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If I have two "entangled" particles and I know the spin state of every one of them. Then, I change the spin state of one of the particles, will it affect the spin state of the other particle even if it is far away? Also, it turns out that entanglement is actually real and can be used for many things, is this just the result of a false interpretation and understanding of Quantum Mechanics by those who claim it or it can be really done? And if it is real, will it just change our interpretation of GR?

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    $\begingroup$ possible duplicate of Quantum Entanglement - What's the big deal? $\endgroup$
    – Mostafa
    Jul 11, 2013 at 22:58
  • $\begingroup$ This is still very unclear and broad (and us closeable), so I'll close it as a duplicate becasue that is indeed one of the ways to interrpet this. I suggest you clarify your question by editing it. $\endgroup$ Jul 12, 2013 at 5:58

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If I have two "entangled" particles and I know the spin state of every one of them. Then, I change the spin state of one of the particles, will it affect the spin state of the other particle even if it is far away?

This is a misconception. There is no way of instantaneously changing the state (from the point of view of an observer in its vicinity) of one particle from an entangled pair by performing operations on the other one. This would imply a violation of relativistic causality, as the tone of the OP's question implies. See the no-signalling theorem.

Also, it turns out that entanglement is actually real and can be used for many things, is this just the result of a false interpretation and understanding of Quantum Mechanics by those who claim it or it can be really done? And if it is real, will it just change our interpretation of GR?

The existence of entanglement is routinely and daily confirmed in quantum optics and atomic physics laboratories and particle accelerators around the world. Macroscopically observable phenomena such as the quantum Hall effect only occur due to quantum entanglement.

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  • $\begingroup$ "There is no way of changing the state of one particle from an entangled pair by performing operations on the other one" This is flat-out wrong. Measurements on one of the particles of an entangled pair absolutely affect the state of the other particle, this is the entire basis behind "spooky action at a distance" and the significance of Bell's inequalities. Yes, quantum mechanics violates relativistic causality (or locality, take your pick). The no-signalling information indicates that information can't be sent, it doesn't indicate that state collapse does not occur. $\endgroup$ Sep 12, 2016 at 1:39
  • $\begingroup$ @aquirdturtle I am afraid you have misunderstood something here. Quantum mechanics is perfectly compatible with relativistic causality, and all other aspects of special relativity. Indeed, relativistic quantum field theories (of which entanglement and Bell correlations are a basic feature) are the cornerstone of the modern understanding of fundamental physics. $\endgroup$ Sep 12, 2016 at 13:15
  • $\begingroup$ The global quantum state of an entangled pair is of course affected by the measurement, and undergoes "collapse". But this state does not describe the state of one particle or another, but rather the total system, while its "collapse" happens on a piece of paper only. The local physics relevant for each particle of an entangled pair is described by its local quantum state, i.e. its reduced density matrix. And it is very easy to show (indeed essential for the proof of no-signalling), that the state of one particle is not affected by any local operation in the vicinity of the other particle. $\endgroup$ Sep 12, 2016 at 13:24
  • $\begingroup$ You are not arguing that "There is no way of changing the state of one particle from an entangled pair by performing operations on the other one.(Full-stop)" you are just repeating the argument for the no-signalling theorem, which is not the same thing. If an observer measures the state of one particle of a well-separated entangled pair, that observer's description of the other particle immediately changes as well. Clearly the observer's description of the state has changed. $\endgroup$ Sep 12, 2016 at 22:07
  • $\begingroup$ Similarly, the description of another observer local to the un-observed particle will change their description of the unobserved particle as soon as they hear of the result on the other particle. In both cases, the state of the particle clearly changes. While I'm sure that you appreciate both of these things, for this reason I find your above statement grossly misleading because it makes no mention of the instantaneous-ness of the change in state or the reference frame for describing the un-observed particle. $\endgroup$ Sep 12, 2016 at 22:09

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