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## Entanglement question

I read an article, than when on observes an entangled particle, the other entangled particle is actually observed at the same time, no matter how far apart. Is that an accurate statement? Please see https://hudsonvalleyone.com/2020/02/...u-can-imagine/

2. For certain values of "observe". If we measure some property of one particle, thereby taking it out of a superposition of states, then the entangled particle will likewise drop out of superposition to exhibit the complementary value of the property we've measured on the first particle.

Grant Hutchison

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Originally Posted by Copernicus
I read an article, than when on observes an entangled particle, the other entangled particle is actually observed at the same time, no matter how far apart. Is that an accurate statement? Please see https://hudsonvalleyone.com/2020/02/...u-can-imagine/
The article says "With entanglement, two particles are born together and secretly share a wave function. If one is observed, its wave function and that of its twin simultaneously collapse. Two items then materialize at the same moment. And they do so regardless of the distance between them.". The other particle is "observed" only in the sense that we know its state from observation of the other particle.

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Originally Posted by Reality Check
The article says "With entanglement, two particles are born together and secretly share a wave function. If one is observed, its wave function and that of its twin simultaneously collapse. Two items then materialize at the same moment. And they do so regardless of the distance between them.". The other particle is "observed" only in the sense that we know its state from observation of the other particle.
I guess that wave function collapse is not a proven theory, but when we measure one of these particles, do we have to do another measurement on the other particle to verify its spin.

5. Originally Posted by Copernicus
I guess that wave function collapse is not a proven theory, but when we measure one of these particles, do we have to do another measurement on the other particle to verify its spin.
Wave function collapse isn't a theory, it's an interpretation of quantum mechanics.
You don't have to do a separate measurement on the other particle, but you can if you want. Quantum mechanics predicts what the experimental result will be, from the result of the observation performed on the first particle.

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Originally Posted by grant hutchison
Wave function collapse isn't a theory, it's an interpretation of quantum mechanics.
You don't have to do a separate measurement on the other particle, but you can if you want. Quantum mechanics predicts what the experimental result will be, from the result of the observation performed on the first particle.

Grant Hutchison
Are you saying we can know both momentum and position for the second particle then?

7. Originally Posted by Copernicus
Are you saying we can know both momentum and position for the second particle then?
No. I don't see how what I wrote could be interpreted that way.

Grant Hutchison

8. Originally Posted by Copernicus
Are you saying we can know both momentum and position for the second particle then?
I'd go as far as saying that what we know about entanglement suggests just the opposite. What you're hoping to claim here sounds a lot like Einstein's position in the EPR thought experiment, trying to suggest that quantum theory was incomplete.

Here's the argument in a nutshell. Position and momentum aren't the only conjugate observables. Although their original argument was framed with position and momentum, it's actually a bit easier to explain in terms of others, for example, the spin in any two orthogonal directions. Since these are discrete, measuring the spin of a particle in the x direction (arbitrarily chosen by the way the experimental apparatus is set up), means that its spin in the y direction (again arbitrary, but at right angles to your first choice) is completely undetermined. Einstein, Podolsky, and Rosen suggested, though, that you could imagine measuring the spin of one entangled particle in the x direction (and thus knowing the spin for both particles), and the spin of the second particle in the y direction (thus apparently knowing the spin of both particles in the y direction). If you wait for the particles to be far enough apart, the measurement of one shouldn't be able to have any effect on the other, and so you've apparently found a clever way to determine the spin of a particle in two separate directions. Since you could in principle do your measurements at any angles, each particle must "know" its spin at every angle, and so the fact that we can't measure it directly is just a matter of classical ignorance rather than any kind of quantum indeterminacy.

But later work on entanglement, especially from Bell, including later verifying experiments, shows that quantum theory doesn't work like this. In particular, measurements at other angles, where there should be partial correlation, shows that the correlation is too good for a model where the particles just "know ahead of time" how they will respond to various measurements. The assumption that the particles have definite properties that do not change in response to things arbitrarily far away (i.e., local reality) cannot hold.

9. Hey, look, I found this as the solution to a homework problem. Take a look here. Looks like a decent upper undergraduate or graduate course on quantum mechanics, using Griffiths as the text, if you wanted to pursue it further. Griffiths is one of the standard texts for this kind of stuff; I'd definitely recommend picking it up and becoming comfortable with it if you're looking for a deeper understanding of the theory behind this.

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