Why do we say that there is no polarization until it is measured

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Discussion Overview

The discussion revolves around the nature of a photon's polarization and the implications of measurement in quantum mechanics. It explores concepts related to Bell's theorem, local hidden-variable theories, and the interpretation of quantum phenomena, particularly in relation to the idea of predetermined states and "spooky action at a distance."

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants question how we can be certain that a photon's orientation is not determined until measurement, suggesting that this is a fundamental aspect of quantum mechanics.
  • Others argue that Bell's inequality demonstrates that local hidden-variable theories, which assume predetermined polarizations, cannot replicate quantum mechanical predictions.
  • It is proposed that if polarization were predetermined, the phenomenon of "spooky action at a distance" would not arise, leading to further inquiries about the nature of this issue in physics.
  • Some participants assert that we do not know the polarization of a photon until it is measured, and that polarization alone cannot account for measurement results at different angles.
  • There is a discussion about the interpretation of Bell's theorem, with some suggesting that the results are not predetermined under certain assumptions, while others highlight the variability in interpretations based on different assumptions, such as locality and non-contextuality.
  • A participant notes that the challenges faced in understanding the quantum mechanics discussed may stem from linear algebra difficulties rather than the quantum concepts themselves.
  • It is stated that we do not know the value of any observable until measurement occurs, reinforcing the uncertainty inherent in quantum mechanics.

Areas of Agreement / Disagreement

Participants express differing views on the implications of Bell's theorem and the nature of measurement in quantum mechanics. There is no consensus on whether photons have predetermined states or how to interpret the consequences of measurement.

Contextual Notes

The discussion includes assumptions about locality and non-contextuality that remain unresolved, as well as varying interpretations of Bell's theorem that depend on these assumptions.

Robin
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How do we know for sure that the photon's orientation isn't determined until we we measure it ?
 
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Robin said:
How do we know for sure that the photon's orientation isn't determined until we we measure it ?

That's what Bell's inequality proves. The assumption that the photons have definite polarizations (we just don't know what they are) is a local hidden-variable theory, which Bell proved cannot reproduce the predictions of quantum mechanics.
 
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stevendaryl said:
That's what Bell's inequality proves. The assumption that the photons have definite polarizations (we just don't know what they are) is a local hidden-variable theory, which Bell proved cannot reproduce the predictions of quantum mechanics.
If it's predetermined then no need for spooky action at a distance. So why the spooky action at a distance issue in Physics ?
 
Robin said:
How do we know for sure that the photon's orientation isn't determined until we we measure it ?
Actually we don't know it. We just know that photon's polarization (if it has such a property) can't be the only thing that determines measurement results at different angles.
 
Robin said:
If it's predetermined then no need for spooky action at a distance. So why the spooky action at a distance issue in Physics ?

Bell proved that the results are NOT predetermined (under certain assumptions).
 
Robin said:
If it's predetermined then no need for spooky action at a distance. So why the spooky action at a distance issue in Physics ?
There isn't really a spooky issue in physics. It's just that you can interpret the Bell violations in various ways by rejecting different assumptions. Most people reject the non-contextuality assumption, since we know that it must be rejected anyway (for different reasons), but you can also reject the locality assumption (together with the non-contextuality assumption). That's what the hidden variables advocates do.

zonde said:
So the tracing operation can produce mixed state corresponding to only one of these two martices, right?
Yes. We have tried to explain this a number of times already in this thread. The reduced density matrix of the EPRB state is the (normalized) identity matrix. All the difficulties you are having in this thread (like partial traces and matrix multiplication) are really linear algebra difficulties and not quantum mechanics difficulties. Teaching these basics through an online forum is quite cumbersome, so I suggest you pick up a some introductory linear algebra textbook (for example Halmos FDVS) and then come back with specific questions.
 
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Actually we don't know the value of any observable until we measure it.
 

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