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What happens if you measure the spin of entangled particle second time

  1. Oct 15, 2013 #1
    This is how I understand it: Bohr argued that universe was inherently unpredictable as was the spin of the particle, and it was only based on probabilities. Einstein argued that the spin of the particle was actually always the same, just that our physics is not capable of determining it.

    But now if we measure the spin of the particle many times over and if it's always the same, then doesn't that mean einsten was right? Alternatively, if the spin of the particle happens to be random every time then doesn't that make Bohr right?
  2. jcsd
  3. Oct 15, 2013 #2
    You can only measure one component of the spin vector at a time. Let's say you measure the z-component over and over, you always get the same result. But let's now say that you measure the z-component, than the x-component, than the z-component. The second measurement of the z-component may be different than the first.
  4. Oct 15, 2013 #3


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    Staff: Mentor

    Sounds that way, but no....

    With the first measurement you'll get spin-up or spin-down with equal probability; and although you know that if someone measures the other particle they'll get the opposite result, the measurement breaks the entanglement so that the two particles will evolve independently from that initial state. If nothing else changes and you do nothing except continue to measure the spin of your particle along the same axis, you'll keep on getting the same result. (If you measure the spin on any other axis, not only will that result be random, but when you go back to measuring on the original axis you'll get a new random result).

    This makes it tempting to think that your particle really was spin-up and the other one really was spin-down all along, and that this whole entanglement thing is no more mysterious than picking up one of a pair of gloves, seeing that it's left-handed, and knowing that other member of the pair must be right-handed. That's basically the hidden-variables argument made by Einstein and others.

    However, if you google for "Bell's Theorem" you'll find the very convincing argument otherwise. The problem becomes apparent if you consider what happens when you and the person measuring the other particle don't always use the same angle; for example, you both could randomly choose to make a measurement at 0, 120, or 240 degrees. It turns out that there is no way to preassign opposite values for the spin along three different axes that will match the quantum-mechanical prediction and experimental results - we have to accept that the result of that first measurement that breaks the entanglement is random.

    So Bohr was right, or at least more right than Einstein.
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