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  1. #1
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    Faster Than Light Communication?

    The following description of the 'EPR' (Einstein/Podolsky/Rosen) hypothesis says that a scientific experiment has shown that faster than light communication can occur, at least at the quantum level. Is this true? The author of this article doesn't cite a reference.

    The EPR Paradox.
    The experiment is so named because it was a thought experiment devised by Einstein, Boris Podolsky and Nathan Rosen. As with Schrödinger's cat-in-the-box experiment, its purpose was to expose the 'foolishness' of the Copenhagen Interpretation. The experiment focuses on the phenomenon of quantum theory known as 'non-locality', which concerns communication between particles. A pair of protons, for example, associated with one another in a configuration called the singlet state will always have a total angular momentum of zero, as they each have equal and opposite amounts of spin. Just as we have seen in the other experiments, the protons will not collapse their probability wave and 'decide' which spin to adopt, until they have been observed. If you measure the spin of one proton, according to quantum theory, the other proton instantly 'knows' and adopts the opposite spin. So far so good, we have come to expect this sort of behaviour from particles, so what is the problem with this particular experiment?

    It is possible, and has been carried out in laboratory tests over a short distance, to split the particles apart and send them in opposite directions and then measure one of them for spin. The instant it is measured, and the spin determined, the other particle adopts the opposite spin. The time interval is zero, the event takes place instantaneously, even though the particles are separated, and theoretically would still do so even if they were separated by a distance measured in light years. This is what upset Einstein, the implication that particles could communicate at faster than light speed, as it is impossible for this to happen according to Einstein's theory of relativity.

    At the time this thought experiment was proposed, in the early 1930's, just about the time of Schrödinger's cat-in-the-box thought experiment, it was not actually possible to physically carry out the experiment. Einstein did not live to see it turned into practical reality, which is probably just as well in light of the results produced. This experiment has now actually been carried out over a distance of 10 kilometres and confirmed as correct. Something here is taking place at faster than light speed, although exactly what seems to be a matter of some debate. Regrettably, due to its very nature, no meaningful communication could be made using such a device. Whether or not it will ever have any useful application remains to be seen, but that is not the point. The point is the experiment has proved Einstein wrong, faster than light speed, at least in the quantum world, is a reality.
    From: http://www.thekeyboard.org.uk/Quantum%20mechanics.htm
    Last edited by FrogSkins; 02-25-2007 at 01:36 PM.
    Scientists animated by the purpose of proving that Nature is purposeless constitute an interesting subject for study. - Alfred N. Whitehead

  2. #2
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    Re: Faster Than Light Communication?

    Quote Originally Posted by FrogSkins
    The following description of the 'EPR' (Einstein/Podolsky/Rosen) hypothesis says that a scientific experiment has shown that faster than light communication can occur, at least at the quantum level. Is this true? The author of this article doesn't cite a reference.



    From: http://www.thekeyboard.org.uk/Quantum%20mechanics.htm
    Unfortunately EPR made mistakes, Einstein basically just didn't like quantum mechanics because he had based his career on showing that predictions and measurements can always be made, and here was QM telling everyone that no there's an uncertainty principle. EPR made many attacks on QM, it's very disappointing considering how brilliant Einstein was.
    He was so convinced of simplicity in the Universe that it closed his mind.
    But anyway to your question, there's no evidence that two entangled particles exchange any information at all, so it doesn't violate GR's predictions on the lightspeed limit.

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    Re: Faster Than Light Communication?

    Thank you Subjunk, that's what I suspected. Proof of that theory would have made headlines.

    The premise of his explanation is faulty, correct? That is: "The experiment focuses on the phenomenon of quantum theory known as 'non-locality', which concerns communication between particles." Non-locality is NOT concerned with communication between particles. What would be communicated?
    Scientists animated by the purpose of proving that Nature is purposeless constitute an interesting subject for study. - Alfred N. Whitehead

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    Re: Faster Than Light Communication?

    Quote Originally Posted by FrogSkins
    Thank you Subjunk, that's what I suspected. Proof of that theory would have made headlines.

    The premise of his explanation is faulty, correct? That is: "The experiment focuses on the phenomenon of quantum theory known as 'non-locality', which concerns communication between particles." Non-locality is NOT concerned with communication between particles. What would be communicated?
    Yeah exactly. They are just entangled. In QM particles are particles and waves (some theories state this more literally than others, but all involve at least wavefunctions) and each "probability wave" stretches across the entire Universe, so it can be said that every particle exists as a wave everywhere in the Universe.
    EPR thought this was wrong; that it only appears this way but the particle was in the same place the whole time, but multiple variations on the double slit experiment showed otherwise.
    I could go into more detail if you want, I'm not sure how interested you are. If you want I can find a good website that explains the various double-slit experiments for you. It's cool stuff!

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    Re: Faster Than Light Communication?

    Quote Originally Posted by SubJunk
    Unfortunately EPR made mistakes, Einstein basically just didn't like quantum mechanics because he had based his career on showing that predictions and measurements can always be made, and here was QM telling everyone that no there's an uncertainty principle. EPR made many attacks on QM, it's very disappointing considering how brilliant Einstein was.
    He was so convinced of simplicity in the Universe that it closed his mind.
    But anyway to your question, there's no evidence that two entangled particles exchange any information at all, so it doesn't violate GR's predictions on the lightspeed limit.
    Your right that the EPR paradox was wrong, but it was wrong because there are non-local effects in QM. Tests of Bell's inequality have shown this.

    Well, they haven't ruled out every possibility, as with everything in QM there are many ways to interpret results, but most accept that there are non-local effects. As for the lightspeed limit, I believe (though I'm not sure) that GR predicts no information can exceed lightspeed, and since the direction in which the wave function collapses is random there is no causal non-local transmission of information. So in a way, causality is violated but not....

    Anyways, you may have guessed from my response I don't totally understand this, but I think I'll be studying it this semester so I'll come back in a few months :)

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    Re: Faster Than Light Communication?

    I found these two links to be helpful in understanding this subject, but it gets awfully complicated!
    http://everything2.com/index.pl?node_id=1321721
    http://everything2.com/index.pl?node...&lastnode_id=0
    Scientists animated by the purpose of proving that Nature is purposeless constitute an interesting subject for study. - Alfred N. Whitehead

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