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Spin and entanglement

  1. Jul 5, 2011 #1
    Why scientists think that quantum entanglement works at the distance? Let's say we have two particles, one spins around axis by x another by -x, we make a measure and find that particle a spins by x, then particle b should spin by -x. How come such non logical interpretation could be made that by measuring particle a, we alter the spin of particle b and this mysterious altering is working immediately (there was an experiment to find the speed, lower speed bound was 10000c, and no higher bound was found, so it could be infinity)?
    Such interpretation doesn't make any sense for me. Am I missing something?
     
  2. jcsd
  3. Jul 5, 2011 #2

    DrChinese

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    Yes. You need Bell's Theorem (1965) to make sense of the situation. What you describe is the understanding circa 1935 when EPR came on the scene. I have a page which may be useful:

    http://www.drchinese.com/Bells_Theorem.htm
     
  4. Jul 5, 2011 #3
    I have read that page.
    Particle isn't in superposition before measure. And quantum entanglement proves that. If particle were in superposition before measuring then quantum entanglement could not be possible, because by measuring one particle we know the state of another particle BEFORE measuring. There is a possibility to have x spin not when measuring a particle, but particle has possibility to have x before it is measured.

    Entanglement is the key to testing Bell's Theorem, but that proves nothing, because it is based on entanglement and misinterpretation.
     
  5. Jul 5, 2011 #4
    Bell assumptions:

    1. It should agree with the predictions of Quantum Mechanics (so as to agree with established experiments).
    2. It should adhere to the principles of relativity (causes cannot propagate faster than the speed of light) - this is called Locality (sometimes Bell Locality). Specifically, a measurement setting for one member of an entangled particle pair should not affect the results of a measurement on the other member of the pair located at a distance. Otherwise, you would have so-called "spooky action at a distance".
    3. There should be simultaneous existence of the elements of reality described above (A, B and C, for example). This is often called "Hidden Variables" or sometimes "Realism".

    A person who believes in assumptions 2. and 3. above is called a Local Realist. These two assumptions are very reasonable, and there were a lot of physicists who believed them before Bell. Why not? You simply accepted the predictions of QM and assumed that 2. and 3. were true too. But... Bell showed that the three assumptions above are actually incompatible when combined. Therefore, at least one must be wrong. Bell derived a specific testable prediction related to this called Bell's Inequality. If Bell's Inequality is correct, then the first assumption is wrong. This was the line in the sand for the Local Realist.

    I find all 3 assumptions true:
    1) Predictions are right, interpretations are wrong.
    2) There is no mystical interaction between particles
    3) Yes, hidden variables, defined properties before measurement
     
  6. Jul 5, 2011 #5

    alxm

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    Then you've either misunderstood what 'superposition' means, or what 'entanglement' means, or both.

    (Also, spin is not a particle's rotation on its own axis and the 'velocity' of spin has no meaning)
     
  7. Jul 5, 2011 #6

    DrChinese

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    Well, you asked a question ("Am I missing something") and I answered it. You are free to disagree. I think you have likely failed to work through Bell sufficiently or you would acknowledge the problem with your position. For example: if 3) is true, please give me a dataset which shows what those values are. I will give you the details if you choose to accept the challenge.
     
  8. Jul 6, 2011 #7
    Let's make though experiment.
    Let's say there are 3 points, A, B and O, A and B has interferometers. O sends entangled photons to A and B, A and B sees interference pattern. Now A starts to observe photons, and pattern disappears, same happens for B, B without measuring doesn't see interference pattern. Now A wants to send a sequence to B, so it measures photons for a sec, stops measuring, then measures again etc. B immediately knows what kind of measuring A does? What am I missing? How information could be transfered faster then light? That disagrees with No-communication theorem.
     
  9. Jul 6, 2011 #8

    DrChinese

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    I will tell you what you are missing, hoping you will listen. Check out this reference, Fig. 2 on page 290:

    http://www.hep.yorku.ca/menary/courses/phys2040/misc/foundations.pdf

    Entangled photons do NOT produce interference patterns the same way that coherent unentangled photons do. This is a peculiarity of entangled photons. So your premise fails at its start.

    P.S. I made this same mistake early on in the game too. :smile: There are plenty of little things like this surrounding entanglement. But all are consistent with normal QM.
     
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