# Need help understanding entanglement

• I
• Orenshved
In summary, the conversation discusses the concept of entanglement in quantum physics and its implications for faster-than-light communication. It also touches on the limitations of classical models to explain quantum phenomena and suggests resources for better understanding the subject.
Orenshved
Hey all, there is something I'm having trouble with here.
If two entangled particles are created and then separated, say one is on Earth and the other in near-earth orbit (like the Chinese experiment), and the one on Earth is measured at say a vertical spin of -1, and at the same time the one in orbit is measured at +1, then what is the "spooky action" here? Isn't it kinda like knowing that with a pair of gloves or shoes that I separate, if in one box I have the left one, then the other box has to contain the other? What exactly is the information that is traveling faster than light? Weren't the two particles split from one particle, and because of the conservation of matter, their sum would be zero?
Before, I assumed (wrongly, I guess), that if you change one of the spins, the entangled twin would change its spin as well (this made sense to me, since it's actually spooky, and has information traveling faster than light). But, from what I understand from this Forbes article, if you change the state for one of them, then the entanglement is broken. Also, it says that it won't even tell me for a fact that one is +1 if the other is -1, just that there is more than a 50% chance that it is. Is this correct?
All of this seems (to me at least, with no formal education in the subject) to be different than what I understood from the quanton physics episodes in the PBS show: Space time, so any help would be greatly appreciated.

Orenshved said:
Isn't it kinda like knowing that with a pair of gloves or shoes that I separate, if in one box I have the left one, then the other box has to contain the other?
No, because if that is all that is going on, there is a limit to how strong the correlations can be between the results, when you run many experiments of this type, over the entire range of possible angles between the directions of the two spin measurements, and collect the statistics. Bell's Theorem, and other related theorems, derive inequalities that express these limits on the correlations. And the predictions of QM (which have now been confirmed in multiple experiments) violate those inequalities. That means the actual results we get, which match the predictions of QM, cannot be explained by a model of the type you describe.

Orenshved
Orenshved said:
from what I understand from this Forbes article
Orenshved said:
the PBS show: Space time
Neither of these are good references if you actually want to learn how QM and entanglement work. You need to be looking at textbooks and peer-reviewed papers.

PeterDonis said:
No, because if that is all that is going on, there is a limit to how strong the correlations can be between the results, when you run many experiments of this type, over the entire range of possible angles between the directions of the two spin measurements, and collect the statistics. Bell's Theorem, and other related theorems, derive inequalities that express these limits on the correlations. And the predictions of QM (which have now been confirmed in multiple experiments) violate those inequalities. That means the actual results we get, which match the predictions of QM, cannot be explained by a model of the type you describe.
Thanks, Peter. About your other response, the reason I'm not reading textbooks or papers is that, as I said, I have no education in Physics or Math, and it seems reasonable to assume that if I want to understand the general idea (I know, I know, "If you think you understand..." and so on) then these are the kinds of things that could help a creative type such as myself (I'm writing a SciFi novel, and want to know that what I'm writing makes sense, without diving too deep).
And again, Thanks. I really appreciate all the help I can get.

Orenshved said:
Isn't it kinda like knowing that with a pair of gloves or shoes that I separate, if in one box I have the left one, then the other box has to contain the other?

It’s like that if we measure only for exact opposites. Say you and I both have spin-detecting devices both pointing straight up and down, and then a source between creates an entangled particle pair and sends one to each of us. Every single time that I measure up you will measure down and vice versa; that’s just like the gloves and shoes.

However, if we choose to point our devices at different angles sometimes we’ll get the same result; both the quantum mechanical prediction and the experimental observation is that the probability of both particles being spin-up relative to the detector is ##\sin^2\frac{\theta}{2}## where ##\theta## is the angle between our detectors - no matter what the actual settings are, all that matters is the angle between them. The more you think about this, the more weird it will seem; it turns out that there is no way to attach fixed properties like the handedness of a glove to the particles in a way that produces this result for all possible angles.

A good layman-friendly introduction is https://static.scientificamerican.com/sciam/assets/media/pdf/197911_0158.pdf and googling for “bell’s theorem” will find much more good stuff.

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## 1. What is entanglement?

Entanglement is a phenomenon in quantum mechanics where two or more particles become connected in such a way that the state of one particle is dependent on the state of the other particle, even if they are separated by large distances.

## 2. How does entanglement occur?

Entanglement occurs when two or more particles interact with each other in a way that their quantum states become correlated. This can happen through various means such as collision, interaction with a third particle, or through a process called spontaneous parametric down-conversion.

## 3. What is the significance of entanglement in quantum computing?

Entanglement is a crucial aspect of quantum computing as it allows for the creation of quantum circuits that can perform calculations on multiple qubits simultaneously. This leads to exponentially faster processing speeds compared to classical computing.

## 4. Can entanglement be observed in everyday life?

No, entanglement is a phenomenon that is only observed at the quantum level and cannot be observed in our everyday lives. It requires precise experimental conditions and equipment to be observed.

## 5. How is entanglement being used in other fields besides quantum computing?

Entanglement is being studied and utilized in various fields such as quantum cryptography, quantum teleportation, and quantum sensing. It also has potential applications in fields such as communication, medicine, and materials science.

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