I Does Time-Symmetry Imply Retrocausality? How does the Quantum World Say “Maybe”?

[DrChinese]

1. In scenarios where the collider loophole applies, whether the systems are classical or quantum, correlations in subsets of Alice's and Bob's outcomes selected by Charles's outcomes can be interpreted as post-selection effects.

2. I.e. The collider loophole places the appropriate entanglement swapping experiments on the same causal footing as conventional entanglement experiments, and hence if an interpretation accounts for conventional entanglement without invoking "action at a distance" (Price + Wharton), it will similarly account for certain entanglement swapping experiments.

3. The CL is insensitive to the spacetime location of the central vertex only if retrocausality is supposed. None of this contradicts Price + Wharton's careful use of the CL.

4. I've hashed out the problems with your understanding of the relation between BSM and SSM signatures in many previous threads, and doing so once again would only bring this thread off course.

1. There is no science to support this statement. Correlating outcomes from Alice, Bob and Charlie is no more "postselection" than any experiment of any type anywhere. In entanglement experiments: the design of the experiment is performed beforehand, and follows established theory. The scientists are testing a specific hypothesis. That being whether the experimenter's choice of a BSM (leads to entangled statistics) or SSM (leads to product statistics).

2. There is no science to support this statement either. It is completely ad hoc as a way to avoid addressing the issue. Entanglement swapping involves scientific issues that are quote different than for tradition PDC entangled systems. For example: a) swapping involves entanglement from different distant sources; b) indistinguishability plays a vital role during the BSM; c) the BSM can occur either before or after the measurements of Alice and Bob; and d) the BSM is performed on an unbiased basis relative to those of Alice and Bob.

3. None of this makes any sense (to me). If the CL existed, no assumptions are needed since the purpose of claiming the CL is to provide the groundwork for a local causal (Einsteinian) explanation - which is not retrocausal. To match experiment, the CL must be insensitive to location/causal ordering. On the other hand: if you accept retrocausality, then what purpose is the CL - since there is no longer a loophole at all.

4. The signature buckets are exactly the same in BSM/SSM experiments such as Megidish et al (see Fig. 3) but yes, slightly different (although of no significance) in Ma et al as you have argued. I agree there is little point of diverting this discussion, especially since you refuse to acknowledge important and well-accepted experimental science.

-DrC

 

[DrChinese]

From Wharton and Argaman, discussion of retrocausal models. This is more what I am familiar with from Ken, and I think this is a good paper.

Bell's Theorem and Locally-Mediated Reformulations of Quantum Mechanics

"These locally-mediated models require the relaxation of an arrow-of-time assumption which is typically taken for granted. Specifically, some of the mediating parameters in these models must functionally depend on measurement settings in their future, i.e., on input parameters associated with later times. This option (often called "retrocausal") has been repeatedly pointed out in the literature... A brief survey of such models is included here. These models provide a continuous and consistent description of events associated with spacetime locations, with aspects that are solved "all-at-once" rather than unfolding from the past to the future. The tension between quantum mechanics and relativity which is usually associated with Bell's Theorem does not occur here."

But the Collider Loophole concept is a bust. It is not generally accepted science, and virtually every reference to Entanglement "Collider Loophole" in Google is to the same handful of papers by Price, Wharton and/or Mjelva.

 

[Sambuco]

Hi everyone!

A new paper by Wharton & Price was submitted to ArXiv ten days ago (https://arxiv.org/abs/2507.15128). There, they discuss entanglement as a kind of preselection due to the preparation of an initial state.

I'm start to think that collider loophole (CL) deserves a thread of its own

Lucas.

 

[Sambuco]

But the Collider Loophole concept is a bust.

The first time I read about collider loophole, I had the same reaction. Now, while when I'm not a "fan" of this idea, I have an opinion more similar to what @Morbert says here:

The CL is not asserted as an interpretation-independent fact. It is instead invoked under appropriate interpretations in understanding entanglement swapping experiments and how they relate to conventional entanglement experriments.

In other words, it may be a useful concept within certain interpretations, such as those based largely on information, where a "mechanism" to explain entanglement is usually denied.

(...) virtually every reference to Entanglement "Collider Loophole" in Google is to the same handful of papers by Price, Wharton and/or Mjelva.

Well, maybe the rest of the world doesn't think that collider loophole is so useful after all...

Lucas.

 

[javisot]

New paper by Zeilinger, "violation of Bell Inequality with unentangled photons"

https://arxiv.org/abs/2507.07756

 

[Sambuco]

As an aside, I'd like to mention something I think we've already discussed, but which is relevant to the current discussion. The "entanglement" in entanglement swapping, and especially in the delayed-choice version (https://arxiv.org/abs/1203.4834) is not an interpretation-independent fact about the quantum state of Alice and Bob's particles. For well-known $\Psi$-ontic interpretations, such as Everettian many-worlds or Bohmian mechanics, the statistical data are predicted by the forward-in-time evolution of the quantum state of the four-particle system according to the Schrödinger equation. In that case, the quantum state of Alice and Bob's particles is separable at any time from the beginning to the end of the experiment. In other words, according to these interpretations, in delayed-choice entanglement swapping (DCES), there is no entanglement swapped.

One way out is simply to abandon the $\Psi$-ontic stance and consider the wave function as mere information, as in $\Psi$-epistemic interpretations, such as Qbism, relational quantum mechanics or the information-based interpretation favoured by Zeillinger himself (https://link.springer.com/article/10.1023/A:1018820410908). However, if we insist on considering this correlation (entanglement) as something "ontic", and we look for some causal explanation with the wave function playing a role in it, then we must accept that the quantum state is not absolute, but relational. Thus, from the perspective of Victor (I follow the names employed by @DrChinese), his decision to make a BSM, together with the measurements outcomes obtained on the two particles he has access to, causes the state of Alice and Bob's particles to be updated, and entanglement arises retrocausally. In other words, there is no justification for accepting the quantum state that Victor assigns to the system, which should evolve backward-in-time, as ontic and rejecting the quantum state Alice and Bob put into the (textbook) Schrödinger equation. I'm not aware of any $\Psi$-ontic interpretation that postulates anything like that. It would be something like a many-many-worlds interpretation.

Lucas.

 

[DrChinese]

Hi everyone!

A new paper by Wharton & Price was submitted to ArXiv ten days ago (https://arxiv.org/abs/2507.15128). There, they discuss entanglement as a kind of preselection due to the preparation of an initial state.

I'm start to think that collider loophole (CL) deserves a thread of its own

Lucas.

I would happy to participate, as the CL really has nothing to do with retrocausality.

Thanks for the reference to the new paper. I think it does a better job of making their case. And for me, it is easier to see and describe its glaring flaws.

 

[DrChinese]

In other words, it may be a useful concept within certain interpretations, such as those based largely on information, where a "mechanism" to explain entanglement is usually denied.

Any interpretation features some kind of narrative that adds "something" to orthodox textbook QM. That narrative is subject to comparison to experiment.

Suppose someone said something like "Forward-in-time-only information-type interpretations fail because they fall victim to the Collider Loophole." Well, I don't have any particular issue with that statement. That's because I believe: "Forward-in-time-only information-type interpretations fail because they are contradicted by existing experiments." That's a much stronger assertion. The collider loophole doesn't even exist/apply in the experiments I cite, it really only exists as a putative concept in the Price/Wharton* papers.


*And to be clear: I am a big fan of most of their other work.

 

[DrChinese]

1. As an aside... For well-known $\Psi$-ontic interpretations, such as Everettian many-worlds or Bohmian mechanics, the statistical data are predicted by the forward-in-time evolution of the quantum state of the four-particle system according to the Schrödinger equation.

2. One way out is simply to abandon the $\Psi$-ontic stance and consider the wave function as mere information, as in $\Psi$-epistemic interpretations, such as Qbism, relational quantum mechanics or the information-based interpretation favoured by Zeillinger himself (https://link.springer.com/article/10.1023/A:1018820410908).

Lucas.

1. I find those to be in direct conflict with DCES experiments such as cited. But unfortunately, I cannot locate sufficiently detailed statements by the well-known advocates of those interpretations in order to highlight such conflict in a suitable manner*. For example, Vaidman's MWI page in Plato skips this entirely. Ditto the page on Bohmian Mechanics. Generally, what papers address swapping employ substantial handwaving without proving any real meat. Referencing Schrödinger equation evolution won't cut it, as there is a remote change of state (by definition) in all swapping experiments.

2. I of course am often surprised by what experimentalists hold as interpretations, and specifically Zeilinger**. All of his work points away from information-type interpretations. I wonder if anyone has seen any recent statements about his preferred interpretation.


*Because the rules of that interpretation seem to change when challenged.
**Who is also of course a master theorist.

 

[MrRobotoToo]

One way out is simply to abandon the -ontic stance and consider the wave function as mere information, as in -epistemic interpretations, such as Qbism, relational quantum mechanics or the information-based interpretation favoured by Zeillinger himself (https://link.springer.com/article/10.1023/A:1018820410908). However, if we insist on considering this correlation (entanglement) as something "ontic", and we look for some causal explanation with the wave function playing a role in it, then we must accept that the quantum state is not absolute, but relational. Thus, from the perspective of Victor (I follow the names employed by @DrChinese), his decision to make a BSM, together with the measurements outcomes obtained on the two particles he has access to, causes the state of Alice and Bob's particles to be updated, and entanglement arises retrocausally. In other words, there is no justification for accepting the quantum state that Victor assigns to the system, which should evolve backward-in-time, as ontic and rejecting the quantum state Alice and Bob put into the (textbook) Schrödinger equation. I'm not aware of any -ontic interpretation that postulates anything like that. It would be something like a many-many-worlds interpretation.

Is this related to the fact that retrocausality violates the PBR theorem's assumption that it's possible to prepare systems independently of each other, thus nullifying it's conclusion that any hidden variable model must incorporate an ontic $\Psi$? Does the relationality of the wave function reduce it to being merely epistemic? (I'm just noticing that the toy HV model I presented earlier makes no explicit use of the wave function)

 

[PeterDonis]

The "entanglement" in entanglement swapping, and especially in the delayed-choice version (https://arxiv.org/abs/1203.4834) is not an interpretation-independent fact about the quantum state of Alice and Bob's particles.

Careful. Write down any quantum state you like: whether or not that state is entangled is a mathematical fact about the state (whether or not it can be expressed as a product of states of subsystems--if it can't, it's entangled), and does not depend on any interpretation.

What does depend on the interpretation is what physical meaning is assigned to quantum states, and which quantum states are taken to be the "correct" ones to describe particular scenarios.

 

[Sambuco]

Careful. Write down any quantum state you like: whether or not that state is entangled is a mathematical fact about the state (whether or not it can be expressed as a product of states of subsystems--if it can't, it's entangled), and does not depend on any interpretation.

What does depend on the interpretation is what physical meaning is assigned to quantum states, and which quantum states are taken to be the "correct" ones to describe particular scenarios.

Thanks @PeterDonis for your comment! I should have said "(...) is not an interpretation-independent fact about Alice and Bob's particles," without including the term "quantum state".
What do you think is best? Should I edit my post to avoid confusion for future readers, or should I leave it as is?

Lucas.

 

[Sambuco]

the CL really has nothing to do with retrocausality

I have a similar view. I think collider loophole could be a useful concept in information-based interpretations, but not in more "realistic" ones, where one attempts to find a mechanism to explain entanglement, and in that attempt, retrocausality appears as a possibility.

"Forward-in-time-only information-type interpretations fail because they are contradicted by existing experiments."

Well, information-based interpretations typically don't pay much attention to the time direction of state evolution, precisely because they assume that states represent mere information. I personally think they fit well with entanglement swapping, where temporal order doesn't change the results.

Lucas.

 

[Sambuco]

1. I find those to be in direct conflict with DCES experiments such as cited. But unfortunately, I cannot locate sufficiently detailed statements by the well-known advocates of those interpretations in order to highlight such conflict in a suitable manner*. For example, Vaidman's MWI page in Plato skips this entirely. Ditto the page on Bohmian Mechanics. Generally, what papers address swapping employ substantial handwaving without proving any real meat. Referencing Schrödinger equation evolution won't cut it, as there is a remote change of state (by definition) in all swapping experiments.

I believe these $\Psi$-ontic interpretations assume that the quantum state evolves (forward-in-time) as predicted by Schrödinger equation, just as in Mjelva's paper. For DCES, the remote change of the state occurs when Alice and Bob measure their particles, causing the global four-particle state (including the other two other particles) to be projected onto the state corresponding to the measurement outcomes observed by Alice and Bob.

Unlike information-based interpretations, such as the one favoured by Zeilinger, these $\Psi$-ontic interpretations conclude that the quantum state of the system, at any time during the delayed-choice version of the experiment, doesn't show entanglement between the particles to which Alice and Bob have access to. This is what I found "problematic" about how these interpretations address entanglement swapping. They offer two completely different stories about what happen in the delayed and non-delayed cases, ignoring the fact that both experiments yield exactly the same results regardless of the temporal order.

Lucas.

 

[Morbert]

4. The signature buckets are exactly the same in BSM/SSM experiments such as Megidish et al (see Fig. 3) but yes, slightly different (although of no significance) in Ma et al as you have argued.

In both the Ma and Megidish experiment, the state of the biparticle system incident on Charles's apparatus is rotated during a successful BSM. In the Ma experiment, this rotation means the relevant polarization buckets in BSM runs cannot be identified with their corresponding buckets in SSM runs. Similarly in the Megedish experiment: The rotation of indistinguishable vs distinguishable photons breaks the correspondence of the polarization buckets in successful vs failed BSMs. They amount to different selection procedures.

No productive discussion about retrocausality or collider bias can be had until you understand this.

 

[Morbert]

Well, information-based interpretations typically don't pay much attention to the time direction of state evolution, precisely because they assume that states represent mere information. I personally think they fit well with entanglement swapping, where temporal order doesn't change the results.

As an aside, forward-in-time accounts based on ordinary undergraduate-level formalisms of QM are perfectly capable of reproducing all correlations seen in these experiments.

 

[DrChinese]

In both the Ma and Megidish experiment, the state of the biparticle system incident on Charles's apparatus is rotated during a successful BSM. This rotation means the relevant polarization buckets in BSM runs cannot be identified with their corresponding buckets in SSM runs. They amount to different selection procedures.

No productive discussion about retrocausality or collider bias can be had until you understand this.

As I have said repeatedly: The difference you describe (BSM vs. SSM) is true but not scientifically significant for the Ma experiment. I acknowledge you dispute this, despite this being one of the most commonly cited of swapping experiments (and by a top team). But for the sake of argument, let's return to the other experiment and see...

What should be relevant and important from your perspective: That same technique (the difference between BSM and SSM) is NOT used in the Megidish experiment. Everything in the setup for both BSM and SSM is 100% identical as far as wave plates, polarizers, BS and PBS, detectors, etc. The setup is in Fig. 2.

Note that there are rotations of wave plates in the main experiment, and this is done in order to perform a full quantum state tomography (QST). So don't let that fool you into seeing a parallel with the Ma experiment. Unlike Ma, the main Megidish experiment does NOT compare BSM to SSM - and the mechanism to perform the QST operates exactly the same in all variations. So there is no difference in count rates for a full QST (approx. 4320 over 6 minutes).

Also capable of confusing: The Megidish experiment uses a very unusual technique to measure the photons' polarizations. They use a single set of polarizing beam splitters to measure all 4 photons. Normally there would be 3 sets (one set each for Alice, Bob and Victor). In Megidish, photon 1 always arrives first to the detectors; photon 4 arrives last; and (when a BSM occurs) photons 2 and 3 physically overlap and arrive in the same time window.

The ONLY difference is that for our relevant variation: They made the 2 & 3 photons distinguishable, casting them into an SSM: "One can also choose to introduce distinguishability between the two projected photons." They do this by adding a small extra distance to one of the paths. From the paper: "we introduced a sufficient temporal delay between the two projected photons (see Fig. 3c)." So one photon (call it photon 2) arrives a bit later than photon 3. The results so obtained are unquestionably apples to apples, in contradiction to your assertion about different buckets. And they show that PHYSICAL overlap in the beam splitter is an absolute requirement for swapping. Added sufficient temporal delay => no BS overlap => no correlation*. The choice of a time delay (or not) is the (only) independent variable. Note also that in this experiment, there is no rapid random switching between BSM or SSM.

No physical overlap, no Entangled State statistics. And although this particular experiment does not feature remote delayed choice (DCES), it should go without question that the results would not change if it had included that. When you combine Ma and Megidish results: Alice and Bob can perform their measurements in the absolute relative past of Victor's choice to entangle (BSM) or not (SSM), leading to Victor's choice being identified as the causal agent for the results.

*Note that on the QST results (Fig. 3), there is correlation in all 3 graphs for the HHHH> and VVVV> bars. These are the cases for which all 4 photons are measured on the same HV basis, which always produces correlation and is usually labeled as separable state. To see proper swapping, the 2 & 3 photons must be measured on a different basis than 1 & 4.

 

[DrChinese]

As an aside, forward-in-time accounts based on ordinary undergraduate-level formalisms of QM are perfectly capable of reproducing all correlations seen in these experiments.

Don't think so. But if you have a reference that actually addresses Ma and/or Megidish in detail, then please share it with me. (Note that orthodox QM does not feature a forward-in-time explanation.)

As we discussed to death in another thread, Mjelva provided a detail account as you mention. It's definitely the best I have seen, and nicely goes through every single step for the Ma and Megidish type experiments for forward-in-time-only interpretations. It also does the same for MWI.

However, it has a fatal flaw (explained in that thread), which is contradicted by experiment*. This is why retrocausal (or acausal as @RUTA calls it) type interpretations have advantages. Only a complete quantum context, including future measurement settings, properly leads to the expectation.


*It took me until about post #86/89 or so in that thread to figure it out! So definitely was not obvious to me...

 

[Morbert]

I acknowledge you dispute this, despite this being one of the most commonly cited of swapping experiments (and by a top team). So for the sake of argument, let's return to the other experiment.

I do not dispute any experimental facts. Everything I have said is fully consistent with Ma's paper. What I dispute is your incorrect understanding of Ma's paper. Like in the Megedish experiment, the time-evolution of the incident 2 and 3 photons breaks the correspondence between polarization buckets across runs with and without a BSM.

The ONLY difference is that for our relevant variation: They made the 2 & 3 photons distinguishable, casting them into an SSM

I should have labelled my edit in my previous post:

Similarly in the Megedish experiment: The rotation of indistinguishable vs distinguishable photons breaks the correspondence of the polarization buckets in successful vs failed BSMs. They amount to different selection procedures.

I.e. What you are doing is looking at the polarization signature of two rotated indistinguishable particles and inferring, counterfactually, the same polarization signature if they had been distinguishable.

However, it has a fatal flaw (explained in that thread), which is contradicted by experiment.

It is not contradicted by experiment.

 

[Sambuco]

Is this related to the fact that retrocausality violates the PBR theorem's assumption that it's possible to prepare systems independently of each other, thus nullifying it's conclusion that any hidden variable model must incorporate an ontic Ψ?

I'm not well versed with all the nuances of the PBR theorem. I believe @DrChinese has studied it in more detail. Do you have anything to say about the relationship between preparation independence and retrocausality.

Does the relationality of the wave function reduce it to being merely epistemic?

Good question! As far as I know, relational (or perspectival) interpretations tend to be $\Psi$-epistemic. Personally, I'm think that it would be possible to formulate a relational $\Psi$-ontic interpretation in a consistent way.

Lucas.

 

[Sambuco]

However, it has a fatal flaw (explained in that thread), which is contradicted by experiment. This is why retrocausal (or acausal as @RUTA calls it) type interpretations have advantages. Only a complete quantum context, including future measurement settings, properly leads to the expectation.

I'm find this statement slightly confusing. In Mjelva's paper, the experimental results in Ma's paper are correctly predicted by taking each of the four entangled states that Victor could obtain after making a BSM on the pair of particles he has access to, and by applying the Born rule by projecting these states onto each of the states of the four-particle system after Alice and Bob measurements were completed (these states are found in eq. (5) of the paper). Everything agrees with the textbook formulation, taking into account the whole context, as you rightly pointed out.

Lucas.

 

[DrChinese]

I do not dispute any experimental facts. Everything I have said is fully consistent with Ma's paper. What I dispute is your incorrect understanding of Ma's paper. Like in the Megedish experiment, the time-evolution of the incident 2 and 3 photons breaks the correspondence between polarization buckets across runs with and without a BSM.

I.e. What you are doing is looking at the polarization signature of two rotated indistinguishable particles and inferring, counterfactually, the same polarization signature if they had been distinguishable.

You are not reading the Megidish experiment! Please read my post where I point you to the exact text and figures involved. I said we'd skip Ma for now.

You: "The rotation of indistinguishable vs distinguishable photons breaks the correspondence of the polarization buckets in successful vs failed BSMs."

No! There is NO rotation occurring in the SSM version that is different than the BSM version. The ONLY difference is the addition of a temporal delay. You are taking a concept from Ma and inserting it in Megidish, but that concept is not present. The use of wave plates is common in experiments using entanglement and entanglement swapping. And while I disagree with your characterization of the Ma experiment setup, it is NOT the same as Megidish at all as to execution specifics.

The BSM trials are absolutely identical to the SSM trials as to rotation (wave plates), detector signatures and buckets. Read it again, you are looking in the wrong spots. Or quote something from the Megidish paper itself that supports your position, as I have taken the time to do.

 

[gentzen]

As an aside... For well-known Ψ-ontic interpretations, such as Everettian many-worlds or Bohmian mechanics, the statistical data are predicted by the forward-in-time evolution of the quantum state of the four-particle system according to the Schrödinger equation.

1. I find those to be in direct conflict with DCES experiments such as cited. But unfortunately, I cannot locate sufficiently detailed statements by the well-known advocates of those interpretations in order to highlight such conflict in a suitable manner*.

*Because the rules of that interpretation seem to change when challenged.

Let me remind you that the current consensus is that Bohmian mechanics makes the same measurable predictions as orthodox interpretations, at least in the non-relativistic, non-QFT case.

Challenging that consensus feels close to "personal research" territory to me. That said, I also believe that this consensus is not fully correct:

Because it was unclear to me how one would model quantum teleportation in Bohmian mechanics (BM), I tried to find existing accounts of how it is done. The only one I found was the link I gave above (Quantum State Teleportation understood through the Bohm Interpretation) from which I took the quote:

Simply by noting the actual position ($x_0$) of the measuring device, the observer, near particles 1 and 2, immediately knows which wavepacket $x_0$ has entered, and therefore which state is active for particle 3. The observer then sends this classical information to the observer at 3 who will then apply the appropriate unitary transformation $U_1\dots U_4$ so that the initial spin state of particle 1 can be recovered at particle 3.

The first step to understand why things are unclear to me, is to understand why the quoted part is not valid in BM, based on my current understanding. The devil is in the details for me. If the goal would just be to model the actually performed entanglement swap experiments in BM, then it is perfectly fine for me to do postselection based on particle trajectories. But for the general case, where different unitary transformations are applied to the "receiving qubit" of the pair of entangled qubits based on measurement results, some modeling needs to be found that makes it sufficiently clear what actually happens in BM. Otherwise, one should not claim to have shown that BM is indeed able to account for this type of experiment.

This is a case where orthodox interpretations do make predictions, but BM has trouble to model the situation, and hence cannot make predictions. But note that the actually preformed experiments can be modelled with BM, and that the mentioned "postselection" occurs both in BM and in the actually performed experiments.

However, it has a fatal flaw (explained in that thread), which is contradicted by experiment. This is why retrocausal (or acausal as @RUTA calls it) type interpretations have advantages. Only a complete quantum context, including future measurement settings, properly leads to the expectation.

I did research time-symmetric and retrocausal type interpretations quite intensely. I also studied "Picturing Quantum Processes" by Bob Coecke and Aleks Kissinger, because the calculus looks nicely time-symmetric too. Luckily for me, it turns out that Bob Coecke (et al?) found out how that time-symmetry gets broken (I think I know how to apply this "solution" to Consistent Histories): By "his" causality postulate:

So, we can more fundamentally interpret the causality equation as follows:
If a state is discarded, it may as well never have existed.

We can also interpret (6.32) directly:
If the output of a process is discarded, it may as well have never happened.
which is a straight generalisation of the interpretation we gave for causal states in Section 6.2.3.

We motivated causality with this motto:
if the output of a process is discarded, it may as well have never happened.
... (6.55)
This also means that if a processes is happening somewhere else, and its output never reaches us, we don’t need to care about it. As we already noted, this is crucial to being able to even do science, in that it allows us to safely ignore parts of the universe that won’t affect us.

Thus, the causality postulate (6.64) for a generic process guarantees that:
probabilities can be consistently assigned to branches.

Definition 8.8 is just a minor update to our original slogan for causality:
If we discard/delete all of the quantum/classical outputs
of a quantum process, it may as well have never happened.
Thus we have succeeded (as promised) in extending the interpretation of causality for quantum maps of Section 6.2.4, to quantum processes which may also involve classical inputs and outputs.

 

[DrChinese]

In Mjelva's paper, the experimental results in Ma's paper are correctly predicted by taking each of the four entangled states that Victor could obtain after making a BSM ... after Alice and Bob measurements were completed (these states are found in eq. (5) of the paper). Everything agrees with the textbook formulation, taking into account the whole context, as you rightly pointed out.

[I don't think it is fair to repeat the full arguments we made over in the other thread in this one. But here's the Cliffs Notes version just to save time.]

Mjelva, after his (3) which I completely agree with, and then after the measurements by Alice and Bob:

"Alice and Bob’s measurements have the effect of projecting the state into one of the following four product states, each with probability 1/4... [4 Product States listed]... On any given run of the experiment, the total system will be in one of the these possible states."

Experimental contradiction of Mjelva: It is an indisputable experimental fact that NONE of these 4 states - and no MIXTURE of these 4 states - can ever lead to an entanglement swap. Every experimenter who has ever worked with PDC entangled pairs can explain why*. That's because normal Type I and Type II PDC produced photon pairs are NOT entangled (and therefore cannot be used for swapping)! They emerge natively in precisely one of the 4 Product States listed by Mjelva**. Accordingly, they require relatively complex/difficult steps to create polarization entanglement. You can randomly mix these 4 Product states (say from 4 different laser sources), and they still would not be polarization entangled unless they were first made indistinguishable.


*PDC details are so taken for granted, experimenters stopped explaining them 10-15 years ago.
** Note that these 4 are counterfactual states, not observed states. A measurement on photon 1 does not produce a specific change (either physical or informational) to photon 2. It must be measured first.

 

[MrRobotoToo]

Do you have anything to say about the relationship between preparation independence and retrocausality.

I'd have to think more about it, although Matthew Pusey--the 'P' in 'PBR theorem'--seems to believe that retrocausality offers a loophole to the theorem.

 

[DrChinese]

1. Let me remind you that the current consensus is that Bohmian mechanics makes the same measurable predictions as orthodox interpretations, at least in the non-relativistic, non-QFT case.

2. Challenging that consensus feels close to "personal research" territory to me. That said, I also believe that this consensus is not fully correct:

3. But note that the actually preformed experiments can be modelled with BM, and that the mentioned "postselection" occurs both in BM and in the actually performed experiments.

4. I also studied "Picturing Quantum Processes" by Bob Coecke and Aleks Kissinger...

1. Don't believe I mentioned Bohmian Mechanics (although I can see why you might think I was including it). I don't feel I understand it well enough to apply the same arguments to BM. I know BM mostly is considered deterministic, but most advocates are careful not to ascribe specific intermediate states during state evolution. They follow orthodox QM on that: No result unless there is a measured result!

If Sheldon Goldstein (or @Demystifier !) ever made the same mistake as Mjelva has, I would definitely address it. Mjelva did specifically use the same [experimentally disproven] reasoning vis-a-vis MWI, which is absolutely forward-in-time-only in all formulations. But not all MWI advocates would necessarily agree with Mjelva's formulation - I'm not sure either way.


2. Experimental citations can't really qualify as "personal research". Somehow, I am about the only person in this thread citing well-accepted scientific teams and their papers.

For example: Mjelva as an authority? The reason his paper is worthwhile is simply because he dared to make explicit what many people already assumed - as responses here demonstrate. I thank him for that. But if someone is going to challenge me on his forward-in-time-only logic in a thread on retrocausality, it would be nice to hear from a suitable experimenter or at least a well-known advocate.

I would say Ma's delayed choice experiment is an example of causal (time) symmetry, regardless of whether you fully accept it or not. See here for a catalog of such. You know... from a well-known advocate.


3. All experiment science is "post-selection" (this should be obvious). You design a hypothesis on a specified dataset, perform tests using an independent variable, and catalog results that match the specifications... after.

Post-selected studies - what I would call the correct usage of the term - is where data is analyzed and a hypothesis is created after completion of the experimental study. This is an especially vicious trap in studies of medicine and human behavior where there are multivariate considerations. The Cochrane Foundation will not approve studies in which the hypothesis is published after data is collected. That generally doesn't happen in quantum physics, for the simple reason that the hypothesis follows established theory.

 

[Morbert]

You are not reading the Megidish experiment! Please read my post where I point you to the exact text and figures involved. I said we'd skip Ma for now.

You: "The rotation of indistinguishable vs distinguishable photons breaks the correspondence of the polarization buckets in successful vs failed BSMs."

No! There is NO rotation occurring in the SSM version that is different than the BSM version. The ONLY difference is the addition of a temporal delay. You are taking a concept from Ma and inserting it in Megidish, but that concept is not present. The use of wave plates is common in experiments using entanglement and entanglement swapping. And while I disagree with your characterization of the Ma experiment setup, it is NOT the same as Megidish at all as to execution specifics.

The BSM trials are absolutely identical to the SSM trials as to rotation (wave plates), detector signatures and buckets. Read it again, you are looking in the wrong spots. Or quote something from the Megidish paper itself that supports your position, as I have taken the time to do.

Similarly in the Megedish experiment: The rotation of indistinguishable vs distinguishable photons

I am saying that, in Megedish's experiment, the rotation of indistinguishable particles (interference terms) vs the rotation of distinguishable particles (no interference terms) plays the same role as rotation vs no rotation in Ma's experiment. It breaks any correspondence between the directly-recorded polarization signatures in BSM and no-BSM runs.

 

[Morbert]

Experimental contradiction of Mjelva: It is an indisputable experimental fact that NONE of these 4 states - and no MIXTURE of these 4 states - can ever lead to an entanglement swap. Every experimenter who has ever worked with PDC entangled pairs can explain why*. That's because normal Type I and Type II PDC produced photon pairs are NOT entangled (and therefore cannot be used for swapping)! They emerge natively in precisely one of the 4 Product States listed by Mjelva**. Accordingly, they require relatively complex/difficult steps to create polarization entanglement. You can randomly mix these 4 Product states (say from 4 different laser sources), and they still would not be polarization entangled unless they were first made indistinguishable.

You're misinterpreting these states as initial states. These states as initial states cannot lead to entanglement swapping however it is interpreted. But these states are not initial states.

 

[gentzen]

1. Don't believe I mentioned Bohmian Mechanics. I don't feel I understand it well enough to apply the same arguments to BM. If Goldstein (or @Demystifier !) ever made the same mistake as Mjelva has, I would definitely address it.

You replied to "... the statistical data are predicted by the forward-in-time evolution ..." which mentioned both MWI and Bohmian Mechanics, and even wrote in your reply

Ditto the page on Bohmian Mechanics.

And the next time "forward-in-time" was mentioned

As an aside, forward-in-time accounts based on ordinary undergraduate-level formalisms of QM are perfectly capable of reproducing all correlations seen in these experiments.

you reacted similarly

Don't think so. But if you have a reference that actually addresses Ma and/or Megidish in detail, then please share it with me. (Note that orthodox QM does not feature a forward-in-time explanation.)

2. I would call experimental citations "personal research". Somehow, I am about the only person in this thread citing well-accepted scientific teams and papers.

It should be pretty clear what I mean by "Challenging that consensus". If you didn't intent to mention Bohmian mechanics, then that is OK for me, and it was just a misunderstanding/miscommunication.

3. All experiment science is "post-selection" (this should be obvious).

Postselection in this context means that you only keep 1 out of 4 results. Partly, this is only a "matter of convenience". But you like the emphasize

Obviously the BSM can be performed remotely, and either before or after the measurements of Alice and Bob.

and then this 1 out of 4 is also a matter of principle, which cannot be avoided, not even with infinite experimental effort.

 

[DrChinese]

I'd have to think more about it, although Matthew Pusey--the 'P' in 'PBR theorem'--seems to believe that retrocausality offers a loophole to the theorem.

This paper is a nice addition to this thread.