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Does entanglement violate special relativity?

  1. Mar 14, 2009 #1
    In this months edition of Scientific American, the main article is on how entanglement violates Special Relativity as it allows for the seemingly instantaneous transmission of information (which spin the other particle has etc.). I asked my physics teacher about this and he said that he did not think this to be a violation of Special Relativity as he didn't see at as being the transmission of information but as the resolution of an uncertainty. I was wondering if anyone could help and shed some light on this for me?

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  3. Mar 14, 2009 #2


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    The simple answer is that since you can't choose which state your observe particle collapses into you have no control on what the state of the other particle would be - so no transfer of information.
    It's like putting a black and a white ball into two separate boxes, posting one to your friend in Australia - when you open yours you know what the colour of his ball is instantly but you can't use this to send him a message
  4. Mar 14, 2009 #3
    But with that analogy both balls always had one definite colour, we just didn't know what it was. I thought the point of entanglement is that the particles themselves don't "know" what their spin is going to be before it is measured? If this is so, then by measuring one we actually affect the other and, therefore, is this not a transmission of information?
  5. Mar 14, 2009 #4


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    They are allowed to affect each other instantly - there is no rule against that.
    Information has a very specific meaning in relativity - sending a random value that you can't influence isn't information.
  6. Mar 14, 2009 #5
    What does relativity define information as then?
  7. Mar 14, 2009 #6
    You can say "what so-called information are you talking about that could conceivably be transferred?" because all our models of Information are classical. Even in quantum computation you have to collapse the qubit by measuring it and then settle for a "0" or "1" classical output. It's essentially incoherent to talk about "quantum information" or what's "inside" a superposed particle. Nobody knows and it's conceivable that nobody ever will.

    That said, there's increasing talk about possible conflict between "spooky action" and SR and the discussion's not going to stop. If all you knew about two entangled particles was what we can imagine them "knowing about themselves" you might not think of them as physically separated, even though the distance between them could measure billions of miles. Two or more entangled particles are a unitary physical system, just as though they were buckets of water interconnected by pipes. But neat though that might sound we still need to deal with the fact that in spacetime terms the distances between the entangled particles measure as the distances between them measure.

    A physicist for whom I have enormous respect, Anton Zeilinger at the IQOQI (Institute for Quantum Optics and Quantum Information) affiliated with the University of Vienna, flat-out describes entanglement as an effect operating "outside of time and space." He can get away with that because he's Anton Zeilinger. All I dare do is quote him. How exactly does that outside-of-spacetime operation operate? No answer yet. Would it avoid contradicting SR? Well, Relativity is a description of Space-Time-Gravitation, not of whatever might lie outside of Space-Time-Gravitation. So presumably it could avoid a contradiction.
    Last edited: Mar 14, 2009
  8. Mar 14, 2009 #7


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    As has been pointed out, it's impossible to use an entangled pair to send any information. To elaborate a little, the point is that you can only make one measurement, and a single measurement tells you very little about a state. For example, if you measure the spin of an electron to be up, then you only know that it's not in the spin down eigenstate. On the other hand, if you do a bunch of repeated measurements on electrons all prepared in the same state, and get spin up every time, it's a good bet the state is at least approximately equal to the spin up eigenstate.

    Turning back to the original scenario, let's say two particle are entangled so that they have opposite spins: if one is up, the other is down. These are sent off to obserers A and B. Then A will try to send a message to B, say an answer to a yes or no question, by the following scheme:

    1. If he wants to send yes, he should measure the electron, collapsing its state.
    2. If he wants to send no, he should not measure it, leaving it in a superposition.

    Now if B measures his electron, let's say he finds it to be spin up. What does this mean? Well, either A wanted to send yes, and measured his electron to be spin down, or he wanted to send no, and B was the one who collapsed the state. These are equally likely, so there's no way for B to know what A did, and so no information is sent.

    On the other hand, let's say B could copy the state of his electron into the states of many other electrons (ie, so that all the electrons have the same state as the original), and measure all of their spins. If he found them all to be spin up (or all spin down), he'd be reasonably sure A collapsed the state, and so wanted to send yes. If the distribution is 50/50 spin up and spin down, he'd know the state was not collapsed, and so the message is no.

    Thus if it was possible to copy states, it would be possible to send information faster than light. Luckily, there's a theorem called the http://en.wikipedia.org/wiki/No_cloning_theorem" [Broken] which says it's not possible to copy states in this way. The proof of this uses simple linear algebra. It's interesting that such a simple, and seemingly unrelated, mathematical property of quantum mechanics is crucial to the consistency of quantum mechanics with special relativity.
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  9. Mar 14, 2009 #8
    What amazes me of that article by David Albert in Scientific American is how categorically he dismiss locality: "...And so the actual physical world is nonlocal. Period."

    Did anyone had a look on this, instead? :
  10. Mar 15, 2009 #9
    The underlying reason for the special relativity constraint is that 'cause and effect' cannot be violated. i.e. the effect cannot be allowed to happen before the cause. This principle results in Lorentz, and its not fundamentally the speed of light at issue but speed of information travel. The speed of gravity information travel must be the same.

    State correlation between entangled particles is not a cause followed by an effect, so there is no 'speed of travel' involved at all.
    Last edited: Mar 15, 2009
  11. Mar 15, 2009 #10
    Lets think of it in terms of information. 3 D space is given by information stored in data (3D space does not actually exist in the normal human-sense, its actually information) and held in the so-called 'information space', then correlating spin entanglement needs not much - 1 bit that both particles point to (and held in the same information space that gives the illusion of 3Space). There is no problem with separation in 3D then, because they 'exist' in information space and there is no physical separation in that space.

    Note SR would be required result in information space processing too, else it would not work - as you said - cause and effect would be wrong. Such a field model requires SR to work.
  12. Mar 15, 2009 #11
    No, that's not transmission of information.
    The transmission of information always involved in transfer of energy
    Can you transmit information without transfer of energy, e.g. wireless communication?
  13. Mar 15, 2009 #12


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    The effect is instantaneous, but the only thing directly affected at the other end is something unmeasurable (the wave function). That's why you can't send FTL messages this way.

    If you want to prove me wrong (or just understand this better), I suggest that you try to design a thought experiment in which Alice can (in principle) send an instantaneous 1-bit message to Bob, for example to let him know the result of a coin flip she just did.
  14. Mar 16, 2009 #13
    I only recently learned of quantum entanglement. My first reaction was that this seemingly instantaneous transmission of data was actually in fact instantaneous.

    I understand that by believing the above, I am believing in an action that violates SR. However, I looked at this situation like this:

    As you approach the speed of light time slows down. I assumed that if you were to *ever* achieve the speed of light then time would therefore stop. Now to me, time = movement. If there is no time there is no movement. So I assumed that to the two particles that are traveling at the speed of light, although from our perspective are light years apart, to them, since they are moving at the speed of light, are right where they started out, that is, close enough to each other so that the transmission of data doesn't violate SR as the distance between the two particles, in their view, is minimal.

    I'm sure I've just confused the ever loving heck out of everyone. But that's how I viewed quantum entanglement.
  15. Mar 16, 2009 #14


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    It's instantaneous, but there's no transmission of data.

    Massive particles can't reach the speed of light, but they can get arbitrarily close.

    The entangled particles aren't moving at all. I mean, they can be, but that would only make the experiment unnecessarily complicated. The typical scenario that's considered in discussions about entanglement involves two particles with the same velocity.
  16. Mar 16, 2009 #15
    Information has no intrinsic mass, but if it were possible for information (eg a series of 1s and 0s) to be massless it would still be limited by special relativity and the Lorentz.

    Why? Because otherwise it could violate cause and effect, and that is the determing factor - (not energy directly).

    Quantum States pass as information very well and have no mass per se - but they could not be 'transmitted' FTL.
  17. Mar 16, 2009 #16


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    I don't think that a reinterpretation of QM can make it local:
    Essentially, since no interpretation can remove nonlocal elements of the theory (such as wave functions in the configuration space or something equivalent), the theory should be considered nonlocal, irrespective of its interpretation.
  18. Mar 16, 2009 #17


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    The answer to the question posed in the title of this thread is: Not necessarily.
    Namely, even if information is transmitted locally, it is still possible that special relativity is obeyed. Special relativity, by itself, does NOT say that information cannot exceed the velocity of light. All it says is that the laws of physics should not depend on the Lorentz frame of coordinates.
    Here is one possibility how can QM be made explicitly nonlocal (with superluminal transmition of information) without violating special relativity:
    http://xxx.lanl.gov/abs/0811.1905 [accepted for publication in Int. J. Quantum Inf.]
  19. Mar 16, 2009 #18
    My question is somehow off-topic but I am really curious if the particle trajectories retain their objectivity in the relativistic form of BM you describe in the paper. Are they uniquely defined for any observer? I remember reading one paper in which it was claimed that this is not the case.
  20. Mar 16, 2009 #19
    Its a fudge that gives the spurious pilot wave credibility. I prefer the little fairy that travels backwards in time meets complex i, makes time an operator which it isn't really and kindly makes everything OK after squaring and adding.
    Last edited: Mar 16, 2009
  21. Mar 16, 2009 #20


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    ...Which has the advantage (if that is important to you) that all apparently non-local effects are explained with complete respect of relativity (making +/- c the "universal" speed limit).
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