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Quantum Entanglement

  1. Mar 21, 2005 #1
    I need a bit of clarity on quantum entanglement... Any of the 'Bell experiments' will do, but for the sake of discussion, I'll reference http://roxanne.roxanne.org/epr/experiment.html [Broken] .

    Now, the data is obviously in favor of the predictions of QM. Admittedly, I am currently unable to work through the equations myself (which could very well be the crux of my confusion)... Regardless, I'm having trouble reconciling with the idea that this experiment rejects locality.

    Correct me if I'm wrong, but in laymen's terms, you've got two photons with circular polarization travelling in opposite directions. - One is going clockwise, the other counter-clockwise.

    The 'classical' prediction would be that when we find one to be clockwise, the other will always be counter-clockwise (and vice versa). So, am I to understand that, in 'reality', both of the particles won't always disagree?

    I suspect that I'm over-simplifying things... In any event, I'm currently around page 114 in Brian Greene's "The Fabric of the Cosmos", and he seems to be talking in circles with his attempt to explain this... Suffice it to say, I welcome any and all feedback on this topic.

    p.s. I realize that asking for a 'simple' explanation in this particular case is probably tantamount to asking for a cup of ice from the top of Mount Everest... but it wouldn't be fun otherwise, right?! :wink:
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Mar 21, 2005 #2
    You will always get the proper correlations either classically or by QM.

    The difference is that if you assume a-priori that the system is already in a state
    before you measure it, the statistics come out different than if you assume
    that the "first" measurement locks in the results.

    The entagnlement means that no matter how far apart the system's
    parts are spatially (light-years say) the correlation is never violated even
    though to "communicate" the measurement event over such distances
    implies some kind of FTL or nonlocal action.

    There are a lot of subtleties here. I'll mention my favorite one, hopefully
    someone can clear it up.

    Becasue the two measurement events do not occur at the same place,
    simultanaity is IMPOSSIBLE to establish. Therefore the best we can
    hope for is correlation, and it is NOT true that one measurement "forces"
    the other because it can't be established which one came first!

    And if that's the case I can always construct a frame in which the two
    measurements are *always* simultaneous. Maybe this is some kind of
    quantum-preferred frame?

    Anyone know if this conjecture holds water?

    If anyone gets a Nobel for this idea, you (possibly) heard it here first. :tongue:
    Last edited: Mar 21, 2005
  4. Mar 21, 2005 #3
    From my admitedly ignorant perspective locality seems to be a concept more driven by our particular cartesian-like perspective on the particular place (and scale) we are in within our universe - a universe that happens to be in the strange configuration that happens to make it appear like that to us :)

    Whether you look at SR or GR or QM, or even Newton and Mach for that matter, the universe is far more connected at all "cartesian" points simultaneously than our day to day experience suggests. Maybe what we call "fields" are better described as spacetime influenced disturbances of this connectivity.

    Oh dear I'm rambling again...
  5. Mar 21, 2005 #4
    Not exactly, and not according to the Copenhagen Interpretation. You only get proper correlations IF you make a measurement of BOTH particles, and ONLY if you bring BOTH of the measurements into proper, or the same time frame. Otherwise, you're just guessing.

    But, I think this is what you brought forward in the rest of your post.
  6. Mar 21, 2005 #5

    I know two things people understood wrong when speaking of Einstein :

    1) When Einstein said "locality", he means : You FORGOT locality. Because every first year student knows that the the correlation is :


    where <AB> is not local, and <A>, <B> are the local parts (obtained without disturbing the second part).

    hence, the correlation is made up of a superposition of local, and non-local parts....(not only non-local)

    2) When Einstein said "God does not play dice"...he is not saying what G-d has to do....but it's rather a prayer...But this remains my opinion.
    Last edited by a moderator: May 1, 2017
  7. Mar 22, 2005 #6
    I think that this is exactly what I needed to hear... - It should give me a foot-hold to move forward. :approve:

    Right, because without measurement, the particle(s) won't 'snap' out of the 'haze of probability'. - But after the appropriate measurements are made, then we always get the proper correlations. (right?)
  8. Mar 22, 2005 #7


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    Classical is a word that can be construed in several ways in this context. The classical formula for correlation is [tex]cos^2 \theta[/tex] and this is the formula used in QM as well. Often, the term classical is also used to describe the "local realistic" position in which there are hidden variables (as Einstein assumed existed). The local realistic position makes predictions which are incompatible with this formula, as Bell showed.

    I have a page that explains the math in fairly simple terms so you can see the difference between the two: Bell's Theorem and Negative Probabilities. Look at the outcome table in b. and compare it to the table in c. so you can see the difference between the local realistic and QM scenarios respectively.
  9. Mar 22, 2005 #8
    To quote Gell-Mann and Hartle about EPR from their paper Quantum Mechanics in the Light of Quantum Cosmology:

    The EPR paradox is a question about reality and realism in the sense of being able to talk about different kinds of particle properties and then combining the properties in one description. EPR was about combining momentum and position and EPRB which is Bohm's version was an improvement in combining x and z spin.

    The interesting thing about quantum theory is locality is fine but realism has gone weird and trying to combine descriptions of different kinds of particle properties isn't allowed in the way we'd expect. :smile:
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