# Counterfactual definiteness and correlation

• B
• entropy1
In summary, it is observer-dependent whether or not correlation exists between measurements on entangled particles.f

#### entropy1

If we assume there is no counterfactual definiteness, would that mean that measurements on entangled particles needn't be correlated, for if you don't compare the results, you just don't know if they do?

Cherye Norris
Counterfactual definiteness (CFD) is really about unperformed measurements. If you measure momentum on A and position on B: you cannot really talk about A's position and B's momentum at the same point in time unless there exists CFD. Classically, objects do not have non-commuting measurement operations. Quantum objects do.

Counterfactual definiteness (CFD) is really about unperformed measurements. If you measure momentum on A and position on B: you cannot really talk about A's position and B's momentum at the same point in time unless there exists CFD. Classically, objects do not have non-commuting measurement operations. Quantum objects do.
So, CFD has necessarily to do with non-commutation?

Because, I was aiming at eternal separation in spacetime of two measurements that were performed. The thing with that would then be that Alice would never know whether Bob actually did carry out a measurement.

Because, I was aiming at eternal separation in spacetime of two measurements that were performed. The thing with that would then be that Alice would never know whether Bob actually did carry out a measurement.

If the two particles are eternally separated ( i.e. in the past and in the future ), then I don't see how you can have entanglement - not only will it be impossible to verify the correlation, but the entanglement cannot exist in the first place, since its creation requires an initial interaction between the particles.

vanhees71
If the two particles are eternally separated ( i.e. in the past and in the future ), then I don't see how you can have entanglement - not only will it be impossible to verify the correlation, but the entanglement cannot exist in the first place, since its creation requires an initial interaction between the particles.
I mean forever separated in the future. (after preparation)

I mean forever separated in the future. (after preparation)

Well, in that case it comes down again to the issue of interpretations. To me, I do not see how you can meaningfully speak of an entanglement relationship, if all you have is two eternally separated observers. The outcome of all measurements is random for Alice, just as it is for Bob; neither of them have knowledge of whether or not the other party has performed a measurement, and, unless told, they have no way to decide whether or not an entanglement relationship exists. It takes either a classical interaction between Alice and Bob, or a third observer ( Charlie ) who can compare measurement results, in order to meaningfully speak of a correlation. In that sense, the state of a system can be considered to depend on the observer, and is not an absolute quantity at all - Alice and Bob ( who are eternally separated ) will argue that they are dealing with free particles, and will mathematically describe the situation as such, whereas Charlie's description of what constitutes reality will involve a composite system of entangled particles. Who is wrong, and who is right ? And who gets to decide ?

entropy1
@Markus Hanke: that is exactly what I mean. So since there is no correlation observed, it is not established. The establishment of the correlation seems to depend on the comparing of notes, the bringing together of information. So, would the measurement devices (and, in line, Alice and Bob) play a role in the existence of correlation? It seems that information being combined is a prerequisite for the existence of correlation, which makes perfect sense of course.

How do we know that measurement devices that measure entangled particles, but never come into contact otherwise, exhibit correlation, and since we'll never know if they do, is it relevant at all? Are things of which we don't have information relevant anyway?

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Are things of which we don't have information relevant anyway?

I don't think there is any physical relevance to either Alice or Bob in isolation. Their reality is the same, regardless of what Charlie - who has information about both particles - concludes ( entanglement or no entanglement ). That is why I think there should be some notion of observer-dependence here, although I don't know what it would formally ( = mathematically ) look like.

I wasn't clear enough in my last response. There was noise and I got distracted.

To put it more clearly: Alice and Bob can:
1. Keep their lightcones separated and never compare notes, or:
2. Compare notes and measure a correlation.
Now it seems to me that one can not conclude, in principle, in case of 2, whether comparing notes produces the correlation or just measures it, or, in case of 1, if there still is also a correlation. It seems as if the comparing of notes (putting together the information/bringing together the lightcones) could (theoretically) play a role in producing the correlation. Is there any physispendence on this?

Thanks!

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If the two particles are eternally separated ( i.e. in the past and in the future ), then I don't see how you can have entanglement - not only will it be impossible to verify the correlation, but the entanglement cannot exist in the first place, since its creation requires an initial interaction between the particles.

Not so fast, grasshopper.

There is no such requirement for entanglement. Yes, that seems to defy logic. But there can be entanglement of photons that never existed in a common light cone. Such is accomplished by entanglement swapping. And you can verify the entanglement at a later time by bringing the distant results together. Even if Alice and Bob never meet.

Truecrimson
And just to make things weirder: you can even entangle particles AFTER they have been measured. Again, this seems to be counter-intuitive but there is no violation of causality in the traditional sense. I.e. there is no grandfather paradox.

That is why I think there should be some notion of observer-dependence here, although I don't know what it would formally ( = mathematically ) look like.

Ordinary QM reflects the observer dependency. You must look at the entire context - including observer measurement choices - to make a quantum mechanical prediction. So that's the math.

Not so fast, grasshopper.

Lol, just as I thought I had the whole EPR-entanglement thing hammered down, someone comes along and throws a spanner in my mental works I am going to have to digest your last three posts, and definitely do a bit more reading on this subject, as I wasn't aware of the phenomena you have mentioned. The learning never ends !

DrChinese
Lol, just as I thought I had the whole EPR-entanglement thing hammered down, someone comes along and throws a spanner in my mental works I am going to have to digest your last three posts, and definitely do a bit more reading on this subject, as I wasn't aware of the phenomena you have mentioned. The learning never ends !

So true! The reference, by the way, is:

Entanglement Between Photons that have Never Coexisted
E. Megidish, A. Halevy, T. Shacham, T. Dvir, L. Dovrat, H. S. Eisenberg
(Submitted on 19 Sep 2012)
The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving, can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the non-locality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spatial separation, but also with temporal separation. Here we demonstrate these principles by generating and fully characterizing an entangled pair of photons that never coexisted. Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime.

Direct interaction is not a requirement, as can be seen. The interesting thing is that the space time diagram of the particle interactions does respect c, but requires forward and backward effect propagation to explain anything. I don't know what that implies exactly, but that is essentially a requirement of entanglement.

Markus Hanke
but requires forward and backward effect propagation to explain anything.
Forward and backward in time? (space?)

Forward and backward in time? (space?)

Yup, that's the odd part. That does not prove anything though, as far as interpretations go.

entropy1
If we assume there is no counterfactual definiteness, would that mean that measurements on entangled particles needn't be correlated, for if you don't compare the results, you just don't know if they do?
Why should not do so

Why should not do so
Because there is word lately about forward and backward 'influence/causality' in time on quantum level! That might mean that what happens later (i.e. the bringing together of information to establish the correlation) has some interaction with what happens earlier (i.e. the preparation of the entangled particles). In that case, the future might be just as important as the past. I image a quantum wave (e.g. a state) propagating forward in time and another one backward. (of course this is heavily simplified)

Can a mentor confirm if what entropy said above was true.. that the bringing together of information to establish the correlation is what created the correlation in entanglement (like some time reversed process)? For instance. Alice and Bob located 14 billion light years away took 14 billion years to travel for classical comparison.. then that bringing togethe of information affects the past? but how could it be?

Can a mentor confirm if what entropy said above was true.. that the bringing together of information to establish the correlation is what created the correlation in entanglement (like some time reversed process)? For instance. Alice and Bob located 14 billion light years away took 14 billion years to travel for classical comparison.. then that bringing togethe of information affects the past? but how could it be?

That conclusion is interpretation dependent. The only thing you can say is that there is an interpretation where the future and the past are part of a joint context in which the quantum "transaction" occurs. Those are what I refer to as the Time Symmetric group, some people call it Retrocausal.

[..]some people call it Retrocausal.
I don't like that idea; it doesn't seem to fit with entanglement. I rather like to think in terms of 'resonance', wherein future probability waves interfere with past ones in a way that due to slight variations (HUP/quantum fluctuations) in the combination of influences from the present as well as the future, the 'outcome' of the both present and future influences reinforce ('resonate') to some outcome at some point. This means that causal influences of the future cancel with causal influences of the present (past), with a net undetectable causation. This seems to correspond most (but not entirely) with TI.

future probability waves interfere with past ones in a way that due to slight variations (HUP/quantum fluctuations) in the combination of influences from the present as well as the future, the 'outcome' of the both present and future influences reinforce ('resonate') to some outcome at some point (eg. as in a [negative] well).

I think I saw that episode of STtNG. ;-)

Is this discussion based on both math and experimental observation? Its very interesting, but its also pretty out there to this lay person.

I think I saw that episode of STtNG. ;-)

Is this discussion based on both math and experimental observation? Its very interesting, but its also pretty out there to this lay person.
It is my own view that I find partly supported by TI. It is an interpretation. QM has already been confirmed. However, TI produces drastically more simple math, I think.

This thread is closed until such time as someone can PM a mentor with a pointer to a peer-reviewed paper showing some math - until then, claims about "resonance" and "drastically simpler math" are meaningless drivel.