Non-locality and Counterfactual definiteness

[Demystifier]

If you believe FTL is involved, how do you explain the impossibility of using these correlations to communicate information?

To communicate information, FTL is not enough. What one needs is controllable FTL.

Let me be more specific. Presumably, FTL happens at the level of hidden variables, i.e. variables which, at least with current technology, cannot be directly observed. (Something like atoms before the 20th century.) Since they cannot be observed, it should be clear that they cannot be controlled. And without a control, they cannot be used for any communication, either faster or slower than light.

Very roughly, this is like asking the following question. If there are eagles flying faster than pigeons, then why can't we use them to send messages with a speed faster than the speed of pigeons? That's because eagles (unlike pigeons) cannot be controlled.

 

[RUTA]

That theory is inconsistent. Can it make valid predictions before Bob measured anything? Are those predictions still valid if Bob measured something else? Then you can't avoid CFD. CFD is a logical necessity of any theory that makes valid predictions for experiments that might not be performed irrespective of ontology.

You supply the specific detector settings and outcomes when computing a probability amplitude using path integral. If you want the amplitude for a different experimental configuration and outcome, you have to do an different computation.

 

[billschnieder]

You supply the specific detector settings and outcomes when computing a probability amplitude using path integral. If you want the amplitude for a different experimental configuration and outcome, you have to do an different computation.

And you can do all those possible calculations and obtain definite predictions, even if only one of them is actualized in an experiment. The rest will be Counterfactual definite. You haven't avoided CFD.

 

[RUTA]

And you can do all those possible calculations and obtain definite predictions, even if only one of them is actualized in an experiment. The rest will be Counterfactual definite. You haven't avoided CFD.

You're not seeing the picture. Imagine a field in the spacetime region between Source emission, detector settings (polarizer or SG magnet orientations, for example) and the detector outcomes for a particular trial in the Mermin device. With CFD that field would contain R and G for each setting for each side (Alice and Bob) no matter what the actual settings are. You can imagine three stripes, for example, on each side with each stripe being R or G corresponding to each possible detector setting. That's CFD or what Mermin calls "instruction sets." Now a realistic no-CFD field would have only one color R or G on each side for the actual outcome corresponding to the actual settings of Alice and Bob.

 

[DrChinese]

And you can do all those possible calculations and obtain definite predictions, even if only one of them is actualized in an experiment. The rest will be Counterfactual definite. You haven't avoided CFD.

In this setup, you have avoided it (as RUTA points out). Like Bohmian mechanics, there is no counterfactual case to consider in a retrocausal/blockworld configuration. All of the settings are readily available; and the Bell premise - that a measurement here does not affect an outcome there - is false (rejected).

 

[zonde]

You're not seeing the picture. Imagine a field in the spacetime region between Source emission, detector settings (polarizer or SG magnet orientations, for example) and the detector outcomes for a particular trial in the Mermin device. With CFD that field would contain R and G for each setting for each side (Alice and Bob) no matter what the actual settings are. You can imagine three stripes, for example, on each side with each stripe being R or G corresponding to each possible detector setting. That's CFD or what Mermin calls "instruction sets." Now a realistic no-CFD field would have only one color R or G on each side for the actual outcome corresponding to the actual settings of Alice and Bob.

But how you come up with prediction?
Say I take many spacetime regions where the source part looks the same and then look at all possible outcomes with different measurement settings. As I don't "see" the field between, I can't group spacetime regions based on differences there. This would be part of the process for coming up with prediction.
But considering the collection of spacetime regions I described we can now talk about CFD.

 

[DrChinese]

But how you come up with prediction?
Say I take many spacetime regions where the source part looks the same and then look at all possible outcomes with different measurement settings. As I don't "see" the field between, I can't group spacetime regions based on differences there. This would be part of the process for coming up with prediction.
But considering the collection of spacetime regions I described we can now talk about CFD.

Keep in mind that RUTA is describing an interpretation of QM. In that: the settings of Alice and Bob are part of the context, even though Alice and Bob may select those settings in the future. So it is no surprise that the observations of Alice and Bob are consistent, they are not independent of the source.

 

[zonde]

Keep in mind that RUTA is describing an interpretation of QM. In that: the settings of Alice and Bob are part of the context, even though Alice and Bob may select those settings in the future. So it is no surprise that the observations of Alice and Bob are consistent, they are not independent of the source.

Yes, I understand that. But consider what we (as observers belonging to that blockworld) can see from that spacetime. And how we would group many similar regions of spacetime. Remember that we are interested in reproducible phenomena or in blockworld terms we are interested in repeating similar spacetime regions.

 

[RUTA]

Yes, I understand that. But consider what we (as observers belonging to that blockworld) can see from that spacetime. And how we would group many similar regions of spacetime. Remember that we are interested in reproducible phenomena or in blockworld terms we are interested in repeating similar spacetime regions.

You compute a probability for each possible outcome in each setting and the distribution is realized in the frequency of appearance in the regions of spacetime. It's true we don't see the screened off situation between the Source and detection events, that's what interpretations deal with, i.e., what is the ontology in that unseen region? Since we're looking at a distribution of various regions of spacetime, each region has a particular outcome with its particular setting, so we can easily choose an ontology without CFD. You run into an issue with no instruction sets (no CDF) when you look at the experiment in a time-evolved fashion (again, read my Insight on Mermin's explanation of the mystery of Hardy's experiment). Get rid of that perspective and instead view the situation spatiotemporally and the mystery disappears.

 

[billschnieder]

Since we're looking at a distribution of various regions of spacetime, each region has a particular outcome with its particular setting, so we can easily choose an ontology without CFD. You run into an issue with no instruction sets (no CDF) when you look at the experiment in a time-evolved fashion (again, read my Insight on Mermin's explanation of the mystery of Hardy's experiment). Get rid of that perspective and instead view the situation spatiotemporally and the mystery disappears.

Please could you define CFD precisely? What you are calling CFD is not CFD.

 

[zonde]

You run into an issue with no instruction sets (no CDF) when you look at the experiment in a time-evolved fashion (again, read my Insight on Mermin's explanation of the mystery of Hardy's experiment). Get rid of that perspective and instead view the situation spatiotemporally and the mystery disappears.

What makes you think I am not viewing the situation spatiotemporally (as blockworld)?

Since we're looking at a distribution of various regions of spacetime, each region has a particular outcome with its particular setting, so we can easily choose an ontology without CFD.

Here you are talking about factual definiteness. Factual definiteness is not contradictory with counterfactual definiteness, do you agree?
If we could see all of the blockworld we would not care about CFD. But we can't. And so we ask, given we see some portion of blockworld can we guess neighboring region? And when we try to do that CFD comes into the picture.

 

[RUTA]

zonde and billschneider read https://www.physicsforums.com/insig...elayed-choice-no-counterfactual-definiteness/ where I explain what the foundations community means by CFD; it's just what Mermin calls "instruction sets." Then read https://www.physicsforums.com/insights/retrocausality/ to see how retrocausality (settings and outcomes 'known' at Source at emission) allows for realism without CFD. zonde you're summing up nicely the problem of the dynamic, time-evolved perspective (Newtonian Schema Universe) versus the blockworld, global constraint perspective (Lagrangian Schema Universe) when you say, "If we could see all of the blockworld we would not care about CFD. But we can't. And so we ask, given we see some portion of blockworld can we guess neighboring region? And when we try to do that CFD comes into the picture." Retrocausality is essentially saying adopt the LSU as fundamental despite our NSU experience and the mystery of no CFD disappears, as you point out. It's that simple.

 

[Ralph Dratman]

To communicate information, FTL is not enough. What one needs is controllable FTL.

The experimental results that tend to puzzle us are correlations that can only be observed in retrospect, long after an experiment has been completed. One might say that entanglement leaves behind non-classical tracks. But maybe that should not be surprising, considering that quantum phenomena apparently "compute" infinitely many paths to completion all at once, producing various possible results whose probabilities are squared sums over all the paths. Parts of those simultaneously-explored paths might be separated by unlimited distances, yet still the sums are computed in finite time.
That alone is what ought to puzzle us about such phenomena. The (harmless) correlations that show up later merely confirm that many things were happening at once.