How Does Entangled Photon Behavior Challenge Local Realism?

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Discussion Overview

The discussion centers around the implications of entangled photon behavior on local realism, particularly in the context of Bell's theorem and the CHSH inequality. Participants explore the timing and nature of polarization collapse in entangled photons, referencing different interpretations of quantum mechanics, including the Copenhagen interpretation.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants assert that a violation of the CHSH inequality indicates local realistic theories are untenable.
  • Others suggest that the discussion should focus on Bell's theorem in general rather than specifically on the CHSH inequality, noting that simpler Bell inequalities may be more accessible.
  • There is a contention regarding whether one photon assumes the same polarization as its partner before detection, with some attributing this view to the Copenhagen interpretation.
  • Participants question the timing of when a photon takes on polarization, with some suggesting it occurs at the moment of measurement, while others propose that it may not correspond to a specific point in time.
  • One participant mentions that experiments support both instantaneous collapse and delayed collapse until detection, indicating uncertainty about the exact timing of collapse.
  • A later post introduces a scenario involving multiple pairs of entangled photons and questions the predictions of quantum mechanics regarding their polarization measurements.

Areas of Agreement / Disagreement

Participants express differing views on the nature of polarization collapse and the implications for local realism. There is no consensus on the timing of polarization assumption or the interpretation of the results from experiments.

Contextual Notes

Participants highlight the complexity of the discussion, noting that the precise mechanism of polarization collapse is not well understood and may depend on interpretations of quantum mechanics.

StevieTNZ
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Obviously a violation of the CHSH inequality means that local realistic theories are untenable.

If we sent two entangled photons towards detectors (far enough away that for information to travel, you'd require it to go faster than light). One reaches a detector before the other, so collapses.

How do we know the partner photon doesn't assume the same polaristion until it reaches its filter and detector?
 
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StevieTNZ said:
Obviously a violation of the CHSH inequality means that local realistic theories are untenable.
By the way, none of what you're saying is based on the CHSH inequality in particular, just Bell's theorem in general. So you might as well work with a much simpler Bell inequality for entangled photons, described in the fantastic explanation "quantumtantra.com/bell2.html" . (The only advantage of CHSH is that it's more practical to test experimentally.)

And incidentally, you should know that there are various fringe people who try desperately to cling to local realism by either exploiting experimental deficiencies in practical Bell tests (e.g. fair sampling), or by theoretical loopholes in Bell's theorem (e.g. superdeterminism); you can read about many if not all these attempts in Bell threads on this forum. But experimental and theoretical advances may make such positions increasingly difficult to maintain.
StevieTNZ said:
If we sent two entangled photons towards detectors (far enough away that for information to travel, you'd require it to go faster than light). One reaches a detector before the other, so collapses.

How do we know the partner photon doesn't assume the same polaristion until it reaches its filter and detector?
That is precisely the Copenhagen view of things.
 
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So its not correct to say that when one photon takes on a polarisation, the other does (even if it's mid-air heading towards a detector)?
 
StevieTNZ said:
So its not correct to say that when one photon takes on a polarisation, the other does (even if it's mid-air heading towards a detector)?
I told you, the idea of the other photon assuming the same polarization as the first photon is precisely the view of the Copenhagen interpretation.
 
lugita15 said:
I told you, the idea of the other photon assuming the same polarization as the first photon is precisely the view of the Copenhagen interpretation.

Yes, but I'm asking when the photon takes on that same polarisation? I thought I made that clear.
 
StevieTNZ said:
Yes, but I'm asking when the photon takes on that same polarisation? I thought I made that clear.
As you said, exactly when the first photon's polarization is measured, at least according to Copenhagen.
 
StevieTNZ said:
Yes, but I'm asking when the photon takes on that same polarisation? I thought I made that clear.

Experiments confirm that it is completely consistent with instantaneous collapse. However, the same experiments also confirm it as consistent with collapse delayed until detection.

So I am not contradicting lugita15, but merely pointing out that to know the precise time collapse occurs is not possible. You would really need to know more about the mechanism, which we don't. For example, the "when" may not really be any specific point in time if you consider a block time structure. Then it is more like "when-where".
 
lugita15 said:
As you said, exactly when the first photon's polarization is measured, at least according to Copenhagen.

i guess it has to be the moment any of the entangled photons is detected (first), else you need to assume a third (undiscovered) entity that holds/stores the (spin info) information (and then transmits) to the photon that is detected later.
 
I wonder in this case:
Two pairs of entangled photons - (VV+HH) description for both pairs.
(VV+HH) TENSOR (VV+HH)

Measure each photon along the 45/135 basis.

Does QM predict that 1/2 the times we get 45 for photon 1 and photon 2 (from pair 1), pair 2's photons take on 135 polarisation, when photon 3 and 4 are measured?
 

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