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Quantum entanglement

  1. Jun 21, 2007 #1
    Ill start this question with an example of entanglement (to make sure I have it right).
    Take two electrons in the ground state of an atom. They are both in the superposition state that is 50% spin up and 50% spin down. You then split these electrons apart, without measuring their state. Now you measure one of them to be spin up. The other one instantly goes from being in a 50/50 superposition to being spin down.

    Now my question is, doesnt this suggest a flaw in quantum mechanics? While it is true that you cant use this phenomenon to transmit information, information IS being transmitted. The far away electron instantly changes it's state in response to your distant measurement. A classical view of this situation explains entanglement in a much better way.

    Assume instead of the electrons being in a superposition (superpositions dont exist classically), they're true spin is just unknown to us due to either some invisible undiscovered particle or just because we havnt measured them. To clarify, they are either up and down or down and up. There is no true randomness here, only apparent randomness. Since they are fermions, we know they have opposite spin however. After seperating them and measuring one of them to be up spin, we instantly know the other is down spin. This thought experiment has the exact same results as the quantum mechanics one. However, this one does not involve the distant electron changing due to your measurements.

    Correct me if I have anything wrong, but if I don't I dont understand how quantum mechanics can be a complete theory. Information IS traveling faster than light because both particle's states change at the exact same time. Just because we cant exploit this fact doesn't mean that it doesn't violate special relativity...
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  3. Jun 21, 2007 #2


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    Information was not travelling faster than the speed of light. The two-electron system IS your entire quantum system and not two separate systems. by measuring one of the e-, you work out what is the state of the entire system is in.
    "change" is not the right word .... they were in a superposition of two states, by measuring one e- all you are doing is discovering which state it is in. Not "changing" the e- afar. it is not like you measure it and find out it is spin down then you try to change it into spin up and then instantly the e- on the other end changed to the opposite spin.
  4. Jun 21, 2007 #3
    Actually, that is not correct. What you suggest is a hidden variables explanation and,a ccording to Bell's theorem, they must obbey some inequalities which are violated in the quantum case. Although the question is not settled, there are experimental indications (not without controversy) that they are violated.

  5. Jun 21, 2007 #4


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    But see, this is why QM is different, and to understand the "weirdness" of entanglement, one has to also understand superposition and why the Schrodinger Cat-type states here also applies.

    As you have stated, we could easily do the analogous classical experiment, where an object that initially had no angular momentum, suddenly fractures into two and the two resulting objects fly apart. Measuring the angular momentum of one immediately determines the angular momentum of the other, no matter how far that object is. So how is this different than what QM has predicted?

    The difference here lies in what John Bell has defined to be "realism", that the physical quantities of the system all actually do exist with definite values. It is just that we don't know what they are before they are measured. So in the classical case above, each of those two particles have time evolving angular momentum values that always preserve the angular momentum conservation even before we measure them. These values exist and are waiting to be measured.

    This is not true for QM. In a superposition, all the possible states are present. Each of the entangled object are in a superposition of all the possible states. The object does not have a specific value of its angular momentum at any particular instant. So one specific value of the angular momentum doesn't exist until the moment of actual measurement. This is the so-called violation of "realism", or what Leggett termed as "local realism". But in fact, from the recent experimental results, even imposing "non-local realism" https://www.physicsforums.com/showpost.php?p=1307660&postcount=41". These specific measurable quntuties are truly undetermined before they are measured even if you allow for non-local interactions. QM truly is incompatible with realism, local or non-local.

    Last edited by a moderator: Apr 22, 2017
  6. Jun 29, 2007 #5
    I've been having trouble with this same problem. This is what I'm getting at is that the two electrons are technically the same. So the probablities of finding each electrons' spin (whether up or down) must add up to 100%? That seems too easy....
  7. Jun 30, 2007 #6
    Wavefunction collapse is, as you point out, instantaneous. It can be instantaneous because it's not something that is, as far as can ever be known, happening in reality. It happens in the imaginary space of quantum evolutions.

    We can't exploit this action-at-a-distance in the real world of our experience -- because as far as anybody knows (or ever will know, if quantum theory is correct) it is only happening in the N dimensional space where quantum evolutions take place.

    Thus, there's no conflict with the principles or definitions or conventions, or conclusions, of special relativity (which theory is essentially about getting us all on the same page wrt a kinematical accounting of how things behave in the 3D space of our experience).
  8. Jul 1, 2007 #7
    Sorry, I'm not exactly the next Newton so if you'll please bear with my newbie questions...

    First of all, I understand that all this is a research front topic, therefore, somewhat difficult to answer with certainty. With that being said, I want to ask just how certain are physicists such as yourself with making that statement in bold. So, if QM is incompatible with even non-local realism, then what type of worldview *is* QM compatable with (presumably with something else that is non-local)?


    p.s. If you give me an answer even half as good as some of the other questions you've answered, I'll be very happy!
    Last edited by a moderator: Apr 22, 2017
  9. Jul 2, 2007 #8


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    Only certain classes of nonlocal theories which satisfy the assumption of "outcome independence" (meaning that the outcome of one experimenter's measurement is independent of the outcome of the other experimenter's measurement) have been shown not to work--there are still viable nonlocal realistic theories which do not assume outcome independence and thus are not ruled out by these results, including Bohmian mechanics. See here for some more info.
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  10. Jul 4, 2007 #9


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    For anyone interested in reading the excellent paper referenced:

    An experimental test of non-local realism

    by Simon Groeblacher, Tomasz Paterek, Rainer Kaltenbaek, Caslav Brukner, Marek Zukowski, Markus Aspelmeyer, Anton Zeilinger

    Abstract: Most working scientists hold fast to the concept of 'realism' - a viewpoint according to which an external reality exists independent of observation. But quantum physics has shattered some of our cornerstone beliefs. According to Bell's theorem, any theory that is based on the joint assumption of realism and locality (meaning that local events cannot be affected by actions in space-like separated regions) is at variance with certain quantum predictions. Experiments with entangled pairs of particles have amply confirmed these quantum predictions, thus rendering local realistic theories untenable. Maintaining realism as a fundamental concept would therefore necessitate the introduction of 'spooky' actions that defy locality. Here we show by both theory and experiment that a broad and rather reasonable class of such non-local realistic theories is incompatible with experimentally observable quantum correlations. In the experiment, we measure previously untested correlations between two entangled photons, and show that these correlations violate an inequality proposed by Leggett for non-local realistic theories. Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned.
    Last edited by a moderator: Apr 22, 2017
  11. Jul 4, 2007 #10
    Single atom entangles two photons

    Here is a interesting article from a couple of weeks ago where this team made a single atom entangle two photons.
    http://groups.google.com/group/sci.physics/browse_thread/thread/fc0500c1ef604b34" [Broken]
    Is this compatible with Bohm's interpretation? I read on another forum that two particle entanglement would prove many-worlds? Does this experiment have anything at all to say about QM interpretations?

    Last edited by a moderator: May 3, 2017
  12. Jul 5, 2007 #11
    There's some pretty heavy philsoophical questions flying around here and AAMOI people might want to know where the problem lies. And what Bells is start here.

    http://plato.stanford.edu/entries/qt-measurement/" [Broken]

    http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BellsTheorem/BellsTheorem.html" [Broken]

    at the moment AFAIK there isn't one or at least one that is widely accepted as the correct intepretation. We simply do not know what is in reality happening. Only mathematically do we have a model and in the Copenhagen interpretation at least it is deemed to be non real or non pictorial. The wave function is not real.

    All we can say is what has already been said, that according to EPR which lead to Bells theorem x or we guess basically that what we assume is true and that information transfer is not faster than light.

    You can get no absolutely positive evidential based assurances the theory is correct(well unless someones willing to stick their neck on the line and assume) It is an incomplete theory and open to intepretation.

    I completely missed this post, I'm going to stick my neck out and say no, absolutely not :biggrin:

    AFAIK Many worlds is and probably always will be a hypothesis based on no empirical evidence.

    Yes discussions about this Issue are What lead Einstein to create the EPR paradox along with Podolsky and Rosen to attempt to debunk QM, so it is very important to the current front runner interpretation Copenhagen.
    Last edited by a moderator: May 3, 2017
  13. Jul 15, 2007 #12
    There's no way of disproving nonlocal realism. The fact is entanglement correlations can be measured between light beams to distances up to 141 kilometres between light beams. This paper's crap.
    Last edited by a moderator: Apr 22, 2017
  14. Jul 15, 2007 #13


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    Welcome to PhysicsForums!

    The question answered in the paper has nothing to do with entanglement over long distances. We already know that entanglement can occur over such distances, and that Bell Inequalities are likewise violated in these cases - disproving local realism.

    The paper is using experiment to disprove non-local realism. If you accept the results - which may or may not rule out all forms of non-local realism - then you would be forced to the conclusion that entanglement is NOT evidence of non-local interaction. Rather, the results would be evidence of an observer dependent reality in which particle attributes do not take on discrete well-defined values until observation/measurement.

    The paper is far from crap, and the science of the experiement itself is sound. The open question is more like: how far can you apply the conclusion?
  15. Jul 15, 2007 #14
    "which may or may not rule out all forms of non-local realism" is the key point here which is not at all the same as saying that the experimental evidence rules out non-local realism which is what the paper is, in effect, saying and, given this, the paper is crap.

    Whereas the science of long distance entanglement experiments is indeed sound. So a more reasonable conclusion is that a form of non-local realism needs to be found that is sound and consistent with the experimental results.
    Last edited: Jul 15, 2007
  16. Jul 15, 2007 #15
    Local realism has not been disproven by Bell. One assumption he uses in the demonstration of his theorem is that detectors' settings are random and uncorrelated with the emission of the entangled particles. In a strict deterministic universe governed by local realism such an assumption is false.

    This post is not necessary directed to DrChinese who is fully aware of this problem (but he chooses to just ignore it) but towards the other participants on this thread who may think that QM contradicts local realism. One may choose to believe that but it has not been proven.
  17. Jul 15, 2007 #16


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    This post is directed to ueit, who continues to pass on opinion that is not generally accepted by the scientific community - and is hoping with a little luck to get the last word in on the matter.
  18. Jul 15, 2007 #17


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    1. The paper is an advancement of our knowledge. It is not yet clear if all non-local realistic theories are ruled out, or some (and therefore leaving the door open). As more and more realistic theories are excluded, we are gaining in knowledge. The paper is sound as is.

    2. It is not a reasonable conclusion that we need a non-local realistic theory when a local non-realistic theory is at least as viable. In such a theory, relativity is respected but there is observer dependence. Can you acknowledge this as a viable option?
  19. Jul 15, 2007 #18


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    Then may I suggest that you write a rebuttal letter to the journal and explain why it is "crap"? I suggest, though, that you first tackle the issue brought up by Leggett and his inequality in your rebuttal.

  20. Jul 16, 2007 #19
    What opinion are you speaking about?

    1. Bell's theorem depends on the assumption that detectors' settings are random and uncorrelated with the emission of the entangled particles.

    2. In a strict deterministic universe such an assumption is not necessary true (not necessary false either but this is enough for my point).

    I think you confuse the above statements which are accepted as true even by Bell in his papers with the toy hypothesis I proposed on this forum (which I freely admit is pure speculation and it was given only as a better way to visualize how the theorem fails if the key assumption is denied).

    If you claim that Bell's theorem proves local realism false you need to show that at least one of the above statements is false. Otherwise, the laws of logic force you to accept defeat.
  21. Jul 17, 2007 #20

    I'd like to add that Gerard ′t Hooft, a Nobel Prize laureate in quantum physics, is in agreement with me in regards to the so-called "proof" against local hidden variables theories and in fact he is working on such an "impossible" theory::biggrin:

    http://arxiv.org/PS_cache/hep-th/pdf/0104/0104219v1.pdf" [Broken]

    Maybe this will change your mind on the matter.
    Last edited by a moderator: May 3, 2017
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