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Why the need for entanglement

  1. Aug 20, 2014 #1
    If I had a factory that produces pairs of gloves. And I packed one box with the left glove and another with the right.
    Then I sent the first box to the north pole and the second to the south pole.
    Now I have no idea which box contains which glove, When sending the identical boxes to their respective locations.

    So now if I open the box in the north pole , and find a left hand glove.
    Then OBVIOUSLY I know what glove is in the box on the South pole, at that instant.
    And behold when I open the box at the south pole it is ALWAYS a right hand glove.

    Why the need to send a signal faster than anything to the other box?
    Why the need for such property , we call entanglement?

    What evidence / experiment caused the scientific world to formulate this spooky action at
    a distance, to explain this logical deduction when measuring/observing a closed system of events?
     
  2. jcsd
  3. Aug 20, 2014 #2
    "And behold when I open the box at the south pole it is ALWAYS a right hand glove."
    apologies should read:
    "And behold when I open the corresponding box at the south pole , it is ALWAYS the othe half of the glove found at the north pole"
     
  4. Aug 20, 2014 #3

    jtbell

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    Read up on Bell's Theorem and related experiments.

    For example, on http://www.drchinese.com/Bells_Theorem.htm , an excellent "Overview with Lotsa Links" which is maintained by our frequent poster DrChinese.
     
  5. Aug 20, 2014 #4
    To put it in a simplified way:

    One cruicial thing you're missing is that in the quantum world there are different measurement bases, and only certain specific bases gives you the perfect anti-correlation that you describe. Other bases can give no correlation at all, which in your case would be equivalent to having a 50% chance of finding either two right-hand or two left-handed gloves. But classically, this can never happen, as you always find one of each, there is never any chance of anything else.

    If they were entangled, then only when you look at your gloves from a certain point of view (a certain basis) would you find correlation, while for another view they can act as though they are completely independent, even though they are the same physically prepared system. This property simply has no cloassical interpretation.
     
  6. Aug 20, 2014 #5
    Thanks ,very comprehensive and informative link.
    The quote below sums it up nicely for me ..

    "It is worth emphasizing that non-separability,
    which is at the roots of quantum teleportation15,
    does not imply the possibility of
    practical faster-than-light communication.
    An observer sitting behind a polarizer only
    sees an apparently random series of 1 and
    & results, and single measurements on his
    side cannot make him aware that the distant
    operator has suddenly changed the orientation
    of his polarizer. Should we then conclude
    that there is nothing remarkable in this
    experiment? To convince the reader of the
    contrary, I suggest we take the point of view
    of an external observer, who collects the data
    from the two distant stations at the end of the
    experiment, and compares the two series of
    results. This is what the Innsbruck team has
    done. Looking at the data a posteriori, they
    found that the correlation immediately
    changed as soon as one of the polarizers was
    switched, without any delay allowing for
    signal propagation: this reflects quantum
    non-separability"




    However I am still not convinced that entanglement is a prerequisite of what we measure experimentally.
    Looking at the combined results "Posteriori" gives us sense of Locality making the photons
    "unneccessarily" to have communicated instantaneously

    OR

    "that they are considered a single non-separable object — it is impossible to assign
    local physical reality to each photon.''

    This is more in line with my view, they are 2 halves of a single entity, they can only behave in a certain way , no matter what you do.

    The left glove will never fit the right hand
     
  7. Aug 20, 2014 #6
    I think the problem is the quantum-mechanical description of the system : before the measurement in A the system is described by a non-separable function, the state of B is not determined in any direction. But directly after the measurement in A the system in B becomes well defined in that direction. In classical world there is no such description before you open the box the property is just hidden and revealed so there is no need for this spooky interaction.
     
  8. Aug 20, 2014 #7

    stevendaryl

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    What this analogy misses is that in an actual experiment involving an entangled pair of particles, the experimenters can choose different types of measurements. Your analogy only has one type of measurement: Determine whether the glove is left-handed or right-handed.

    Let's make your analogy more complicated by adding the element of choice. Suppose that there are a pair of couriers: One delivers three boxes to the north pole, labelled red, green and blue. The other delivers three boxes to the south pole, similarly labelled. The experimenter at the north pole, call her "Alice", picks a box and opens it. The experimenter at the south pole, call him "Bob", picks a box and opens it. The couriers only allow them to open one box a piece.

    The rules are:
    1. If Alice and Bob pick the same color glove, they always get the opposite handedness: one gets a left-handed glove, the other gets a right-handed glove.
    2. If Alice and Bob pick different colors, they always get the same handedness: either both left-handed, or both right-handed.

    If you think about this scenario, I think you will agree that there is no way to accomplish it without either guessing ahead of time which color Alice and Bob will pick, or by somehow teleporting gloves. You can't just start with three pairs of gloves, and for each color, either send the left one to Alice and the right one to Bob, or vice-verse.

    Using quantum mechanics, you can't precisely mimic this new scenario, but you can come close:
    1. If Alice and Bob pick the same color glove, they always get the opposite handedness: one gets a left-handed glove, the other gets a right-handed glove.
    2. If Alice and Bob pick different colors, they usually (75% of the time) get the same handedness: either both left-handed, or both right-handed.
     
  9. Aug 20, 2014 #8
    Your analogy does not include the fact that the gloves need to be a in superposition of fitting the left and right hand. That might be worth noting.
     
  10. Aug 20, 2014 #9
    Hi Stevendaryl

    The rules are:
    1.
    If Alice and Bob pick the same color glove, they always get the opposite handedness: one gets a left-handed glove, the other gets a right-handed glove.
    2.
    If Alice and Bob pick different colors, they always get the same handedness: either both left-handed, or both right-handed.


    I agree with rule 1 but not with rule 2

    A possible scenario is :
    2 left handed gloves ( say Red and Blue) in the North pole, which corresponds to 2 right handed gloves ( Red , Blue) in the South pole.
    So Alice picks the Red glove in the North pole ( left handed) and Bob picks the Blue glove
    in the South pole (Right Handed).

    Even with this element of choice , why should entanglement be a pre-requisite?






    which is
     
  11. Aug 20, 2014 #10
    "Your analogy does not include the fact that the gloves need to be a in superposition of fitting the left and right hand. That might be worth noting"

    No sure what is meant by "Superposition" in the context of this sentence, could you elaborate?
     
  12. Aug 20, 2014 #11

    stevendaryl

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    What do you mean, you don't agree? I'm just giving you an example of a distant correlation that cannot be explained by simple classical means. QM has similar distant correlations (not exactly as extreme as that one).
     
  13. Aug 20, 2014 #12

    stevendaryl

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    I don't think that it's fair to require that. Superpositions are part of the QM model, but they aren't directly observed. The question is: what observations force us to consider superpositions?
     
  14. Aug 20, 2014 #13

    stevendaryl

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    I don't see the point of your scenario, since it doesn't relate to the QM situation. As I said, here is a glove scenario that is almost exactly analogous to the QM case:

    1. Alice and Bob are each presented with three possible boxes marked Red, Green, or Blue.
    2. If they choose the same color, then they always find gloves with opposite handedness.
    3. If they choose different colors, they find gloves with the same handedness 75% of the time, and different handedness 25% of the time.

    There is no way to create such a situation using three pairs of gloves, unless you know ahead of time what colors Alice and Bob will choose (or if you can magically teleport gloves around). But you can create an analogous situation using entangled pairs:

    • Instead of choosing a color, Alice and Bob choose one of three directions for measuring spin: 0 degrees, 120 degrees or 240 degrees (in the x-y plane, with 0 degrees being the y-axis).
    • Instead of left-handed and right-handed gloves, they get spin-up or spin-down particles.
     
  15. Aug 20, 2014 #14

    bhobba

    Staff: Mentor

    This is a variant of the famous Bertlmann's Socks the great physicist John Bell talked about and it indeed sheds considerable light on quantum entanglement:
    http://cds.cern.ch/record/142461/files/198009299.pdf

    It's such a pity that a man of such rare insight, and a virtual shoo-in for a Nobel prize, died young.

    Whether such violates locality, and is spooky action at a distance, depends a lot on your definition of locality.

    I hold to the cluster decomposition property:
    https://www.physicsforums.com/showthread.php?t=547574

    According to that locality basically only applies to uncorrelated systems - correlated systems may still be non-local. Entangled systems are correlated - so its OK to view them as non-local if you wish - I personally do.

    But it purely depends on how you view it. The Consistent History guys view it differently:
    http://quantum.phys.cmu.edu/CQT/index.html

    See Chapter 24 on the EPR:
    http://quantum.phys.cmu.edu/CQT/chaps/cqt24.pdf

    Thanks
    Bill
     
    Last edited: Aug 20, 2014
  16. Aug 20, 2014 #15
    previously you stated :

    If Alice and Bob pick different colors, they always get the same handedness: either both left-handed, or both right-handed.

    later you stated that:

    If Alice and Bob pick different colors, they usually (75% of the time) get the same handedness: either both left-handed, or both right-handed

    I agree with the latter but disagreed with the former, to answer your question on what I disagreed on.

    However the % ratio from my calculation is 66% of the time get the same handedness not 75 %.
    How do you get to 75% ?

    Is this not a classical correlation to entanglement?
     
  17. Aug 20, 2014 #16

    stevendaryl

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    What does it mean to disagree? I was giving you a scenario that I made up. How can you disagree with something I made up? If I say: "Suppose I have two apples..." how can you disagree and say that no, it's three apples?

    The 75% comes from quantum mechanics. That's the issue about quantum entanglement: it can produce correlations that simply cannot be reproduced using classical means.

    Specifically, if in the spin-1/2 EPR experiment, Alice measures the spin of one particle along one axis, and Bob measures the spin of the other particle along another axis, then the probability that they will get the same result (spin-up or spin-down) is [itex]sin^2(\frac{\theta}{2})[/itex] where [itex]\theta[/itex] is the angle between their two axes. If [itex]\theta=120^o[/itex], then you get a probability of 0.75.
     
    Last edited: Aug 20, 2014
  18. Aug 20, 2014 #17

    stevendaryl

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    The notion of "nonlocality" that is relevant for entanglement is that it's possible to have information about a composite system that cannot be factored into information about the two component systems.
     
  19. Aug 20, 2014 #18
    Experiments are far less spectacular than Alice and Bob adventures. I don't see why make up stories when we can describe actual experiments. In the experiment there is a photon A and polarizer A on one side, and on the other side there is a photon B and polarizer B. Photon A will try to pass through polarizer A, and photon B will try to pass through polarizer B. If both manage to pass or if both fail we record '1', it's a match, and if one goes through but not the other we record '0', it's a mismatch. This is repeated with 10,000 more photons, the number of matches and mismatches are compared and then somehow interpreted to imply all kinds of crazy stuff.

    I'm not impressed. The result is so very indirect and only vaguely related to what is being inferred from it. There is Malus's law in classical physics which can calculate probability for a photon to pass through a polarizer. Can it be demonstrated the outcome of the experiment in not predetermined by the angles set on the polarizers and Malus's law before the experiment even begins?
     
  20. Aug 20, 2014 #19

    stevendaryl

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    The original poster gave a classical analogy of EPR, and I was just pointing out that the actual EPR was more complicated, because the two experimenters have to make choices as to what to measure. If the choices are fixed ahead of time, there is no problem.

    YES! That's the whole point of Bell's proof. Classical probabilities provably cannot explain the results of EPR (without either assuming action-at-a-distance, or assuming that the settings of the polarizers are known ahead of time; the polarizer settings can be changed in the middle of the experiment).
     
  21. Aug 20, 2014 #20

    Nugatory

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    h
    We create a pair of entangled particles, and then randomly choose to measure their spin on one of three axes: 0, 120, and 240 degrees. The choice of axis is analogous to the choosing the color of the box. The measurement result will be either spin-up on that axis or spin-down on that axis, and this is analogous to finding a left-handed or a right-handed glove in the box that you open.

    The quantum-mechanical prediction is that the correlation will depend on the square of the cosine of the angle between the two measurements, which for these separations works out to 75% the same result, 25% opposite results.

    But as you have just calculated, there is no way of getting beyond 66% if the handedness of the gloves is determined when they go into their boxes at the source, analogous to the spin of the particles being set when the entangled pair is created.

    The experiments have been done, and the quantum mechanical prediction has been confirmed.
     
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