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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Zo the Relativist
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1possible duplicate of Quantum Entanglement - What's the big deal? – Mostafa Jul 11 '13 at 22:58
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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. – Manishearth Jul 12 '13 at 05:58
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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.
Mark Mitchison
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"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. – aquirdturtle Sep 12 '16 at 01:39
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@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. – Mark Mitchison Sep 12 '16 at 13:15
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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. – Mark Mitchison Sep 12 '16 at 13:24
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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. – aquirdturtle Sep 12 '16 at 22:07
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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. – aquirdturtle Sep 12 '16 at 22:09
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@aquirdturtle In my answer, "state of the particle" means the reduced density matrix of the particle. What you are apparently talking about is the global quantum state of an entangled pair (which refers to the total system and not to individual particles). I think that is the source of the confusion here. Otherwise I don't understand what you can reasonably object to in this answer. It is indisputable that the local physics at A is not affected by measurements at B. Sure the description of the system by some observers may change upon measurements, but this change has no invariant meaning. – Mark Mitchison Sep 12 '16 at 22:12
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I would agree to the statement "the local physics at A is not instantaneously affected by measurements at B.", pointing out the necessity of the word instantaneously (or the longer but more accurate "not affected until a measurement of B lies within the light cone of the measurement of A"), and also point out that "the local physics" is very different from "the state", as I just pointed out in my last comment. – aquirdturtle Sep 12 '16 at 22:22
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@aquirdturtle As far as I can tell, you are saying that an observer should update his or her description of the system (the state) when new information about the outcomes of measurements becomes available. This is nothing but a tautological restatement of the definition of a quantum state. The OP seems to be asking whether entanglement implies some kind of physical connection that causes the observable properties of one particle (eg. spin) to change upon measuring the other. We seem to agree that it does not. So I'm a bit lost, what are you actually trying to add? – Mark Mitchison Sep 12 '16 at 22:28
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