in classical probability, just like you can use correlations to "state teleport" in classical probability. Entanglement swapping and quantum teleporting are just quantum generalisations of these simple protocols.

 

[WernerQH]

The thread is supposed to be about whether there are hidden assumptions in Bell's theorem which leave the possibility open for a local hidden variable theory.

I've been following this thread since its beginning, and I thought that @Demystifier (post #4) and @DrChinese (post #5) had already summarized what I'd wanted to say about the topic. (Not that the coffin for local hidden variable theories needed any more last nails!) Questioning the appropriateness of the "algebra of real numbers" seems hardly relevant to a physicist like myself, but the "Event-by-event simulation of Einstein-Podolsky-Rosen-Bohm experiments" by Zhao et al. piqued my interest. Also a computer (properly programmed!) cannot violate Bell's theorem.

The tricky part seems to be the artificial time delays they introduce in their simulations, which depend on the detector settings:

There are not many reasonable options to choose the functional dependence of T.

We found that T(x) = T0|sin 2x|^d yields the desired results [15].

It's not clear to me why these time delays should be introduced in such a simulation at all. The dependence on the polarizer orientations introduces an additional (anti-)correlation that together with the assumed strict orthogonality of the photon states $\xi_n$ helps beat the Bell inequality. For the particular (peculiar!) case $d=4$ the integrals (eq. 26) are dominated by small values of $\theta$.

The reason why people still search for loopholes in Bell's theorem must be psychological. Some of them seem to think of locality and determinism as the essence of physics. The Hess et al. paper is similar in spirit to Superdeterminism, but is different in how it deals with Probability Theory. While Superdeterminism seems to reject Probability Theory altogether, Hess et al insist that it is applied incorrectly. But as to the question of why the correlations are stronger than what local models can account for, Hess et al. are just as vague:

Possible time dependences within the light cone are numerous. We just list here a smorgasbord of those that matter for sets of particles with spin. The earth rotates around itself and this rotation introduces a time dependence on [...]

(In other words: anything might happen.)

How plausible are alternative abstractions which don't, or which don't and support locality.

The mathematical abstractions must of course match the physical concepts of the theories. And, at least in my view, among the "hidden assumptions" there is an elephant in the room: the existence of photons. It is compelling to explain the observed correlations in the experiments of last year's Nobel prize winners in terms of photons. How else could you explain them, if not through photons having polarization? But this explanation raises more questions than it answers. What are the properties of those photons? They seem to be "undefined" or uncertain most of the time, and properties of the detectors as much as the properties of those photons. Photons have paradoxical properties (especially when they are "entangled"). It reminds me of the paradoxical properties of the ether, which was an equally compelling idea for Maxwell and Michelson. (How else can a light wave propagate? What could an electric field be if not the stress in an elastic medium?) Today we interpret the Michelson-Morley experiment as evidence against the existence of the ether. But this was not Michelson's view! I like to believe that when quantum theory is finally understood, the Aspect et al. experiments will be seen as evidence against the existence of photons.

 

[DrChinese]

1. My main interest was to clarify the foundational assumptions. In this context, as much nuance as possible is appreciated, meaning that even if a majority of those in the community have decided certain assumptions or nuances can be glossed over or abstracted, I would be interested to hear the basis for glossing over or abstracting these nuances or assumptions in as much objective depth as possible.

This could mean exploring not only the assumptions but also the definitions. Locality and realism have always been confusing for me in this regard, because it seems that either a lot that I don't know is being assumed (well almost certainly this is true), and/or these concepts are not fully agreed on or objectively defined.

In terms of locality and Hess's argument that Bell's theorem assumes the algebra of real numbers applies, I wonder how would you even define locality if your hidden variable theory doesn't follow the algebra of real numbers. It seems like an issue where you first need the hidden variable theory with its explicit mathematical abstractions known before you can meaningfully define locality?

2. Anyways, sorry for the interruption, carry on.

@Jarvis323

1. a. I believe the correct foundational assumptions go back to the 1935 EPR paper. They defined Realism ("Elements of Reality") and assumed Locality (simply that an action here cannot affect an outcome there). They also explicitly assumed that all possible elements of reality should be considered as simultaneously real. I.e. they conclude that outcomes of all possible measurements must therefore be predetermined, and since QM does not predict those outcomes, it must be incomplete.

b. Bell successfully attacked that conclusion, and note that discussion of "algebra of real numbers" is not a component of either EPR or Bell. The De Raedt team is trying to attack Bell with their simulations, trying to demonstrate that there is a local realistic solution that reproduces the QM statistics. In a way, they are able to show that... but their simulation in turn exploits a variety of experimental type assumptions: detection loophole, varying angle settings and issues around that, etc. But subsequent experiments where there is detection of all events renders their simulation moot.

c. My point has been that the advent of experiments featuring entanglement swapping from independent distant sources, there just isn't a mechanism to base ANY "local" simulation on. In the context of their simulations, what happens with the BSM on [2 & 3] is not relevant as long as it is distant from the Bell test. It is then only the [1 & 4] photons - which originate distant from each other but are now entangled (and still distant) - that are of interest. Clearly, such photons should not be entangled if they have never even existed in a common light cone AND you assert strict locality. You don't really need to go any further.2. It is we who are interrupting you! Apologies, I am ready and willing to discuss the De Raedt papers further if you like.

3. It's not clear to me why these time delays should be introduced in such a simulation at all.

3. I completely agree. They have a number of little tricks like this that appear to make the simulation work. What they have done is essentially fine tune their model away from a straight simulation and towards a series of detection and measurement loopholes. All ones that they dream up.

The problem with that is that there are literally hundreds/thousands of different Bell-type tests, and their models are only "correct" for one/few at a time. Any individual local realistic computer program they invent can be falsified by some other Bell test.

 

[DrChinese]

1. The objective state change is due to the selection of a subensemble by projecting photons (2&3) to a Bell state (in the discussed paper by Pan et al to the polarization-singlet state).

2. All you should read is the meaning of the microcausality condition as ruling out causal connections of space-like separated events within relativistic QFT (and that's why it's called a local relativistic QFT at least in the HEP community). For that it's sufficient to read the chapter labelled with "September 30" (pp. 21-28). These are 7, not 355 pages.

https://arxiv.org/abs/1110.5013

1. We agree. The projection causes the state change.

2. I read those pages previously, and it definitely does NOT say anything that applies to an entanglement swap. Long distance Entanglement Swaps are experimental reality, which trumps words in an outdated and irrelevant college lecture.

3. Very briefly you can "correlation swap" in classical probability, just like you can use correlations to "state teleport" in classical probability. Entanglement swapping and quantum teleporting are just quantum generalisations of these simple protocols.

3. Assuming I understood your comment: Obviously false. There can be no classical analog of swapping that generates perfect correlations and violates Bell inequalities. Not when the individual components have never existed in a common light cone. That's the whole point of this thread. That is what the De Raedt simultations (OP) attempted to demonstrate. So you will need to give up some backup here. There are dozens of papers that say exactly the opposite. Here for example:

https://arxiv.org/abs/1211.3560
Entangled states cannot be classically simulated in generalized Bell experiments with quantum inputs
"A natural way to compare quantum resources against classical ones is via the simulation [6] of quantum measurement scenarios [2]. There, one tries to simulate the correlations obtained by measuring a given quantum state in a Bell scenario [7, 8], allowing the parties to use in the simulation shared randomness and possibly classical communication [9]. If no communication is allowed, the set of simulable correlations is bounded by Bell inequalities [10], whereas an infinite amount of communication allows the simulation of any correlations."

No communication is possible in the swapping regime we've discussed, as the each of the observations are recorded before any classical communication can arrive.

 

[LittleSchwinger]

Assuming I understood your comment: Obviously false. There can be no classical analog of swapping that generates perfect correlations and violates Bell inequalities

That's not what I mean. Classical probability theory of course doesn't violate Boolean inequalities. It's that you can correlation swap and teleport states in classical probability theory as well. The quantum part is generating correlations beyond the classical bounds, not so much the swapping or teleporting. So entanglement swapping is:

Correlation swapping [Has a classic analogue]
+
Boolean inequalities violations [Has no classical analogue]

When you study Probability theories in general Entanglement swapping and teleporting are lifts/generalisations of classical protocols. That's what I meant by entanglement swapping is a generalisation of classical protocols.

 

[Fra]

1. We agree. The projection causes the state change.

I still have the impression that again the disagreement is to a large extent focused around the terms "caused by" vs "explained by". There seems to be agreement as to what actually happens (objectively), but not howto "understand it", to bring in yet another word.

It's like causation is a sacred word for physicists, who has the right to use it?

/Fredrik

 

[gentzen]

So I ask once again - since you have evaded the central argument (supported by the Monogamy of Entanglement, which is not disputed by you) I have demonstrated objective action at a distance via the BSM contribution to a distant entanglement swap:

Subensemble Before (Initial Preparation): $$\hat{\rho}=\hat{\rho}_{12} \otimes \hat{\rho}_{34}.$$
Subensemble After (Final Observation): $$\hat{\rho}'=\hat{\rho}_{23} \otimes \hat{\rho}_{14}.$$

That is not right. The subensemble already was in state $$\hat{\rho}'=\hat{\rho}_{23} \otimes \hat{\rho}_{14}$$ before the final observation. Only the full ensemble was in state $$\hat{\rho}=\hat{\rho}_{12} \otimes \hat{\rho}_{34}.$$

 

[PeterDonis]

The subensemble already was in state $$\hat{\rho}'=\hat{\rho}_{23} \otimes \hat{\rho}_{14}$$ before the final observation.

What subensemble? There can't be any such subensemble in the initlal state. That's the point of the monogamy of entanglement argument.

 

[gentzen]

QM (or QFT) predicts statistics of measurement results. Its predictions are not concerend with objective states of individual pairs of photons.

This depends on which interpretation you are using. You appear to recognize that in your next paragraph.

I guess you mean

So your statements about objectively different states are interpretation dependent, believe it or not.

I was thinking here about interpretations with an instantaneous collapse of the wavefunction.

Is my statement that "QM (or QFT) predicts statistics of measurement results" is interpretation dependent? Interesting question! I do use the minimal statistical interpretation for comparing predictions of QM with the predictions of different interpretations. So it seems that my statement is only acceptable if one subscribes to the minimal statistical interpretation. But does this mean that I would subscribe to QBism, if I used the Brier Score to compare the predictive performance of QM (or some interpretation) for individual systems? (The connection to the Brier Score is Mermin's statement "That probability-1 assignments are personal judgments, like any other probability assignments, is essential to the coherence of QBism.") Probably not, I guess it only means that I accept that the intepretation has some point. It doesn't imply full acceptance, or rejection of other interpretations.

 

[PeterDonis]

I was thinking here about interpretations with an instantaneous collapse of the wavefunction.

In such an interpretation the instantaneous collapse changes the state, so there is no problem at all with the entanglement changing; certainly instantaneous collapse is not the same as "just select a subensemble".

Is my statement that "QM (or QFT) predicts statistics of measurement results" is interpretation dependent?

No, that statement is not. But your next statement that I quoted, that the predictions "are not concerned with objective states of individual pairs of photons", is.

 

[gentzen]

Subensemble Before (Initial Preparation):

That is not right. The subensemble already was in state

What subensemble?

I guess the subensemble which was selected by the final observation. If you want, you can also say that it is unclear what DrChinese means by Subensemble Before (Initial Preparation), because in a certain sense, the subensemble has not been determined yet at that point.

 

[PeterDonis]

I guess the subensemble which was selected by the final observation.

But if the final observation (the BSM measurement) changes the state, then you can't say that it's just selecting a subensemble from the initial state.

you can also say that it is unclear what DrChinese means by Subensemble Before (Initial Preparation), because in a certain sense, the subensemble has not been determined yet at that point.

You can just pick the subset of runs for which the BSM measurement gives an "event ready" signal, and look at the initial state for that subset of runs. Which will be the same as the initial state that was prepared--with 1 & 2 maximally entangled and 3 & 4 maximally entangled--since the same state is prepared for all the runs. And then the monogamy of entanglement argument goes through for just that set of runs.

 

[gentzen]

But your next statement, that the predictions "are not concerned with objective states of individual pairs of photons", is.

Oh, I see. Yes, that statement is interpretation dependent, because some interpretations might indeed be concerned with such predictions.

 

[DrChinese]

That is not right. The subensemble already was in state $$\hat{\rho}'=\hat{\rho}_{23} \otimes \hat{\rho}_{14}$$ before the final observation. Only the full ensemble was in state $$\hat{\rho}=\hat{\rho}_{12} \otimes \hat{\rho}_{34}.$$

False. Each and every pair of the [1 & 2] pairs, and each and every pair of the the [3 & 4] pairs, are maximally entangled to start with. Therefore, not a single one of those pairs contained a photon entangled with another quantum object anywhere in the universe. I.e. there is no such initial subensemble of [1 & 4] entangled pairs as you claim.

If what you said were true, then some of the [1 & 2] pairs would not demonstrate perfect correlations - which we all know is incorrect (because they all do). Alternately, the following would need to be true (for psi+ case):

• [1 & 2] are perfectly correlated (maximally entangled) and will violate a CHSH inequality.
• [1 & 4] are perfectly correlated (maximally entangled) and will violate a CHSH inequality.
• [2 & 4] are perfectly correlated (maximally entangled) and will violate a CHSH inequality.

But no two of these are allowed simultaneously. That's explicitly ruled out by monogamy of entanglement. If [1 & 2] violate a CHSH inequality, then [1 & 4] cannot also (and vice versa).

This is true of each and every initial pair, as well as any subensemble of same you care to select.

 

[gentzen]

You can just pick the subset of runs for which the BSM measurement gives an "event ready" signal, and look at the initial state for that subset of runs.

Yes, then you are right, and the state that DrChinese gave for that (sub)ensemble was correct.

 

[Simple question]

The two photons are "connected", because they are prepared in an entangled state

In single pair experiment, yes. Do you even realize that in swapping 1&4 are NOT entangled ? which is the point ?

but this does not imply that a measurement on one photon has an instantaneous or faster-than-light influence influence on the other photon. The entangled state describes correlations observed for the outcome of such measurements. Einstein called it "inseparability".

Nobody is interested by explanation. Actually there is NONE. You opinions about instantaneous action, whatever that means, are immaterial.

I don't understand what you mean by "non-local".

The same as everybody else, including you. Example:

The experimentalists measure one photon at one place and the other at another far distant one, i.e., the detectors used to register the photons have a well-defined position and are well separated from each other, i.e., you perform local measurements, and the setup is such that the measurement events ("clicks") are space-like separated.

But as usual, in the very next sentence you contradict yourself

In this sense the observables measured on the photon are "local" in the usual sense of QFT.

No, in every sense possible, those observations and ticks are non-local. That's why you cannot observe Bell's violation but by waiting for those non-local records, to be confronted/compared (by non-FLT/classical means)). Only then non-local correlation are observed. Only the final merging of those non-local post observation actually reveal entanglement.

QED cannot predict non-local phenomena, because it's a local QFT by construction.

Correct

The contradiction is on your side! What's described by an entangled state are correlations of observables referring to parts of the entangled system which are measured at far distant places.

Now, who is contradicting himself again ?

Nature behaves precisely as predicted by QM, including stronger-than-classical correlations between far-distant observables, which however are not spooky in any sense.

So it's not classical, but not spooky, and NATURE behave like QM predicted, and not the other way around ?

The projection is perfectly causal. It's achieved by registering photons 2 and 3 at different detectors and thus ensuring that they are found to be in the polarization-singlet Bell state. There's nothing acausal here. How to you come to this idea?

I follow the thread. Do you ? 2&3 ordering with 1&4 is IRRELEVANT. In other word acausal

I've no idea, what you want to say with this.

Don't worry, cryptographer, quantum-computer scientist, and bankers do.

 

[mattt]

I guess we all agree on the mathematics, but different people describe this mathematical piece with very different words.

Some questions to try to reach an agreement:

1. Do Alice and Bob ( communicating among themselves, but not with Victor) have a way of knowing if Victor made/is making/will make a BSM measurement on pairs 23 ?

If not, even if our description of the state of a subensemble has really changed, are we comfortable saying that "something has changed/is changing/will change backwards for Alice and Bob 14 particles", even if there is no way to verify ( by means of measurements) this claim?

 

[PeterDonis]

I guess we all agree on the mathematics

Please note that equations in images are not acceptable. Please use the PF LaTeX feature to post equations directly. There is a "LaTeX Guide" link at the bottom left of each post window.

 

[vanhees71]

In single pair experiment, yes. Do you even realize that in swapping 1&4 are NOT entangled ? which is the point ?

Sigh, I repeatedly explained this. The point of swapping is that you start with a state of the form
$$\hat{\rho}=\hat{\rho}_{12} \otimes \hat{\rho}_{34},$$
i.e., in the initial state photons 1 and 2 are not entangled in any way with photons 3 and 4. Photons 1&2 and photons 3&4 are maximally entangled in the polarization-singlet state.

When projecting photons 2&3 to the polarization-singlet state, which occurs in 1/4 of the cases, the so prepared ensemble is in the state
$$\hat{\rho}'=\hat{\rho}_{23} \otimes \hat{\rho}_{14}.$$

Nobody is interested by explanation. Actually there is NONE. You opinions about instantaneous action, whatever that means, are immaterial.

Then, why do you deny the simple fact that in QED there are no actions in a distance, i.e., no FTL signals, etc? If it's anyway immaterial you need not to deny mathematical properties of the theory!

The same as everybody else, including you. Example:

But as usual, in the very next sentence you contradict yourself

No, in every sense possible, those observations and ticks are non-local. That's why you cannot observe Bell's violation but by waiting for those non-local records, to be confronted/compared (by non-FLT/classical means)). Only then non-local correlation are observed. Only the final merging of those non-local post observation actually reveal entanglement.

That's not, what I wrote.

Correct

Now, who is contradicting himself again ?So it's not classical, but not spooky, and NATURE behave like QM predicted, and not the other way around ?I follow the thread. Do you ? 2&3 ordering with 1&4 is IRRELEVANT. In other word acausal

What do you mean by ordering?

Don't worry, cryptographer, quantum-computer scientist, and bankers do.

 

[vanhees71]

View attachment 320844
I guess we all agree on the mathematics, but different people describe this mathematical piece with very different words.

Some questions to try to reach an agreement:

1. Do Alice and Bob ( communicating among themselves, but not with Victor) have a way of knowing if Victor made/is making/will make a BSM measurement on pairs 23 ?

If not, even if our description of the state of a subensemble has really changed, are we comfortable saying that "something has changed/is changing/will change backwards for Alice and Bob 14 particles", even if there is no way to verify ( by means of measurements) this claim?

I can't read the image, but I hope that all agree on the mathematics, although several people seem to contradict mathematical facts (microcausality) of QED. I've no clue why.

ad 1) No Alice and Bob need to know, whether the photons they discover are chosen due to Victor's Bell measurement, i.e., if the photons they consider are only those, where Victors pair 2&3 has been found to be in the polarization-singlet state (i.e., whether both of his detectors registered a photon). To see the violation of Bell's inequality you have to communicate the outcomes of all three, Alice, Bob, and Victor.

Of course the state has changed, because we have chosen a subensemble due to Victor's projection measurement. You can say that this is a preparation procedure for the new state given the original state of the four photons, and QED predicts that for this subensemble the pair 1&4 is also in a polarization-singlet state.

 

[Simple question]

Spoiler alert: a contradiction is coming.

Sigh, I repeatedly explained this. The point of swapping is that you start with a state of the form
$$\hat{\rho}=\hat{\rho}_{12} \otimes \hat{\rho}_{34},$$
i.e., in the initial state photons 1 and 2 are not entangled in any way with photons 3 and 4. Photons 1&2 and photons 3&4 are maximally entangled in the polarization-singlet state.

And you are telling us that QFT/QED allows you to evolve that prepared state deterministically, and microcausaly, leading to an absolute determination of what can be observed.

When projecting photons 2&3 to the polarization-singlet state

When what ? Stop hand-waving. Show me your computations that tell when this happens. It follows microcausality remember ? It must be easy for you.

, which occurs in 1/4 of the cases, the so prepared ensemble is in the state
$$\hat{\rho}'=\hat{\rho}_{23} \otimes \hat{\rho}_{14}.$$

Not it is not. Update of knowledge is not part of your favorite philosophy.

Then, why do you deny the simple fact that in QED there are no actions in a distance

I don't, but you do. You've just written that "projection" change preparation state.

Here. Let me help you out with words: QM include spooky correlation at a distance. And they cannot be causally ordered, they just are non-local.

, i.e., no FTL signals, etc? If it's anyway immaterial you need not to deny mathematical properties of the theory!

I don't deny that either, so your point is moot.
What is not, is that you don't understand the implication of micro causality, beyond the no FLT signaling.

That's not, what I wrote.

I quoted verbatim your post #262.
But I am not the only one noticing you also have an idiosyncratic way to understand what "quoting" means.

 

[Fra]

QM include spooky correlation at a distance. And they cannot be causally ordered, they just are non-local.

I guess I do not understand the phrasings.

FTL influence would indeed be "spooky", but we agree there is no such thing.

But what part of a plain correlation deserve to be labeled spooky? Is it spooky we have a hard time to understand a mechanism that violated bells inequality? Is that it? Spooky as in not obeying bell realism?

/Fredrik

 

[Simple question]

I guess I do not understand the phrasings.

FTL influence correlation would indeed be "spooky", but we agree there is no such thing.

Fixed. And this is tested in the lab. So why bother mincing words ?

But what part of a plain correlation deserve to be labeled spooky?

The part that bothered Einstein.

Is it spooky we have a hard time to understand a mechanism that violated bells inequality? Is that it?

Yes, entanglement is spooky.

Spooky as in not obeying bell realism?

or Einstein locality. As per Bell's theorem.

 

[Fra]

FTL influence correlation would indeed be "spooky", but we agree there is no such thing.

Fixed. And this is tested in the lab. So why bother mincing words ?

I try to avoid mincing words, on the contrary do I try to understand the meaning behind. But your editing above still makes we wonder. Am I right to think that you by "FTL correlation" means "spacelike correlation"?

Using the FTL word seems to imply a communication, why else use the term. The only communication I see going on here is the between the observer Victor to the Observer that compares observations from Alice, and Bob (1&4) and the KEY info from Victor that is required to define the postselected ensemble. This is supposedly a classical message. Without receiveing this key, no observer can infer any entanglement.

The part that bothered Einstein.

Ok, we can call this part spooky.

or Einstein locality. As per Bell's theorem.

For me Einstein locality just means there are no FTL causations between remote systems? But where correlations have a previous common cause, they are not a violation of Einstein locality.

While a Bell style HV might have been one way to solve Einsteins original issue. Give his record of doing away with the realism of space and time, not once but twice, had he lived on and digested bells theorem, my bet is that he would have done away with realism of HV as well. Doing away with realism is possibly "spooky".

/Fredrik

 

[vanhees71]

Spoiler alert: a contradiction is coming.

And you are telling us that QFT/QED allows you to evolve that prepared state deterministically, and microcausaly, leading to an absolute determination of what can be observed.

After projecting (2&3) to the polarization-singlet state also (1&4) are in this state. That's due to the interaction of photons (2&3) with the beam splitter and the detectors. This is, of course, in principle described by QED since QED of course also applies to interactions of the em. field with matter.

When what ? Stop hand-waving. Show me your computations that tell when this happens. It follows microcausality remember ? It must be easy for you.

There's no need to do any calculations to know that there's no instantaneous interaction between the photons (2&3) and the equipment used to project them to the said polarization-singlet state with photons 1&4 and the equipment used to measure their polarization at their far-distant places. That's implemented in QED via the microcausality condition. If the registration events of photons (2&3) and 1 and 4 are space-like separated there cannot be causal influences between these measurements.

Not it is not. Update of knowledge is not part of your favorite philosophy.

Of course, as soon as for a given photon pair (2&3) the observer at the place knows that also (1&4) are entangled in the polarization-singlet state. Of course is an update of knowledge part of the minimal interpretation of QT, which I'm follow as an interpretation. That's not philosophy that's physics!

I don't, but you do. You've just written that "projection" change preparation state.

Of course, selecting a subensemble from a given ensemble leads to another state of the subensemble. That I've written.

Here. Let me help you out with words: QM include spooky correlation at a distance. And they cannot be causally ordered, they just are non-local.

Correlation but no interaction/causation! So indeed, finally you agree with what I say for years!

I don't deny that either, so your point is moot.
What is not, is that you don't understand the implication of micro causality, beyond the no FLT signaling.

If there's no FTL (faster-than-light) signalling, then space-like separated detection events cannot cause each other. That's a tautology.

I quoted verbatim your post #262.
But I am not the only one noticing you also have an idiosyncratic way to understand what "quoting" means.

 

[vanhees71]

FTL influence correlation would indeed be "spooky", but we agree there is no such thing.

Fixed. And this is tested in the lab. So why bother mincing words ?

I try to avoid mincing words, on the contrary do I try to understand the meaning behind. But your editing above still makes we wonder. Am I right to think that you by "FTL correlation" means "spacelike correlation"?

The editing in fact is crucial! There's no FTL influence due to microcausality, but there are correlations between far-distant parts of an entangled quantum system. It's of course pretty misleading to call that "FTL". It's just described by the state of the system at a given time (in some arbitrary reference frame).

 

[Fra]

It's of course pretty misleading to call that "FTL".

I totally agree.

But I was trying to be diplomatic in my comment as there seems to be disagreement what is misleading and what is clarifying I suspect this itself is conditional on ones particular state of confusion.

/Fredrik

 

[WernerQH]

After projecting (2&3) to the polarization-singlet state also (1&4) are in this state. That's due to the interaction of photons (2&3) with the beam splitter and the detectors.

Is this "projecting" something physical or mathematical? Have you changed your position with respect to the "collapse of the wave function"?? The word "after" is misleading here. I think QED forces us to consider the entire pattern of events in space-time, and their possible causal connections. The entire sequence, or "history", if you like. The sub-ensemble then encompasses only those sequences where something special (the "Bell state measurement") happened.

The way QED does its magic of correctly mirroring Nature's fine book-keeping has to do with the propagators reaching also into the backward light cone. I don't think you can make the book-keeping consistent if you allow only a local, continuous description that can only evolve forwards in time.

 

[gentzen]

Is this "projecting" something physical or mathematical? Have you changed your position with respect to the "collapse of the wave function"??

I don't think that vanhees71 has changed his position. As he said, he always uses the minimal statistical interpretation. There is no "collapse of the wave function" in that interpretation, but the ensembles and subensembles are of prime importance, and are what gets assigned a state (and what gets "prepared").

The way QED does its magic of correctly mirroring Nature's fine book-keeping has to do with the propagators reaching also into the backward light cone. I don't think you can make the book-keeping consistent if you allow only a local, continuous description that can only evolve forwards in time.

I guess you are simply thinking of something else here than vanhees71. His description is just QED, and not something "local" in the typical Bell-theorem interpretation of that word.

 

[vanhees71]

Is this "projecting" something physical or mathematical? Have you changed your position with respect to the "collapse of the wave function"?? The word "after" is misleading here. I think QED forces us to consider the entire pattern of events in space-time, and their possible causal connections. The entire sequence, or "history", if you like. The sub-ensemble then encompasses only those sequences where something special (the "Bell state measurement") happened.

The way QED does its magic of correctly mirroring Nature's fine book-keeping has to do with the propagators reaching also into the backward light cone. I don't think you can make the book-keeping consistent if you allow only a local, continuous description that can only evolve forwards in time.

It's something physical, because you select only those four photons to be measured, for which the pair (2&3) was found to be in the polarization-singlet state.

Of course you have to consider the entire pattern of events in spacetime, and what's for sure within standard local (=microcausal) QFT is that space-like separated events cannot in any way causally influence each other.

The history is (in any reference frame)

at $t=t_{12}$ pair (1&2) was created in an entangled state (say the polarization-singlet state for simplicity) at a place A'
at $t=t_{34}$ pair (3&4) was created in an entangled state (say the polarization-singlet state for simplicity) at a place B'

The time order of this creation processes is irrelevant. To have them for sure not in causal contact (that's what's aimed at in the entanglement-swapping experiment) you must ensure these creation events to be space-like separated. That can be achieved by simply choosing the inertial reference frame (lab frame) such that $t_{12}=t_{34}=0$.

Photon 1 will be manipulated with beam-splitters/polarizers and detected at time $t_1$ at a far distant place A

Photons (2&3) will be subject to the projection measurement to the polarization-singlet state at a place C, which can be very far distant from A, at times $t_{2C}$ and $t_{3C}$

Photon 4 will be manipulated with beam-splitters/polarizers and detected at a far distant place B at time $t_B$.

For sure for both photons (23) to be detected at C it needs at least a time $\text{max}(A'C,B'C)/c$ since the corresponding wave packets travel with $c$.

It's also for sure that photons 1 and 4 need the minimal times to reach their detectors given by the speed of light and the distances from their point of creation to the place of detection.

The temporal order of all these measurements is, however, completely irrelevant for the outcome of the photon statistics of the pair (14) given that you select only those for which the pair (23) was found to be in the polarization-singlet state. The result of all measurements on (14) is that they are also in the polarization-singlet state.

The fact that the time order for all these measurement is completely irrelevant for this outcome together with the assumption that standard QED is correct and thus that space-like separated events cannot be causally connected then ensures that all the measurements cannot causally influence in any way each other. Nevertheless through the selection of the pairs for which (23) was found to be in the polarization-singlet state also the before completely uncorrelated pairs (14) are foudn to be in the entangled (and thus maximally correlated) polarization-singlet state.

These arguments show clearly that under the assumption that standard QED is right that these correlations are not due to a causal influence of the measurements, and indeed QED, which was used to come to this prediction, tells us that the correlations are due to the preparation of the pairs (12) and (34) in the polarization-singlet state in the beginning, but these pairs being completely uncorrelated, i.e., in the initial state $\hat{\rho}=\hat{\rho}_{12} \otimes \hat{\rho}_34}$.

 

[WernerQH]

I guess you are simply thinking of something else here than vanhees71. His description is just QED, and not something "local" in the typical Bell-theorem interpretation of that word.

No, I was thinking of QED. It is true that the field operators satisfy local and even deterministic equations. But they are only statistical representations of possible field configurations. There's more to QED than field operators and their equations of motion: the Born rule forms an integral part of the theory. And applying the Born rule breaks the "locality" of the theory. What's been causing this ongoing kerfuffle is vanhees71's insistence on calling the entire theory "local" based on a heuristic used in the derivation of just a piece of it.

Consider one radioactive atom surrounded by several detectors. At most one of them can register the decay. How do the detectors "negotiate" which one will do this?
Of course, total energy has to be conserved. But adding up the energies absorbed by different detectors is definitely a nonlocal operation.

 

[WernerQH]

It's something physical, because you select only those four photons to be measured, for which the pair (2&3) was found to be in the polarization-singlet state.

[...]

These arguments show clearly that under the assumption that standard QED is right that these correlations are not due to a causal influence of the measurements [...]

I have absolutely no doubts about QED. Thank you for explaining everything once again, but I'm still unable to see your statements as a coherent whole.

 

[gentzen]

No, I was thinking of QED. It is true that the field operators satisfy local and even deterministic equations. But they are only statistical representations of possible field configurations. There's more to QED than field operators and their equations of motion: the Born rule forms an integral part of the theory.

And vanhees71 agrees with this statement. He uses the minimal statistical interpretation to interpret the probabilities from the Born rule.

And applying the Born rule breaks the "locality" of the theory. What's been causing this ongoing kerfuffle is vanhees71's insistence on calling the entire theory "local" based on a heuristic used in the derivation of just a piece of it.

No, the kerfuffle is pretty independent of vanhees71's use of the word "local". He even admits the existence of that non-local part of QM (or QFT), he just prefers to call it "far-distant correlations":
https://www.physicsforums.com/threa...ntum-foundations.1045477/page-10#post-6812647

The question which caused the kerfuffle is more subtle. Even if vanhees71, or martinbn, or I, or LittleSchwinger, or ... would explain it in perfectly clear words, it is still not sure that those who didn't find the answer themselves would get it. vanhees71 is just more motivated to try to explain it, so you get the impression that the problem would be with him.

Consider one radioactive atom surrounded by several detectors. At most one of them can register the decay. How do the detectors "negotiate" which one will do this?
Of course, total energy has to be conserved. But adding up the energies absorbed by different detectors is definitely a nonlocal operation.

There is just one particle from the decay, so the different detectors don't need to negotiate. (Or at least there will be just one particle, if you really have a one-particle state. But your question basically already presupposes that.) Of course, that particle doesn't have properties like a classical particle. It has fewer properties, and those fewer properties even can have strange non-local quantum correlations with properties of other particles. You want to know where those non-local correlations and their quantum randomness comes from? In the end, the information content of the particles must be limited, at least if the energy is limited. So they share their randomness... And where does the randomness itself comes from? I don't know.

 

[WernerQH]

There is just one particle from the decay, so the different detectors don't need to negotiate.

So it's the particle that decides? Does it send out tentacles to each detector? In a continuous, tentative way? Sorry about the sarcasm, I'm too much afflicted with classical thinking.

Of course, that particle doesn't have properties like a classical particle. It has fewer properties, and those fewer properties even can have strange non-local quantum correlations with properties of other particles.

I have no problem with non-local correlations. I think that's what QFT is about. But we should be more explicit about the "correlata". What is it that is correlated? "Particle" is a classical concept, and talk about particle properties (undefined? uncertain? non-local?) just confuses the issues. I'm very much in favour of the (minimal) statistical interpretation. But I think it needs to be supplemented with a minimal ontology: that there are no particles, but just emission and absorption events (localized short-lived currents). Those events are what is correlated, and QED describes just that. What we call photons or electrons is something that we read into the patterns of events in space-time.

You want to know where those non-local correlations and their quantum randomness comes from? In the end, the information content of the particles must be limited, at least if the energy is limited. So they share their randomness... And where does the randomness itself comes from? I don't know.

I don't believe in particles, and therefore they don't need to carry information.
QFT is just fine as statistical theory.

 

[gentzen]

So it's the particle that decides? Does it send out tentacles to each detector? In a continuous, tentative way? Sorry about the sarcasm, I'm too much afflicted with classical thinking.

The way you formulated your question, the implicit assumption of a one-particle state was already there. That is the reason why you know that at most one of the detectors can register the decay. So the property that there is only one particle is certain, and hence that is also the explanation for what you would observe in that case.

But we should be more explicit about the "correlata". What is it that is correlated? "Particle" is a classical concept, and talk about particle properties (undefined? uncertain? non-local?) just confuses the issues.

I don't believe in particles, and therefore they don't need to carry information.

It is not really important whether it is a particle, a quasi-particle or an excitation which carries energy, momentum, spin, and information. The preparation is such that the environment (say a crystal) is in its ground state, and therefore cannot carry information. Only the particle/quasi-particle/excitation has the energy to be in that non-ground state, and therefore carries information. So the particle does not need to be real here, it is sufficient that it is a suitable concept to describe the physical situation caused by the preparation.

I'm very much in favour of the (minimal) statistical interpretation. But I think it needs to be supplemented with a minimal ontology: that there are no particles, but just emission and absorption events (localized short-lived currents).

You are in favor of a statistical interpretation. My impression is that neither vanhees71 nor you fully grasp that "minimal" part. A. Neumaier repeatedly tried to explain that it doesn't apply to the continuous evolution in time of a single system (like the universe), but only to ensembles. But with an ontology, I don't see what would stop you to apply it precisely to such a situation. Or maybe I don't grasp the "minimal" in your minimal ontology.

P.S.: I doubt that anybody else in this thread had this sort of objection to the position of the "postselection" camp. Your objection seems pretty independent of the kerfuffle to me.

 

[vanhees71]

There is just one particle from the decay, so the different detectors don't need to negotiate. (Or at least there will be just one particle, if you really have a one-particle state. But your question basically already presupposes that.) Of course, that particle doesn't have properties like a classical particle. It has fewer properties, and those fewer properties even can have strange non-local quantum correlations with properties of other particles. You want to know where those non-local correlations and their quantum randomness comes from? In the end, the information content of the particles must be limited, at least if the energy is limited. So they share their randomness... And where does the randomness itself comes from? I don't know.

This randomness is just a generic property of Nature. You could as well ask in classical physics, why particles have properties like mass, electric charge, etc. Of course in both classical an quantum physics the "answer" to that question according to modern physics is that a great deal follows from symmetry principles, but then you can ask, why the symmetry principles are the specific ones that describe Nature. In this sense the symmetries are just a description of basic properties of Nature, which cannot be explained by any "more simple" other generic feature of Nature.

After all physics is an empirical science, i.e., its descriptions of phenomena in terms of mathematical theories/models are based on quantitative observations of phenomena, often in experiments, where a well-defined sufficiently separated piece of matter is accurately investigated, and what came out of many such observations in connection also with theoretical analysis and theory/model building is the inherent randomness of Nature, which is described by quantum theory. In this sense QT is just a very efficient book-keeping tool of a vast of single empirical discoveries in a handful of "fundamental rules" or "postulates". That this works for such a vast number of observations is simply a miracle, which cannot be explained. Wigner called it "the incomprehensible effectiveness of mathematics in the natural sciences". Not also Weinberg's dictum about "the incomprensible ineffectiveness of philosophy in the natural sciences", which is, however, another story ;-).