# I The mechanism of entangled particles

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1. Aug 31, 2016

### 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?)

2. Aug 31, 2016

### StevieTNZ

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.

3. Aug 31, 2016

### entropy1

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

So...
Nevermind.

Last edited: Aug 31, 2016
4. Aug 31, 2016

### Heinera

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.

5. Sep 1, 2016

### 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.

6. Sep 1, 2016

### 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.

Last edited: Sep 1, 2016
7. Mar 11, 2018

### Derek P

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.

8. Mar 11, 2018

### Mentz114

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.

9. Mar 11, 2018

### entropy1

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?

10. Mar 11, 2018

### Derek P

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.

11. Mar 11, 2018

### Mentz114

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.

12. Mar 11, 2018

### Derek P

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

13. Mar 11, 2018

### Mentz114

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

14. Mar 11, 2018

### entropy1

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.

15. Mar 11, 2018

### Derek P

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.

16. Mar 11, 2018

### Mentz114

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 !

17. Mar 11, 2018

### Derek P

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.

18. Mar 11, 2018

### Mentz114

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 ?

19. Mar 11, 2018

### Derek P

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.

20. Mar 11, 2018

### Mentz114

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

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