Quantum Entanglement, ideas?

Discussion in 'Physics & Math' started by queeg, Jun 3, 2013.

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  1. queeg Registered Member

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    ok all, we know that entanglement of particles happens. a phenomenon in which two entities are inexorably linked no matter how far away from each other they may be. so far there is no explanation as to why or how this phenomenon occurs. my question is, has anyone got any theories as to how entanglement works as it does, and how it seems to skip the distance problem in space. i dont care how whacky your theory sounds, as nothing is as strange as entanglement anyway.

    thanks folks
     
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  3. eram Sciengineer Valued Senior Member

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    The entangled particles are connected via a "portal". Does that sound wacky enough?

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  5. Markus Hanke Registered Senior Member

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    You don't need any fance new theories, it is a simple matter of correlations within quantum systems.
    Suppose you have two completely separate particles - they are then brought together and interact in a certain manner to become entangled ( never mind the exact mechanisms for now ). Let us take for example two electrons. If you bring them together in the same system, they become subject to the Pauli exclusion principle, meaning that the two electrons cannot be the exact same in all quantum numbers. Let's pick spin for example - bringing them together, and demanding all other quantum numbers to be the same, would mean that the electrons must have opposite spin; it is important to understand that at this stage we don't know yet which electron has which spin ( since we haven't made any measurements ), but we do know that the spins must be opposite. Creating entanglement now means that a correlation is established between the two particles, in that their spins are opposite. That correlation persists, even if we separate the particles very far apart - that is the meaning of entanglement. Due to that very correlation, if we now measure the spin of one of them, we automatically know what the other particle's spin must be, regardless of how far away it is. That is simply because the quantum states of the two particles are correlated back to the point of their entanglement, so no exchange of information between the two is necessary when we perform the observation. The correlation happens at the point of entanglement, not at the point of measurement.

    I think entanglement is very straightforward, and not at all mysterious or strange, and in particular it does not require any wacky theories or breaking of physical laws to be perfectly well explained; but maybe that is just me

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  7. eram Sciengineer Valued Senior Member

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    moved below
     
    Last edited: Jun 3, 2013
  8. Cheezle Hab SoSlI' Quch! Registered Senior Member

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    But you didn't tell the whole story. You can't ask a particle to tell you the orientation of its spin. You can only set up a detection at some possibly random direction and ask a yes or no question, "is your spin in this direction." And if the direction you choose is not the exact same or exact opposite direction of the actual spin, the particle can still only answer with a yes or no with some probabilistic distribution. If you ask it if it is at an orientation that is 90 degrees off of its actual spin it will give a 50/50 answer yes or no. The strange part is that if you ask the two correlated particles the exact same question they always have opposite answers. If you ask it if it is at an angle that is only slightly off of its actual angle, it will answer yes mostly and no very rarely. But if one gives that rare answer no, then the other has to say and equally improbable yes. The only way to actually determine the orientation of the spin is to ask a large number of identically prepared particles yes and no answers until you can figure it out. This is the seemingly mysterious problem not that the particles are anti-correlated, which is a given.
     
  9. przyk squishy Valued Senior Member

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    Is there something wrong with the account given by the theory that predicted entanglement decades before it was detected experimentally?
     
  10. przyk squishy Valued Senior Member

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    If you're suggesting that entanglement is just correlation caused by past interaction, then obviously the problem is more subtle than that otherwise we wouldn't have had the EPR argument (arguing that textbook quantum physics is nonlocal) or Bell's theorem (answering that it is actually the statistical predictions made by quantum physics that are characteristically nonlocal, and not merely quantum physics as described in textbooks).

    Bell's theorem goes something like this: suppose in an experiment you observe some correlations, which you summarise in the form of a table of probabilities \(P(xy \,\mid\, ab)\). The idea is that x is some result you might see in some location A, and a is some information in the region A that might be affecting the outcome x. Similarly, y is some result you might see in a distant location B that might depend on some information b. You're confident that a shouldn't be exerting a causal influence on y and that b similarly shouldn't be affecting x. For instance, in a Bell experiment, a and b might be choices about which axis you measure the spins of two particles along, and x and y would represent which way they go in Stern-Gerlach magnets. If the measurements are made nearly simultaneously and you don't believe in superluminal communication, then you shouldn't expect that the choice of which measurement you're making on one spin should affect the outcome on the distant spin.

    If you try to explain the correlations you see in terms of shared history or a past interaction, then in mathematical terms this means that there should be some information -- call it \(\lambda\) -- in the common past that, if you knew it, it would let you factorise the probability distribution. So conditioned on \(\lambda\) you find \(P(xy \,\mid\, ab;\, \lambda) \,=\, P_{\mathrm{A}}(x \,\mid\, a;\, \lambda) \, P_{\mathrm{B}}(y \,\mid\, b;\, \lambda)\). The correlations you see should then just be an average over all the possible \(\lambda\)s:

    \(P(xy \,\mid\, ab) \,=\, \int \mathrm{d}\lambda \, \rho(\lambda) \, P_{\mathrm{A}}(x \,\mid\, a;\, \lambda) \, P_{\mathrm{B}}(y \,\mid\, b;\, \lambda) \,.\)​

    The point of Bell's theorem is that not all probability distributions can be written this way, including some of the ones predicted by quantum physics involving joint measurements on entangled states. Entangled spin states with certain spin measurements performed on them provide a simple and well-known example, for instance. The GHZ paradox is a more striking example.
     
    Last edited: Jun 3, 2013
  11. Markus Hanke Registered Senior Member

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    I should probably have mentioned that I am a proponent of RQM, the relational interpretation of quantum mechanics. In that interpretation entanglement neither violates locality, nor does it necessitate information exchange between the particles. RQM interprets quantum states not as absolute and intrinsic to the system, but as relations between the system and its oberserver. Different observers thus may see different aspects of a system ( different states, or superpositions ), much like in relativity not all observers necessarily agree on the physical attributes of a given frame. In fact, the whole of RQM is a distinctly "relativistic" flavour.

    If you take this interpretation to its logical conclusion, then the EPR paradox is really no paradox at all, because the states of the two particles are definable only through their relations to an observer who performs a measurement on them. The only physically meaningful way for those observers to compare their finding is by going through "traditional" channels of communication. Thus, locality if preserved for the particle systems, without requiring superluminal communications. The entanglement is then reduced to a backwards correlation.

    I do concede, however, that whether or not Rovelli's RQM can be considered mainstream is a matter of debate. To me, it seems to work pretty well though, and RQM should ( to the best of my knowledge ) be indistinguishable from standard QM, while avoiding many of its conceptual difficulties such as the EPR paradox.
     
  12. eram Sciengineer Valued Senior Member

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    Brian Greene talks about this, "two gloves" vs "spooky action". (jump to 27:40 or 30:33)

    [video=youtube;EGhQmNZhlqw]https://www.youtube.com/watch?v=EGhQmNZhlqw[/video]


    Your idea sounds similar to Einstein's "two gloves" concept. But by 37:00, they state that Aspect and Clauser's experiments have proven that there really is "spooky action at a distance."
     
  13. Fednis48 Registered Senior Member

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    That's fascinating! I'll definitely have to look into this RQM you speak of; entanglement always bugged me a little.
     
  14. Bryan.sanctuary Registered Member

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    Entanglement is a property of quantum mechanics, but not of Nature

    You are one of many who realize that entanglement makes no sense for separated singlets etc. The hopeless "explanation" is "quantum weirdness".

    I cannot accept non-locality and so worked on this for a long time. First you can show that entanglement is separable into a sum of products if you make some small changes to the definition of spin I am not allowed to put links here, so look up Structure of a spin ½ by Sanctuary on the ArXiv

    Second I am waiting for the referee comments for a paper I just submitted: (look up quantummechanics.mchmultimedia by Bryan Sanctuary

    Basically, as many have said, including Isaac Newton, non-locality is an absurd notion and cannot be explained. Persistent entanglement (quantum channels) also do not exist. What I do is assume a spin has two axes of quantization rather than one. Then all of quantum weirdness vanishes.

    Hope this helps.
     
  15. eram Sciengineer Valued Senior Member

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    Why do you consider non-locality absurd? Personal preference?

    In the video above, John Clauser shares your views. But he did the experiments and it proved him otherwise.
     
  16. Fednis48 Registered Senior Member

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    There is no way you could get around quantum entanglement by something as simple as redefining spin. You can make a Bell-like inequality with any pair of non-commuting operators, so by redefining spin, you're just getting around the results of a particular experiment without addressing the fundamental issue.
     
  17. Layman Totally Internally Reflected Valued Senior Member

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    I had an idea that will knock your socks off. It is unlike any scientific theory ever developed that describes quantum physics. Okay, you ready for this? Here it goes.

    Space-time dilation/contraction is REAL! When I mean real, I mean that as an object travels at a relative speed to another object close to the speed of light, the difference's in their length and time are actually smaller! Are you following all this? This can be too much to handle for some people.

    Okay, now assuming that space-time dilation is real, say an object travels close to the speed of light relative to another object at a constant speed. Then each object can assume it is at rest and that the other object is contracted. They both look to be shorter in the direction of motion relative to the other object.

    Then say one of those objects is an electron in free space or a near perfect vacuum. ( Speeds of electrons in mediums can tend to get a bit tricky) Then say this electron is traveling at a constant speed relative to an observer at close to the speed of light. Well, it just so happens that any object can assume that it is rest if it is traveling at a constant speed. So then lets say that the electron "thinks" that it is at rest even though it is traveling at a speed close to the speed of light relative to another observer in another frame of reference.

    Then ask yourself what would the electron "see" if it assumed that it was at rest? It would "think" that the observer in the other frame of reference was traveling at a constant speed close to the speed of light relative to it! The other observers entire frame of reference would look to be traveling the speed of light relative to the electron! So then the electron would "notice" that the entire frame of reference of the other observer was contracted!

    Now that the other frame of reference is traveling at a speed close to the speed of light relative to the electron then that entire frame of reference would be contracted to almost zero from the electrons frame! The frame of reference of the observer would then "look" to only be a plane to the electron. The electron would "observe" itself to be almost at every location in its direction of motion at the same time!

    The difference we see in the movement of the electron would, from the electrons frame of reference, not even be a direction. It would be like telling a flat lander that he is actually moving up off the plane but he doesn't notice it because from his frame of reference it is contracted to almost zero. The flat lander (electron) cannot decipher events that happen further up differently than events that are happening on the plane. They are all happening together at the same location simultaneously!

    Then when the electron in flat land is changed in spin in one location it then happens at all locations further "up" simultaneously! To the electron they are simply the same location. There is no distance that it would instantly act at...
     
  18. przyk squishy Valued Senior Member

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    Fair enough, though I think the loopholes in Bell's theorem are quite well understood. Basically the only mathematical way you can violate a Bell inequality with a Bell-local model is if you allow the probability distribution over the past information (the \(\rho(\lambda)\) in post #7) be correlated with the settings a and b. This could be the case in so-called "superdeterministic" models or if you allow some form of retrocausality.

    Another possible way around Bell's theorem is to try to argue that the concept of Bell locality simply doesn't apply to the model you're considering. I think it might be possible to argue this in the case of the Everett interpretation of quantum physics for instance. There, quantum states are viewed as the only things that really exist, so the end result in a Bell-type experiment would just be some joint state \(\rho_{\mathrm{AB}}\), with the state also including the two observers and their measurement apparatus. In Bell speak, \(\rho_{\mathrm{AB}}\) would be a nonlocal "beable" -- nonlocal because in general you can't completely specify a quantum state as a quantum state in region A and another one in region B (i.e. \(\rho_{\mathrm{AB}} \,\neq\, \rho_{\mathrm{A}} \otimes \rho_{\mathrm{B}}\)). By contrast, the as and bs and xs and ys in the definition of Bell locality are supposed to be local beables, so there's an argument to be made that the definition simply can't be applied. But then you're really just trading one type of nonlocality for another.

    That said, for someone not already very comfortable with quantum physics, I think any of these approaches would look "wierd".
     
    Last edited: Jun 4, 2013
  19. przyk squishy Valued Senior Member

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    Yeah, the definition of Bell locality (which I gave in [POST=3075945]post #7[/POST]) refers only to notions from probability theory and deliberately doesn't assume anything about the nature of spin or how it should be modelled. Consequently, it's not something you can get around just by imagining spin should be modelled a bit differently than it already is in quantum physics.
     
  20. Grumpy Curmudgeon of Lucidity Valued Senior Member

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    Physicists tell us that there are more than the three dimensions of space and one(half, I think)of time. These extra dimensions are said to be "rolled up" in the quantum realm, not expressed in our Universe but still there. These dimensions can be found everywhere in our Universe, all of those points(of the Universe)are the same point in those dimensions(or at least one of them, the one entangled particles are communicating through)at distances less than the Planck length. It seems as if entanglement violates Einstein, but not if these Quantum dimensions are considered, as lightspeed is not violated through the rolled up dimension. We know that both Einsteinian physics and quantum physics are amazingly accurate descriptions of the behavior of matter at their respective sizes. What we don't know is how to reconcile the two. Entanglement may be a sign of how they interact.

    As to a half dimension of time, we can move both directions in all the other dimensions but only one way(forward)in time. I think time is in two halves, the forward half in our Universe, the other half rolled up but accessible inside Black Holes. Scientists tell us that it is possible that White Holes also exist, connected to Black Holes but spewing out mass and energy. But we only know of one(the Big Bang was a White Hole). I think the other half of the time dimension, like the other rolled up dimensions, has no size in our Universe and short circuits back to the BB. So everything that has ever, or will ever fall into a BH is spewing out of the BB as energy and time/entropy are in a completed loop. Time gets funny in BHs, some think time stops inside the Event Horizon, that everything that has ever fallen into a BH is still falling(from the perspective of the Universe, from it's own perspective the fall is near instantaneous)and will never reach the Singularity(scientese for WTF). Whatever, entanglement shows there is much more to learn about our Universe and if it violates our rules it is our rules that are in error.

    Grumpy

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  21. eram Sciengineer Valued Senior Member

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    So string theory is the answer to entanglement?
     
  22. ash64449 Registered Senior Member

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    you somewhere replied that it is theory of everything.sure it must have answers
     
  23. Layman Totally Internally Reflected Valued Senior Member

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    It could have answers but odds are that it will never provide them. It would just take too long to get the answers from it. I think the answer lies in correctly applying SR to quantum mechanics. A restart from the ground up of modern physics. In other words taking back Einsteins ramblings about how God doesn't play dice with the universe and instead apply SR to particles so that it shows why God play's dice with the universe.
     
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