Entanglement/Nonlocality question

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Discussion Overview

The discussion revolves around the EPR paradox and the implications of measuring entangled particles, specifically focusing on the behavior of spin measurements in different experimental setups. Participants explore concepts of entanglement, nonlocality, and the interpretations of quantum mechanics related to these phenomena.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that measuring the spin of entangled particles A and B along the same axis will yield opposite results, while the impact of intermediate measurements along different axes is debated.
  • One participant suggests that the relationship between the spins of entangled particles could be viewed as breaking upon measurement, questioning the interpretation of action at a distance.
  • Another participant emphasizes that Einstein, Podolsky, and Rosen did not label their reasoning as a paradox, but rather argued for the incompleteness of quantum mechanics.
  • It is proposed that the interpretation of the EPR paradox as action at a distance arises from the nature of the entangled wave state existing at separated points in spacetime, leading to discussions on quantum non-locality.
  • Some participants mention that to explain the results of measurements, one must consider the entangled system as a whole rather than assuming independent realities for each particle.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the EPR paradox and the implications of entanglement, indicating that multiple competing perspectives remain without consensus.

Contextual Notes

There are unresolved questions regarding the nature of measurement and the implications of intermediate measurements on entangled states, as well as the definitions of locality and realism in quantum mechanics.

Jewlian
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I'm sure something like this has been posted before but I would appreciate if you humor me. I've been reading the Wikipedia article on the EPR paradox and it greatly confused me.

Let's start with the basic premise that we have two entagled particles A and B.

Expriment 1:

We measure the spin of particle A along the x-axis. Then we measure the spin of particle B along the x-axis. As I understand it the measured x-axis spin of B will always be the opposite of the measured x-axis spin of A.

Experiment 2:
We measure the spin of particle A along the x-axis. Then we measure the spin of particle A along the y-axis. Then we measure the spin of particle B along the x-axis. Would the measured spin of particle B along the x-axis still be the opposite of the measured x-axis spin of A ?
 
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My answer would be: it depends on the details of the experiment.
 
Jewlian said:
I'm sure something like this has been posted before but I would appreciate if you humor me. I've been reading the Wikipedia article on the EPR paradox and it greatly confused me.

Let's start with the basic premise that we have two entagled particles A and B.

Expriment 1:

We measure the spin of particle A along the x-axis. Then we measure the spin of particle B along the x-axis. As I understand it the measured x-axis spin of B will always be the opposite of the measured x-axis spin of A.

Experiment 2:
We measure the spin of particle A along the x-axis. Then we measure the spin of particle A along the y-axis. Then we measure the spin of particle B along the x-axis. Would the measured spin of particle B along the x-axis still be the opposite of the measured x-axis spin of A ?

Yes. The intermediate measurement of Alice along the y-axis has no effect on the results.
 
I'm really missing something, I don't get the paradox here. Why is it interpreted as action at a distance. Why can't we say that "When particles are entangled they have the opposite spins on all axis. Then the first observation causes this relationship to break"?
 
Einstein, Podolsky and Rosen, who invented this type of reasoning, did not say that there is a "paradox". They simply argued that quantum mechanical description is not complete.
 
Jewlian said:
I'm really missing something, I don't get the paradox here. Why is it interpreted as action at a distance. Why can't we say that "When particles are entangled they have the opposite spins on all axis. Then the first observation causes this relationship to break"?

There is no paradox. It is just that, as Bell showed, the system cannot be explained by a locally realistic model. This means that if you assume that each of the widely separated particles has a physical reality independent of the other particle, then you cannot explain the results. In order to explain the results, you have to treat the total system of the two particles together, which is why we say that the two particles are 'entangled'.
 
Jewlian said:
I'm really missing something, I don't get the paradox here. Why is it interpreted as action at a distance. Why can't we say that "When particles are entangled they have the opposite spins on all axis. Then the first observation causes this relationship to break"?

The interpretation is a result of the idea that the entangled wave state exists at (space-like) separated points in spacetime. Of course, this is actually true of the state of any individual particle as well. So this is sometimes referred to as quantum non-locality. This means that there is apparent non-locality but with limits consistent with the quantum world. Causes and effects won't propagate faster than c, but wave function collapse will.
 

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