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I understand the idea of quantum entanglement - where what happens to one particle in one location instantly effects another particle in another location, even if separated by millions of miles.

But the question is how does it do that? Are the particles connected through a wormhole or something like that?

glS
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Joshua Flaugher
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  • It doesn't happen that way. There is no action at a distance due to entanglement and there are no two particle states. I think you should read this: http://physics.stackexchange.com/questions/128376/entangled-electrons/128377#128377. Until you understand what @AcuriousMind wrote there, you will not understand what entanglement is and is not. – CuriousOne Jun 16 '16 at 00:40
  • Okay - you're saying there is no action at a distance - other people say there is - or at least when they explain entanglement it appears as if they're describing action at a distance. I mean, I'm not a scientist and when I read science books it has to be the type of book that explains science in laymen terms - so maybe I understood it wrong, but I don't think so.

    In fact I think the Youtube video I'm linking you to describes action at a distance, am I wrong?

    https://www.youtube.com/watch?v=PWuCXcoXNts

     – Joshua Flaugher Jun 16 '16 at 02:06
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    The key question for causality is this: can it be used as an FTL garage door opener? No, it can not be. The individual measurements on each end of an entangled pair are random sequences. They don't contain any information that could be used to generate an open/close signal. Only when we combine the results from both ends do we get a positive correlation, but this requires us to send a classical signal with electromagnetic waves. That's the use of entanglement for cryptographic communications: the classical message contains no information by itself, and the quantum signal can't be intercepted. – CuriousOne Jun 16 '16 at 02:17
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    @CuriousOne I can just see a Douglas Adamsesque story here: the civilization that destroyed itself spending all its efforts on developing a faster than light communication system whilst neglecting their crops and food supplies: when they finally got there, they wasted their efforts producing FTL garage door openners and retrodictive stock quotation systems, instead of transporting a messenger back in time to tell them how bloody stupid they were and that they'd better begin seeing to their basic needs sometime soon. – Selene Routley Jun 16 '16 at 02:41

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No, it simply means that there is a particular kind of statistical correlation between measurements made at the two points. If you measure the state of one, you can infer something about the state of the other; you do not set or influence the state of the other. This is almost exactly the same phenomenon as an experiment wherein one candidate is given a green ball and another a blue in boxes and told to go to different places before opening the boxes; perhaps separated by many light years. On the openning of their boxes, they each know what result the other must see when the other opens their box. But nothing one party does can influence that result. So the correlation is a non causal link.

The only difference between this situation and the measurement of entangled quantum particles is roughly that the measurement results have no meaning until the measurement is done. For a more technical discussion with a worked example, see my answer to the Physics SE question "Does quantum entanglement arise from quantum theory or is it merely an experimental observation?"

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Selene Routley
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  • ok - I get it now..... then why is there so much theoretical talk about possibly using quantum entanglement like a star trek like transporter? – Joshua Flaugher Jun 16 '16 at 02:25
  • @JoshuaFlaugher Have a look at the Wiki on Quantum teleportation, especially the discussion "Protocol". You need a classical communication channel as well as the entangled quantum states to do teleportation. The causal link is the classical communication channel alongside the quantum one. So, in theory, this could be like a Star Trek teleporter, but it is replicating the quantum state of a system elsewhere (whilst destroying the original copy) through classical information transmitted at no faster than the speed of light. – Selene Routley Jun 16 '16 at 02:32
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    Ah - thanks. You have successfully taught a chimp to read Shakespeare. I can't say I fully understand quantum physics (I suppose no one really does), but you've made this aspect of it a lot less foggy for me. – Joshua Flaugher Jun 16 '16 at 03:04
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I guess when you say "Are the particles connected through a wormhole or something like that", you are implying faster than light (FTL) signal.

The need for FTL signal, to explain entanglement behavior, actually arises from a confusion. It arises due to mixing of two types of correlations involved in entanglement.

  1. Anti-correlation (real time) - This means if you measure spin of an entangled particle in a certain direction, then the other particle will certainly have opposite spin if measured in the same angle. You can measure the two particles almost simultaneously to eliminate possibility of any signal at light speed, and the anti correlation is still found true. That makes some people think/say that there must be an FTL communication enabling this "real time" anti-correlation.

Actually, anti-correlation can be easily pre-determined like a pair of shoes and there would be no need of FTL signal. At the time of creation of entangled electrons, due to conservation of angular momentum, the wave function, imparted to the pair is capable of this behavior. So there is no problem with this case.

  1. Statistical Correlation - When spin of various entangled pairs is measured at two different angles A and B, then the correlation (same spin outcome) is given by QM predictions - square of cos((A-B)/2) times the number of pairs measured. Another scenario of statistical correlation is - If you keep measuring spin of an entangled electron in any specific angle, then 50% times, the spin will measure up and 50% times, it will measure down.

Statistical correlation is the one that perplexes people, because, it would require previous measurements to influence the new measurements. However, this would not require an FTL signal either. Because, to statistically influence the outcome of the new measurements, there would always be sufficient time for the light speed signal from previous measurements to travel to wherever it may be needed for the influence. (I am not implying that there is a light speed signal that influence the subsequent statistical outcome. All I am trying to say is that FTL signal is not really required)

Real time behavior would not require FTL signal because anti-correlation can be easily pre-determined.

Statistical behavior would not require FTL signal because it is statistical and there is plenty of time anyway.

Therefore FTL signal is not needed at all to describe entanglement behavior - unless you mix the two types of correlation.

However, based upon Bell's Inequality, some claim that in order to exhibit the statistical correlation, the anti-correlation can not be pre-determined. But that is a different issue for now.

kpv
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