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Necessity of Bell's experiment

  1. Jul 17, 2012 #1
    Hello, I was just wondering why do you need Bell's experiment to prove that there are no local hidden variables? If you do ANY experiment with the same initial conditions, and you don't get the same result, then it's clear the universe is not deterministic! is it not?

    I guess you can say, how can you control the whole universe? But if you create a photon, then you just need to control the past light cone since the photon was created, which is considerably much easier.

    I guess then tha Bohms theory suggest that what causes things to look underteminsitic is things outside this cone affecting the result.

    Anyway, my question is that if any experiment would be valid for proving this, why is Bell's experiment all that important?

    It seems to me that Bell's inequality experiment is just proving that the particles are correlated because the inequality assumes the particles are random (doesnt matter wether statistically or fundamentally) but uncorrelated, and thus it will be violated by particles that are correlated.
  2. jcsd
  3. Jul 17, 2012 #2


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    There are stochastic type theories which postulate that you do not have access to sufficient information to prepare the initial conditions in the manner you describe. Therefore there is apparent indeterminism even though the underlying physical processes are purely deterministic. Bell demonstrates this is not possible for local physical operations, or more accurately, is incompatible with quantum mechanics.
  4. Jul 17, 2012 #3


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    It is not. The problem is that you can never know that ALL the initial conditions were the same. A typical experimental apparatus has at least 10^23 degrees of freedom, while you can really control only a few of them. Considering only the past light cone does not change this fact significantly.
  5. Jul 17, 2012 #4
    Is the main difference that in QM theres always half probability of going through with entangled photons no matter the polarization, but as in classical M photons have a certain polarization the probability depends? Anyway i cant see how they would yield different results. Probably that was the genius of Bell

    Btw sorry for answering late
  6. Jul 17, 2012 #5
    to that respect


    ...Finally, our results demonstrate that one doesn’t need the “big guns” of Bell’s theorem to rule out locality for any theories in which ψ is given ontic status; more straightforward arguments suffice. Bell’s argument is only necessary to rule out locality for ψ-epistemic hidden variable theories...
  7. Jul 18, 2012 #6
    You cannot know that the initial conditions are the same if you don't know if there are no hidden variables. :wink: Moreover, you seem to think that the main issue is determinism. It's not. Instead, it's about what seems to be, as Einstein called it, "spooky action at a distance".

    Bell: "What is held sacred is the principle of "local causality" or "no action at a distance". [..] It is remarkably difficult to get this point across, that determinism is not the presupposition of the analysis. There is a widespread and erroneous conviction that for Einstein determinism was always the sacred principle."
    - cdsweb.cern.ch/record/142461/files/198009299.pdf
  8. Jul 20, 2012 #7
    Ok, I get it. Local hidden variables won't give the same result as quantum mechanics, so it's a no-go theorem. I wanted to check this and I calculated the probability of the spin of one of two entangled electrons being up along z, and the other up along 1/sqrt(2)*z+1/sqrt(2)*x, and got 0.07322, and then did a classical case in which electrons have certain correlated spins but randomlly distributed, and got 0.36254. So if my working is right, this proves they are incompatible!
  9. Jul 23, 2012 #8

    ...It is important to comment on some of the facts that are commonly overlooked in obtaining the conclusion that quantum theory violates local causality. Firstly, not needed are Bell’s inequalities . Secondly, not needed is a ‘free will’ assumption whereby one assumes a form of
    independence between λ and the settings a, b. Thirdly, there is no need for an analysis of the ‘collapse of the wavefunction’ as a real physical process...

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