Entanglement/Nonlocality question

[Jewlian]

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 ?

 

[arkajad]

My answer would be: it depends on the details of the experiment.

 

[DrChinese]

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.

 

[Jewlian]

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"?

 

[arkajad]

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.

 

[phyzguy]

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'.

 

[DrChinese]

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.