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What is so special about quantum entanglement?

  1. Dec 1, 2011 #1
    From my understanding of it thus far (and please pardon me if this is the result of a novice perspective of the subject):

    a) if a pair of particles are entangled they are known to be in the same state or exactly opposite states

    b) changes to the state of one of the two particles introduce noise that in turn breaks the entanglement - thus making it difficult to entangle large numbers of particles

    What is special about this?

    Isn't entanglement the same thing as if I were to spin two basketballs in the same direction, measure the rate of spin of one and say their the same from that measurement while simultaneously measuring the sideways motion of the other? (assuming they were in fact spun with the same sideways movement and rate of spin for all measurable precision)

    Am I wrong, can an entangled pair undergo changes from MULTIPLE interactions while transmitting the results, or is it just a neat way to encode the starting parameters of a system and get around Heisenberg's Uncertainty Principle without discrediting the man in the process?
  2. jcsd
  3. Dec 1, 2011 #2

    Transmit results? Can you please elaborate.
  4. Dec 1, 2011 #3
    No isn't a very elaborate answer - how does it differ from the analogy I gave? (are there known changes to one particle guided by the interactions the other particle experiences)

    In terms of transmitting results, if you impart a value to any entangled parameter of a particle - can you make the change appear on the other end repeatedly (for the same pair of particles - without breaking entanglement). For that matter, is information (change in any given parameter describing the particle) even transmitted from one to the other when it occurs?
  5. Dec 1, 2011 #4
    You didn't ask that originally. I cannot reply thoroughly right now, though.
  6. Dec 1, 2011 #5
    That is what I asked the first time, though I used a metaphor to describe it (not to belittle the subject, but from looking at what has been observed of quantum entanglement over the years it seems to be that simple - like trying to preserve Heisenberg's Uncertainty Principle in spite of a deterministic universe).
  7. Dec 2, 2011 #6
    I understand that entanglement doesn't violate HUP - I suggested entanglement exists purely as a statistical anomaly that is backed by HUP as a mechanism keeping it alive.

    My view of these experiments to date:

    A single photon is the minimum emission/absorption energy that can be produced or measured - never a particle.

    A wave front as a plane wave passing through a slit (polarized light being an example) is clipped by the walls of the slit as it passes through following the conservation of energy (if it can't be absorbed it can only continue or be reflected - whether this occurs through a compression suggesting substance to the wave front or simple tunneling I'm not guessing at here - personally I dislike how prominent a role locality played in the Michelson–Morley experiment when gravity hasn't even been factored in - but that's a separate discussion entirely).

    The rate of a photon making it through is still going to be cos(polarization)^2 because it's the cross-sectional area of the passing wave front when laid over the opening of the slit.
  8. Dec 2, 2011 #7
    Also, I apologize for any tangents or poorly phrased statements - I've been up about a day and plan on passing out shortly.
  9. Dec 2, 2011 #8

    Ken G

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    Classical entanglements are certainly possible, the common example is a pair of socks (if I end up with the left one, I know the one I don't have is the right one, etc.). Those are all more like your basketball example, and all the information involved in the entanglement can be thought of as local to each subsystem in the entanglement-- each basketball "knows" its own spin, each sock "knows" if it is a right or left sock. Quantum entanglements have a very different character-- as elucidated in Bell's theorem, in a quantum entanglement, the system must be treated holistically. That means the "information" we are using to talk about the entanglement is not "known" by either particle alone, it requires that we correlate the outcomes of the two experiments to find what is unusual about the entanglement. It's actually a rather subtle business, but important for issues like quantum teleportation and quantum cryptography, which are already technological fields (albeit in their infancies).
  10. Dec 2, 2011 #9


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    Welcome to PhysicsForums, Physt!

    Entanglement would allow one to get around the Heisenberg Uncertainty Principle if it were as you describe. In fact, the EPR paper (1935, and Einstein is the E in the initials) said as much when it claimed QM was incomplete.

    As KenG points out, you need to learn about Bell's Theorem (1965) so you can see why this view is NOT correct. I have a fairly easy to follow proof of that on my website. See below. Once you follow Bell, we can answer other questions about quantum entanglement.

    Bell's Theorem with Easy Math
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