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Random switch setting in bell theorem

  1. Mar 8, 2015 #1
    I was trying to understand Bells theorem and I found this site
    easy to understand.

    I found the "The Second Analogy: More Boxes" in it easy to grasp. But one thing I didn't understand in it, where it uses the word random and expect all possible setting occur equally. Like, here it says,

    "But the switch settings are made at random, so we expect each of the six possible results in the above table to occur with equal frequency. So both lights flash the same color one-third of the time."

    Here when we say the switch setting are random, then how is it, that all possible setting occur equally. When it is random, it is possible, only one switch setting can occur all the time OR how is it that all possible setting occur equally?

    Thank you.
  2. jcsd
  3. Mar 8, 2015 #2


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    Staff: Mentor

    If the switch settings are random and you repeat the experiment many times, you will expect see to all settings about equally often - google for "law of large numbers". It's not necessary that they all occur exactly the same number of times to see a violation of the inequality.
  4. Mar 9, 2015 #3
    Thanks for the reply.
    Why do we take switch setting at random and also why are the three different state of particles(here speed, size and shape) are also random?
    The device can also record the state of the entangled particles created and the switch setting. So that later it can be checked what was going on.

    Also is this experiment similar to double slit experiment?

    I understand bell's theorem is all about proving einstein idea and entanglement(quantum physics). For e.g according to quantum physics when pair of object are entangled, say a pair of gloves i.e if one is right hand then the other will be left hand. According to QM the other gloves is not determined before one of the gloves is checked. But according to einstein, they both are determined before the experiment is done.
  5. Mar 9, 2015 #4


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    Staff: Mentor

    Read through the material here: http://www.drchinese.com/Bells_Theorem.htm

    The essential point of Bell's theorem is that if the properties of both members of the entangled pair are determined before the measurement (as Einstein and many others expected) then the results of randomly chosen measurements of a large number of pairs will have certain statistical properties: they will obey Bell's inequalities.

    Quantum mechanics predicts that under certain circumstances when there are (at least) three possible measurements, the results will violate the inequalities.
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