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I’ve been learning about quantum entanglement and I’m struggling to understand the full picture. Here’s what I’m thinking:

In entanglement, we have two particles (let's call them A and B) that are described as a single, correlated system, even if they are far apart. For example, if two particles are entangled with total spin 0, and I measure particle A to have clockwise spin, I immediately know that particle B will have counterclockwise spin, and vice versa.

However, here’s where my confusion lies: It seems like the only reason I know the spin of particle B is because I measured particle A. I’m wondering, though, isn’t it simply that one particle always has the opposite spin of the other, and once I measure one, I just know the spin of the other? This doesn’t seem to involve influencing the other particle "remotely" or "faster than light" – it just seems like a direct correlation based on the state of the system, which was true all along.

So, if the system was entangled, one particle’s spin being clockwise and the other counterclockwise was always true. The measurement of one doesn’t really influence the other, it just reveals the pre-existing state.

Am I misunderstanding something here? Or is it just a case of me misinterpreting the idea that entanglement “allows communication faster than light”?

allexj
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1 Answers1

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Here is what you're missing.

You take one entangled particle. I'll take the other. I'll take it someplace very far away, in fact.

Now take your measuring apparatus, orient it north/south, and measure your particle's spin. I'll do the same. 100% of the time, we will get opposite results.

Now orient your apparatus, say, northeast/southwest, while I continue to hold mine north/south. We make our measurements, and now we get opposite results about 70% of the time.

Or orient your apparatus east/west. Now we get opposite results 50% of the time.

Or orient your apparatus north-northeast/south-southwest. Then we get opposite results about 85% of the time.

Likewise if you keep your apparatus fixed north/south and I start changing my apparatus's orientation.

Go try and explain that as simple discovery of pre-existing correlations, using just classical concepts.

So the particles don't have pre-existing spins that we are simply discovering. But there is even more to the story: It turns out that with a little analysis of the particular correlations we observe, you can prove that the particles don't even have a pre-existing joint probability distribution for the spins they'll exhibit when measured in different orientations.

So the first (mild) surprise is that there are no pre-existing spins; the second (far more unsettling, at least before you start thinking like a quantum mechanic) is that there's not even a probability distribution for those spins.

Finally, you're last paragraph says this:

The measurement of one doesn’t really influence the other, it just reveals the pre-existing state.

It is certainly true that the measurement of one does not influence the other. It is also true that there is no pre-existing joint probability distribution for the outcomes of the spin measurements (which rules out any sort of pre-existing "state" in the sense that you appear to be using that word). And it is also true that according to classical physics and probability theory, those two statements contradict each other. That's why quantum mechanics takes some getting used to.

WillO
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