# Interpretation of polarisation experiment

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Consider a fully entangled pair of polarized photons, A and B, fired at two detectors with polarisation filters in front of them. I have to get a little philosophical to understand the way the interpretations of this experiment play out. My knowledge is still very basic but I'm working on it. I'm curious though.

When does the polarisation get determined? I have several different ways of looking at this:
1. Collapse. A reaches the filter and 'collapses' into a parallel or perpendicular polarisation. At this very point B takes on this direction of polarisation also. However, in a different reference frame B could be the one that collapses. So there this interpretation seems have a kind of arbitrarity in it.
2. Determinism. A reaches the filter and we interpret that its path was entirely deterministic, so that its polarisation can be traced 'back in time' to the creation of A, thereby fixing the polarisation direction of B. This is a kind of retrocausality. However, this interpretation has the same arbitrarity as (1), for which photon first reaches its filter depends on the reference frame. However, B's path is in this view as deterministic as A's.
3. Uncertainty. There is no way to determine when the polarisation got determined. The state of the pair is fully mixed, and no knowledge is available about either photon state with respect to their polarisation until observed. And if this is true, then there is no need to further try to define or investigate it, for it is all we know.
Now, firstly, admin, I want to stress I am not putting forward any theory of my own. I just don't know how to phrase the question other than this. Is (3) closest to the truth?

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Nugatory
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I just don't know how to phrase the question other than this. Is (3) closest to the truth?
#3 is the closest to what the mathematical formalism will tell you. As for whether that's the truth you're looking for..... We don't know.

bhobba and entropy1
Gold Member
#3 is the closest to what the mathematical formalism will tell you.
Do you mean that (1) and (2) are not entirely valid?

Do you mean that (1) and (2) are not entirely valid?
Well QM formalism doesn't include a system taking on a definite observable (in this case polarization). As far as the formalism goes, the photon is in a superposition of passing and not passing the filter.

entropy1
Gold Member
Well QM formalism doesn't include a system taking on a definite observable (in this case polarization). As far as the formalism goes, the photon is in a superposition of passing and not passing the filter.
So, if we write for A after passing the filter, $$pol(A)=\frac{1}{\sqrt{2}}(cos(\alpha) |H \rangle +sin(\alpha) |V \rangle)$$, assuming H and V parallel and perpendicular to the filter's direction, which value do we assign α? Does this depend on, or get determined by, interaction with the detector?

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bhobba
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When does the polarisation get determined?

The QM formalism is silent on that. Its the realm of interpretations.

Thanks
Bill

vanhees71
Gold Member
The QM formalism is silent on that. Its the realm of interpretations.
That's interesting. That means that even the result of the measurement isn't determined, isn't it? (like being in superposition or like having multiple worlds to be in) Or does decoherence make the result determined?

bhobba
Mentor
That's interesting. That means that even the result of the measurement isn't determined, isn't it?

The formalism is silent on that.

Decoherence does not solve the issue. In fact despite great progress that is what the measurement problem has morphed into - why exactly do we get outcomes at all - all the theory, even with decoherence, predicts is a probability of an outcome.

Thanks
Bill

entropy1
That's interesting. That means that even the result of the measurement isn't determined, isn't it? (like being in superposition or like having multiple worlds to be in) Or does decoherence make the result determined?
Do remember that Bohmian Mechanics, produces (thus far) the same predictions as Quantum Mechanics. However, one day there -may- be some person or persons who find a prediction differing from QM.

Gold Member
Do remember that Bohmian Mechanics, produces (thus far) the same predictions as Quantum Mechanics.
The 'problem' with that, it seems to me, is that it is not better (or less) than other interpretations, so it has no preference. Since there are multiple but different interpretations, each valid in their own right, in my view, each disregards some part of reality. Either that, or the nature of reality is such that there is no way to establish a consesus interpretation (except for the formalism).

But I may be off my limits here.

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vanhees71
Gold Member
2021 Award
The QM formalism is silent on that. Its the realm of interpretations.

Thanks
Bill
In my opinion it's the realm of speculations. QT tells you precisely the probabilities for coincidence experiments of the polarization states of the photons prepared in the entangled polarization state. All you know before measuring is that they are prepared in this state, and the photon polarizations of the single photons in the pair is totally undetermined, but what's also clear from the preparation is the 100% correlation between the outcomes of polarization measurements on the single photons. That's all that's known from the quantum formalism, and that's the physics according to theory and that's what's confirmed by countless experiments with very high significance. Everything beyond that is speculation. Some, particularly philosophy oriented people, call it interpretation ;-)).

bhobba
bhobba
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In my opinion it's the realm of speculations.

Indeed it is.

However they are interesting in that they elucidate exactly what the formalism implies and what it doesn't. It does not imply nature is not deterministic for example because of interpretations like BM.

We both hold to the Ensemble interpretation which is very minimal and face QM issues head on without speculation. So did Einstein which some do not know. He believed QM correct, but incomplete. For him the ensemble interpretation was the only one that didn't assume what he thought were unnatural assumptions. Although because of Kochen-Specker he would likely modify his view a bit.

Thanks
Bill

vanhees71
Gold Member
That leads me to the question why they are speculative; is it because the interpretations contradict and don't entirely cover the thing they are trying to describe, and thus are arbitrary in that sense, or is it because the formalism suggests that no interpretation can be given a priori to the formalism? I imagine many physics-afficionado's must have bumped this wall at some point... (when still inexperienced for instance) (like me )

bhobba
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That leads me to the question why they are speculative;

Its because no one can figure out how to test them.

Thanks
Bill

entropy1
vanhees71
Gold Member
2021 Award
The answer to a question like "When does the polarisation get determined?" becomes physical, if you an define and experiment in the lab (not in the head of a theoretician) that answers this question. After that you can try to describe it with the theory.

The real-world experiments, I'm aware of are such that A and B measure the polarization state of their photon keeping the time of their detection events such that one knows that the respectively measured photons belong to the same entangled pair. How the polarization gets determined, is not (and I'm not sure whether it can even in principle!) resolved as a dynamical process in time at all.

bhobba and entropy1