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How is quantum entanglement a fact?

  1. Oct 9, 2011 #1
    I have read and I believe that I pretty well understand quantum entanglement but the more I learn the more it sounds like it is a way of perceiving how something happens than "spooky action at a distance". By this I mean how can this be considered a destruction of locality instead of common sense. An example would be a stationary particle decays into two smaller particles that fly off in opposite directions, one with an "up" spin and one with a "down" spin, to conserve angular momentum. The uncertainty principle states that those particles can be moving at any speed anywhere until measure them. Which sounds much more logical than scientific. And when we measure one particle we automatically force that particle into a state and it automatically changes the other particle to correspond to the first one. Physicists seem to be living more in a world of probability and statistics than the real world.

    It makes more sense to say that the two particle have a definitive momentum and position at any point in time and that we find those positions and momentum when we measure them (however we cannot know both at the same time). And when we know one we then know the other, not that the particles appear in a position when we look and change for our benefit. Physicists appear to have gotten lost in the math and forgotten what the real world is like.

    Please reply back with explanations to quantum entanglement that explain better why quantum entanglement is a truly strange phenomena and why physicists are right about quantum entanglement. Because I obviously do not seem to understand it.

    Your comments would be much appreciated, Thank You!
  2. jcsd
  3. Oct 9, 2011 #2


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    Unfortunately this model makes some predictions that disagree with certain kinds of experiments. Search this forum for discussion of "Bell's Theorem" (no connection with me). Look for posts by DrChinese and follow the link in his signature, to his Web site. You'll find a lot of material there, including some relatively easy-to-digest examples.
  4. Oct 10, 2011 #3
    What you proposed is the hidden variable explanation for a quantum measurement. That has been disproven. Entangled system constitute a super position. There can be no mathematical function which predicts the outcome of a measurement. That is why physicists believe that it is truly random. This is the conclusion of bells experiment.
  5. Oct 10, 2011 #4


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    Really? Which ones would be guilty of that? After all, the math is what is used to make predictions. Not a bad way to represent the "real world", whatever you think that is.

    As mentioned above, Bell precludes your description and I thought you had already been pointed in that direction previously. Check out:


    It does require you to follow a minimum of math, nothing more complicated than some simple statistics. You should deduce from the chart presented that your reasonable assumption does not lead to the actually observed statistics.
  6. Oct 10, 2011 #5
    OK so what I think you are specifically asking about is the Bell’s theorem. Put simply, the theorem concludes that QM must violate locality or counterfactual definiteness. The 2 together are often called local reality. I recommend you read http://en.wikipedia.org/wiki/Bell's_theorem plus anything from DrChinese. He’s your best resource on this.

    The Bell theorem (1964) is a response to the EPR paradox (1935). The EPR argument is that the results of an experiment must be set when the 2 entangled particles separate. This conclusion fits our classical concept of physics and is often called the hidden variable theory. Bell came up with a way of demonstrating that this cannot be true since the results are dependent on the offset angle at which you take the measurements. Further, he concluded that some fundamental concept of reality must be broken.
  7. Oct 10, 2011 #6
    I can’t stop typing. I must be board today.

    Let’s start with a more specific thought experiment for you. Let’s say 2 entangled photons travel in opposite directions and they both go straight into polarizers. And let’s say those polarizers are set at the same angle so that if the left photon goes through, then you can be curtain that the right one made it through. And if the left photon does not make it through, then you can be sure that the right one didn’t make it either. This is called a 100% correlation.

    Now let’s turn one polarizer 90 degrees. Since you know how you’re polarized lenses work, I bet you can figure out that if the left photon makes it through, then you can be certain that the right one won’t make it. And if the left one is blocked, then the right one must have made it through. This is a 0% correlation.

    Now you’re going to have to do some work. What if we had turned one of the polarizers only 22.5 degrees? What do you think the correlation would be then? The first time I tried tackling this, I figured that the angle of the photons must be set randomly. So the correlation must be 75%. That makes sense because if the polarizer is turned an additional 22.5 degrees, then they’re offset by a total of 45 degrees and the correlation would be 50%.

    Let’s think of this a different way now. For a century before the EPR paradox, it was known that the amount of light that gets past 2 polarizers offset by a particular angle is cos(angle)^2. So if the polarizers are offset by 45 degrees, then 71% of the light gets through. (Not 50%!) And if they’re offset by 22.5 degrees, then 92% gets through. This does not match what I figured out earlier. One of these must be wrong.

    I can tell you that the experiment has actually been carried out by sending one pair of entangled photons through polarizers one at a time (Alain Aspect 1981). And if the polarizers are offset by 22.5 degrees, then the correlation is 92%. And if the offset is 45 degrees, the correlation is 71%. Such experiments have been done in all different ways with the same conclusion. And this is just what Bell predicted 17 years earlier.

    If you double the angle, it has more than double the effect. HOW CAN THIS BE? The only way that could be is if one photon is somehow affected by the angle of the other polarizer. Once again HOW CAN THIS BE? You might conclude that information is sent instantaneously between the photons. That would break locality and special relativity. Or you might go with some kind of information time travel. That would break counterfactual definiteness.

    No experiment either performed or imagined is able to determine which of the 2 principals must go. And that leaves just enough room for quantum mechanics and special relativity to remain compatible.
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