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https://www.google.com/amp/s/amp.livescience.com/28550-how-quantum-entanglement-works-infographic.html?espv=1

In this article, it says "In quantum physics, entangled particles remain connected so that actions performed on one affect the other, even when separated by great distances. "

I've had a constant confusion about whether the actions of one actually travel across distance, or if the states are merely totally correlated because of their interaction at the origin, and then the knowledge of the state of the other is known solely because that correlation is preserved. If it's no longer preserved, this means the particle is no longer correlated, and sometimes people explain this by saying that it became entangled with something else.

Can some one verify this intuition or explain why it's wrong?

scl
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  • The largest distance we have tested entaglement is until the International Space Station. Right now we have observed that the information "travels" from one particle to another at a speed 10,000x speed of light. We need to test at much much higher distances to check whether the information, in fact, "travels" or is instantaneous which means something else is going on. – Ali Kazi Nov 14 '18 at 04:44
  • But the information is imparted on both particles at the origin, no? – scl Nov 14 '18 at 04:53
  • If you take a pair of shoes and send one shoe to the moon, when you look at the shoe on the ground, it's always going to be the opposite foot of the one on the moon. It doesn't mean they have any causal effect on each other. It's just because the state is preserved across that distance – scl Nov 14 '18 at 04:57
  • @sci The question may be partly addressed here: https://physics.stackexchange.com/a/440362/206691 – Chiral Anomaly Nov 14 '18 at 05:01
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    @sci The fact that entanglement is not just ordinary correlation is explained here: https://physics.stackexchange.com/a/438137/206691 – Chiral Anomaly Nov 14 '18 at 05:02
  • Your understanding of "correlation is preserved" is correct. see my recent answer here to a similar question https://physics.stackexchange.com/questions/439450/how-do-we-know-quantum-entanglement-works-no-matter-the-distance/439469#439469 – anna v Nov 14 '18 at 05:17
  • Dan, is the fact that entanglement is not ordinary correlation similarly "explained" by the bloch sphere and epr paradox? – scl Nov 14 '18 at 05:25

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Your shoe example is only one part of entanglement.

It is referred as perfect anti correlation - Suppose you and your friend have same shoe size. You take a pair of shoes of your size and randomly send one shoe to your friend. And then you and your friend try the shoe at your respective end. You will always find that if the shoe at your end fit your left foot, then the other shoe will fit your friend in the right foot, and vice versa. This is simple and will happen irrespective of distance between you and your friend and also irrespective of who tries the shoe first. This is simple, not mystery.

Then comes the next part - Statistical correlation - Here is not a correct example, but it should convey the idea. Suppose you and your friend have different shoe size. Now pick numerous pairs of shoes of random sizes and repeat the above experiment. From (say) 100 pairs, some shoes will fit you and some shoes will fit your friend. Say statistically, 20 shows fit you and 30 shoes fit your friend.

It is this statistical correlation (how many shoes fit you and how many fit your friend) that makes the entanglement baffling. The statistical percent is given not by classical math, but by quantum mathematics. And the Quantum mathematics is proven right in all experiments, given sufficiently large number of pairs.

So, no information is transmitted between the pairs, but the quantum particles follow quantum math at statistical levels. By performing variations of experiment, an impression is created as if information is instantaneously (or say at 10K times c) transmitted from one pair to another. That is not true. The real instinctive explanation is yet to be found. There are different views that try to explain the statistical correlation but none satisfies the instinct even though the math is always followed.

kpv
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  • But the statistics is a separate concern from the anti-correlation, right?

    The EPR Paradox is easily explained by the idea that you're projecting a position on a sphere onto 2 bits. This would happen whether you were looking at 2 entangled particles or a single particle from 2 angles at the same time (which is impossible given the way we measure the particles)

     – scl Nov 14 '18 at 05:37
  • @sci: Yes, it should be a separate concern but it is not. Because if you separate the two, then the anti correlation becomes very simple and we can say that the particles had all needed info at origin. To demonstrate that this is not the case, they perform the statistical experiments by setting classical statistical limit (Bell's inequality), And the experiments find that the classical limits are not followed, rather QM predictions are followed. Setting classical limits assumes independent outcomes from pair to pair, which may not be the case in reality. – kpv Nov 14 '18 at 05:45
  • @sci: An example of not independent outcomes can be - Suppose by trying out so many shoes, your foot gets swollen little by little. So, the statistical limit that was set by assuming a fixed foot size, is not valid any more! The classical probability of you fitting a certain size can change over the course of experiment. So, setting the classical limit and the expecting it to be not broken, may be a faulty premise. That is why, I do not think Bell's inequality adds any value to the whole thing. – kpv Nov 14 '18 at 05:52
  • Can you address here? https://physics.stackexchange.com/questions/440850/epr-paradox-entanglement-spheres-and-information – scl Nov 14 '18 at 05:54
  • @sci: Note that Bell, as mathematical theorem has no issues but its application to entanglement is likely misplaced. – kpv Nov 14 '18 at 05:55
  • But still the EPR paradox can be explained by assuming a random orientation in 3d space is being mapped to a single boolean value – scl Nov 14 '18 at 05:56
  • @scl: I am not skilled to comment on your sphere example in other question. – kpv Nov 14 '18 at 06:01