The mechanism of entangled particles

[entropy1]

First let me ask this:

Consider a pair of entangled photons fired at a respective detector after passing respective polarisation filters.

If a photon passes a polarisation filter, is it in a superposition of having passed and not having passed?

Is the measuring device (that detects the photon) in a superposition of having detected and not having detected? (or is it allowed to think of it that way?)

 

[StevieTNZ]

If a photon passes a polarisation filter, is it in a superposition of having passed and not having passed?

Yes.

Is the measuring device (that detects the photon) in a superposition of having detected and not having detected? (or is it allowed to think of it that way?)

The fundamental Schrodinger equation governing what is going on shows the measurement device being in a superposition of detection + non-detection. Where collapse occurs, to make the photon pass the filter or not, is subject to the still unsolved measurement problem.

 

[entropy1]

Sorry, I fear that what I was about to write will be seen as posing a theory.

So...
Nevermind.

 

[Heinera]

NOTE: I am trying to find where my reasoning fails.

There is nothing that fails in that reasoning, since it is a perfectly valid interpretation that the combined mesurement results are not fixed until they are brought together and observed jointly. But there are also a bunch of other interpretations for this scenario.

 

[vanhees71]

No! If you have an ideal polarization filter it either absorbs a photon or it let's it through, and then either there is no photon left or the photon is in a particular linear-polarization state, determined by the orientation of the polarizer. It's a paradigmatic example of a von Neumann filter measurement as a state-preparation procedure.

 

[entropy1]

Ok. Since i get responses I will dare to put forward a mechanism for the purpose to determine at which points it goes erroneous:

Suppose we have photon A and photon B. They are measured by apparati A resp. B. Now apparati A and B both become in superposition of detecting their respective photon, since the photons passed their polarisation filters. Now suppose that when the measurement outcomes (of A and B) are brought together, both A and B (apparatus) will collapse in a definite 'detected' or ''not-detected' state respectively, thereby retrocausally fixing the events at the polarisers and detectors. The correlation between photon A and B can be established at the joining of measurement outcomes by the knowledge (at this point) of the positions of the filters and the shared wavefunction of A and B (entanglement)(which, icw the positions of the filters, can generate the correlation).

NOTE: I am not trying to put forward a theory. I am asking if the interpretation put forward is a valid one, and if not, why not!
NOTE: I can elaborate a little, but leave it at this for the moment.

 

[Derek P]

Ok. Since i get responses I will dare to put forward a mechanism for the purpose to determine at which points it goes erroneous:

Suppose we have photon A and photon B. They are measured by apparati A resp. B. Now apparati A and B both become in superposition of detecting their respective photon, since the photons passed their polarisation filters. Now suppose that when the measurement outcomes (of A and B) are brought together, both A and B (apparatus) will collapse in a definite 'detected' or ''not-detected' state respectively, thereby retrocausally fixing the events at the polarisers and detectors. The correlation between photon A and B can be established at the joining of measurement outcomes by the knowledge (at this point) of the positions of the filters and the shared wavefunction of A and B (entanglement)(which, icw the positions of the filters, can generate the correlation).

NOTE: I am not trying to put forward a theory. I am asking if the interpretation put forward is a valid one, and if not, why not!
NOTE: I can elaborate a little, but leave it at this for the moment.

Yes, if you treat the detector states as collapsing then it is easy to construct retrocausal scenarios. Which is as good a reason as any not to believe in collapse.

 

[Mentz114]

Yes, if you treat the detector states as collapsing then it is easy to construct retrocausal scenarios. Which is as good a reason as any not to believe in collapse.

Thinking in classical mode gets one into this futile sort of discussion.
In the DCQE setup, accept that 1) entangled objects share a wave-function 2) any change to the WF affects the entangled pair immediately 3) there is no meaning in assigning precedence to this disentangling event so the classical cause and effect chain does not apply. Therefore there is no retrocausaiity.

 

[entropy1]

Thinking in classical mode gets one into this futile sort of discussion.
In the DCQE setup, accept that 1) entangled objects share a wave-function 2) any change to the WF affects the entangled pair immediately

It sometimes seem to me that the WF is used as an excuse for the confrontation with non-locality. Non-locality is an undesirable explanation for entanglement, but since we have the WF the problem is sorted. Isn't that kind of a decoy substitute?

 

[Derek P]

Thinking in classical mode gets one into this futile sort of discussion.
In the DCQE setup, accept that 1) entangled objects share a wave-function 2) any change to the WF affects the entangled pair immediately 3) there is no meaning in assigning precedence to this disentangling event so the classical cause and effect chain does not apply. Therefore there is no retrocausaiity.

Let's see now. A photon lands in a dark stripe of the |A>+|B> pattern and is registered. A month or two later (OK it's a few nanoseconds) its partner faces a choice - and (within experimental limits) never chooses the |A>+|B> detector. Either it knows what the signal photon did or both their choices were pre-ordained. Or the later choice determined the earlier one.

 

[Mentz114]

It sometimes seem to me that the WF is used as an excuse for the confrontation with non-locality. Non-locality is an undesirable explanation for entanglement, but since we have the WF the problem is sorted. Isn't that kind of a decoy substitute?

We ONLY have the WF and it has nothing to say about causality nor the order of events. Assuming an order leads to the problems.

 

[Derek P]

It sometimes seem to me that the WF is used as an excuse for the confrontation with non-locality. Non-locality is an undesirable explanation for entanglement, but since we have the WF the problem is sorted. Isn't that kind of a decoy substitute?

Yes. QM is a total tarradiddle invented purely to avoid the awkward facts that nature imposes!

 

[Mentz114]

Let's see now. A photon lands in a dark stripe of the |A>+|B> pattern and is registered. A month or two later (OK it's a few nanoseconds) its partner faces a choice - and (within experimental limits) never chooses the |A>+|B> detector. Either it knows what the signal photon did or both their choices were pre-ordained. Or the later choice determined the earlier one.

Exactly. If you assume an order of events you have a problem.

 

[entropy1]

We ONLY have the WF and it has nothing to say about causality nor the order of events. Assuming an order leads to the problems.

I think that was not the point I was trying to make in 2016. That the WF doesn't adress that is even more convenient.

 

[Derek P]

Exactly. If you assume an order of events you have a problem.

If you assume events you have an order and it is wrong. It's not the ordering that must go, it's the "events" themselves.

 

[Mentz114]

If you assume events you have an order and it is wrong. It's not the ordering that must go, it's the "events" themselves.

That would suggest that nothing happens. According to the WF description we are assigning 2 events where there is only one. One event cannot have a precedence with itself (obviously). In QT it appears that an event can be its own cause and effect !

 

[Derek P]

That would suggest that nothing happens. According to the WF description we are assigning 2 events where there is only one. One event cannot have a precedence with itself (obviously).

You know better than that. Events of the kind we are talking about punctuate the unitary evolution of the system. Plenty happens with unitary evolution, just no collapses that we would call a detection event.

 

[Mentz114]

You know better than that. Events of the kind we are talking about punctuate the unitary evolution of the system. Plenty happens with unitary evolution, just no collapses that we would call a detection event.

I stand by what I said. You are doing contortions to hang on to the traditional notions of causailty and locality.

Bell's theorem eliminates the kind of reality you are proposing. Separability is impossible - so why try to separate inseparable states ?

 

[Derek P]

I stand by what I said. You are doing contortions to hang on to the traditional notions of causailty and locality.

Then you have not understood.

Bell's theorem eliminates the kind of reality you are proposing. Separability is impossible - so why try to separate inseparable states ?

I'm not proposing such a reality. It demonstrably exists. Bell's theorem relies on classical statistics. It doesn't work if properties are indefinite.
As to separating inseparable states, that's a different matter.

 

[Mentz114]

Then you have not understood.

I'm not proposing such a reality. It demonstrably exists. Bell's theorem relies on classical statistics. It doesn't work if properties are indefinite.
As to separating inseparable states, that's a different matter.

Bell's theorem always applies. But if you don't accept it that is your choice.

 

[Derek P]

Bell's theorem always applies. But if you don't accept it that is your choice.

Don't be insulting please. A theorem is a theorem. This discussion is over.

 

[Mentz114]

Don't be insulting please. A theorem is a theorem. This discussion is over.

No insult intended. I should have said 'if you don't think it applies ...' etc. I don't understand why being 'classical' should make it inapplicable.

 

[Derek P]

No! If you have an ideal polarization filter it either absorbs a photon or it let's it through, and then either there is no photon left or the photon is in a particular linear-polarization state, determined by the orientation of the polarizer. It's a paradigmatic example of a von Neumann filter measurement as a state-preparation procedure.

I know that you know this but it is worth saying, for the sake of the OP, that technically we don't know that it is either/or. Observation will certainly yield an either/or situation but unitary QM says it's a superposition. The OP wanted to know whether the photon stays in superposition, ditto the detector. Technically they do not because the filtering process creates an entanglement which must include the filter. Fast forward to an improper mixture which certainly looks like an either/or situation but in reality is not. You can then sprinkle collapse of the superposition into the mixture to make it proper if it takes your fancy but it is not obligatory.

 

[Derek P]

Yes.
The fundamental Schrodinger equation governing what is going on shows the measurement device being in a superposition of detection + non-detection. Where collapse occurs, to make the photon pass the filter or not, is subject to the still unsolved measurement problem.

The measurement problem may be defined that way, but measurement theory explains the appearance of collapse without needing to assume that it actually occurs. Of course the superposition applies to the extended system - photon (optional) plus filter plus detector plus observer plus environment plus any surviving Schroedinger cats. But with that caveat, measurement theory seems to be able to get rid of the "measurement problem" - basically by eliminating collapse as a superfluous hypothesis.

 

[Lord Jestocost]

Of course the superposition applies to the extended system - photon (optional) plus filter plus detector plus observer plus environment plus....

One should be cautious when mentioning the “observer” in such a list. The observer is – so to speak – a self-knowing unit which differs in a fundamental way from the rest.

“The observer has a completely different impression. For him it is only the object x and the apparatus y that belong to the external world, to what he calls 'objectivity.' By contrast he has with himself relations of a very special character. He possesses a characteristic and quite familiar faculty which we can call the 'faculty of introspection.' He can keep track from moment to moment of his own state. By virtue of this 'immanent knowledge' he attributes to himself the right to create his own objectivity…….” (F. London and E. Bauer, “The Theory of Observation in Quantum Mechanics” (1939))

 

[Derek P]

One should be cautious when mentioning the “observer” in such a list. The observer is – so to speak – a self-knowing unit which differs in a fundamental way from the rest.

Making the observer fundamentally different from the rest of the universe is the second biggest source of confusion in quantum physics. The first being rank disbelief in superposition.
Physics deals with the physical side of the observer, not with any subjective experience that the observer may or may not have. So in physics the observer is exactly the same as all the other subsystems.
It's probably better not to quote material that is 79 years out of date when the entire paradigm has been overhauled several times since then.

 

[Mentz114]

[..]
The first being rank disbelief in superposition.
[..]

That is not the mainstream view. Physicists believe in superposition in the sense that some states may be written in different bases and therefore be superpositions and simple states at the same time. What the formalism gives us is the possible set of outcomes of measurements in all bases and the probability of each outcome.
Trying to apply this to chairs and elephants is what causes rank disbelief. There is at least one other reason to dismiss macroscopic superposition but that is hardly necessary.

 

[entropy1]

Is the measurement problem in essence not a 'history-selection-problem'?

 

[Lord Jestocost]

It's probably better not to quote material that is 79 years out of date when the entire paradigm has been overhauled several times since then.

In comment #25, I forgot to mention:

Original publication: Fritz London and Edmond Bauer, 1939, “La théorie de l’observation en mécanique quantique”, Paris: Hermann.

English translation, “The theory of observation in quantum mechanics”, in Quantum Theory and Measurement, J.A. Wheeler and W.H. Zurek (eds), Princeton: Princeton University Press, 1983, 217–259.

 

[Derek P]

That is not the mainstream view. Physicists believe in superposition in the sense that some states may be written in different bases and therefore be superpositions and simple states at the same time. What the formalism gives us is the possible set of outcomes of measurements in all bases and the probability of each outcome.
Trying to apply this to chairs and elephants is what causes rank disbelief. There is at least one other reason to dismiss macroscopic superposition but that is hardly necessary.

Well it's understandable. For very good quantum reasons we don't see superpositions very often. And we've had hundreds of millions of years of evolution fine-tuning our brains to deal with the phenomena that are emergent. But just because my simian brain doesn't respond well to the idea of Schroedinger's cat doesn't warrant my rejecting the logic. (Yes I do know the shortcomings of SC as originally posed!)
I am curious as to the "other" reason to dismiss macroscopic superposition.

 

[Derek P]

Is the measurement problem in essence not a 'history-selection-problem'?

It is, but there are so many completely different ways the selection could occur that lumping them all together is not really helpful. And, of course, interpretations such as MWI take the selection out of the realm of physics - there isn't any selection - and make it something akin to the anthropic principle. We see the state that we are part of.

 

[Mentz114]

I am curious as to the "other" reason to dismiss macroscopic superposition.

Superposition depends on the linearity of the WF. There is good reason (theoretically) to believe that linearity fails at high enough energies.
It is based on this paper

Schrödinger equation revisited
Authors : Wolfgang P. Schleich, Daniel M. Greenberger, Donald H. Kobe, and Marlan O. Scully

This is off-topic so if you want to discuss - please start a new thread.

 

[PeterDonis]

This is off-topic so if you want to discuss - please start a new thread.

New thread is here:

https://www.physicsforums.com/threads/is-non-linear-quantum-mechanics-even-plausible.942330/

 

[StevieTNZ]

It's probably better not to quote material that is 79 years out of date when the entire paradigm has been overhauled several times since then.

The reference provided by the honourable member is certainly not unworthy to be cited, as it is still relevant (at least in my opinion).

 

[Lord Jestocost]

Physics deals with the physical side of the observer, not with any subjective experience that the observer may or may not have. So in physics the observer is exactly the same as all the other subsystems...

That’s your point of view and that’s o.k. Why not? But others have different points of view. Let’s cite, for example, Erwin Schrödinger:

“One can only help oneself through something like the following emergency decree: Quantum mechanics forbids statements about what really exists – statements about the object. Its statements deal only with the object-subject relation. Although this holds, after all, for any description of nature, it evidently holds in a much more radical and far reaching sense in quantum mechanics.”

— Erwin Schrödinger, 1931, letter to Arnold Sommerfeld