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I know I am missing something and this question is probably very silly, but I would like to understand. Quoting an article:

If one photon is measured to be in a +1 state, the other must be in a -1 state. Since the outcome of one photon affects the outcome of the other, the two are said to be entangled.......when you measure the state of one photon you immediately know the state of the other....If we’re light years apart, we each know the other’s outcome for entangled pairs of photons, but the outcome of each entangled pair is random (what with quantum uncertainty and all), and we can’t force our photon to have a particular outcome.

I just cannot see the "magic" here. Using a stupid analogy:
There are two balls, black and white, wrapped in a piece of cloth. You take one and I take the other. Whenever and wherever I unwrap the one I took, I will immediately know which one you have.

What is so special about that in the world of particles, how does the outcome of the first affect the other?

John V
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  • The analogy does not work, because in the analogy, you assume that the color of the two balls is something predetermined that does not change. But photons, which are quantum objects, are not like that: they don't have a fixed polarization direction until you measure it, and when you measure it, there are certain probabilities for certain polarization directions. – Marius Ladegård Meyer Nov 14 '19 at 13:48
  • Now, the weird thing is that when you measure on one of the photons and then measure on the other photon after that, the two measurements are correlated no matter how far apart the two photons are when you measure them, even when they are so far apart that no signal could possibly travel fast enough from the first photon you measure on, to the second photon to "enforce" the correlation. This is what Einstein called "spooky action at a distance". – Marius Ladegård Meyer Nov 14 '19 at 13:55
  • This video was helpful for me: link – Marius Ladegård Meyer Nov 14 '19 at 13:55

3 Answers3

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What is "magic" about entanglement is that entangled particles have a quantum state based on, or dependent on, the other particle(s) that it is entangled with. If you look at two random particles that aren't entangled, the quantum state of the first will tell you nothing of the second. However, if you look at first of an entangled pair, you will know the state of the second without ever observing it.

CuriousOne
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"There are two balls, black and white, wrapped in a piece of cloth. You take one and I take the other. Whenever and wherever I unwrap the one I took, I will immediately know which one you have."

How about this:

There are two balls, wrapped in cloth. You take one and I take the other. We repeat this every day. And this happens.

  • Whenever we both unwrap the balls with our left hands, we find that they have opposite colors.
  • Whenever we both unwrap our balls with opposite hands, we find that they usually --- but not quite always --- have opposite colors. (So maybe somehow using your right hand occasionally, but not often, changes the color of the ball somehow?)
  • Whenever we both unwrap our balls with our right hands, we find that they usually have the same color. (Now try to find an explanation for this that's consistent with the other observations.)

    • This, unlike the article you quoted, captures exactly what some people find mysterious about entanglement.

    WillO
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    • Thank you. To understand it a bit more, how does this left/right hand affect that? In the original example, there is no constraint on how to measure (or unwrap, in this case). I still somehow do not get how the outcome of the first affects the other. In normal objects, the color is predetermined and when the first is black, you know the other will be white. – John V Nov 15 '19 at 08:34
    • "in the original example there is no constraint on how to measure" --- nor is there here. You choose what/how to measure. In the hypothetical ball example, you choose which hand to use. In the real quantum example, you choose which way to tilt your measuring apparatus. – WillO Nov 15 '19 at 14:07
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    The analogy you should consider is the case in which you and I both hold a coin. You toss your coin and I toss mine. Whether you get a heads or a tails I always get the opposite result. The results of our two supposedly random processes are perfectly correlated- that is the magic.

    Marco Ocram
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    • This seems to me to be a terrible analogy. You get to measure the components of an entangled pair once; after that it's not entangled any more. So maybe you want to assume we use a different entangled pair for each flip? But then your scenario is perfectly well explained by hidden variables, just as the OP suggests. – WillO Nov 14 '19 at 16:18
    • Sorry, @WillO, I don't really understand your objection. For example, there are no hidden variables in the tossing of coins. But never mind- we are probably talking at cross purposes. – Marco Ocram Nov 14 '19 at 16:34
    • Yes, there are no hidden variables in the tossing of coins. There are also no coins that behave as you describe. The point is that if there were coins that behave as you describe, then their behavior would be perfectly explicable via hidden variables. The problem with your example is not that it's hypothetical. The problem is that the hypothetical behavior fails to illustrate what's unique about entanglement. – WillO Nov 14 '19 at 16:58
    • We definitely are talking at cross purposes. There is no pair of balls that behave as you describe (ie sometimes being opposite colours and sometimes being the same colour). You have designed your analogy to be more aligned with the quantum mechanical effect of entanglement. I have designed mine to illustrate what is wrong with the OP's analogy in an easy to understand way. We are addressing different pedagogical aims. – Marco Ocram Nov 14 '19 at 17:11
    • Of course there are no balls that behave as I describe. That is, after all, the whole point. I have described hypothetical balls that behave exactly as quantum objects do. But there are no such balls, which shows that quantum objects are not like balls. This addresses the OP's issue. You have described hypothetical coins that behave in a way that is not at all like the behavior of quantum objects, and therefore have nothing to do with the OP's issue. I'll be happy to let you have the last word at this point, since I don't think I'll have anything to add to this. – WillO Nov 14 '19 at 17:58
    • Hi @WillO. Please, your analogy is excellent in its way. Mine is attempting to achieve a different outcome, namely explaining in a more naive way why the OP's analogy is wrong. I think our two approaches are complementary, and perhaps it would be good to combine them. Can you think of a way to do that? I'll try to come up with something that includes the best of both. All the best. – Marco Ocram Nov 14 '19 at 19:36