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Simultaneously Measuring Entangled Particles

  1. Aug 25, 2012 #1
    Let's say you have entangled particles emitted from a source. What happens if you measure the spin of both entangled particles simultaneously with respect to the reference frame of the source?

    Would the spins measured still be opposite (anti-correlated)? Would you even get a definite measurement? Thanks.
     
  2. jcsd
  3. Aug 25, 2012 #2
    They will be opposite spins as the theory goes...


    How ever - im not sure we really can measure them instantly at the same time, even computers probably are not always going to ping dead on time (even if were talking unimaginably small timing differences)

    Me personally the more exciting question is how are they connected over these distances. Is it a new energy that can travel faster than light, are they connected in a higher dimension outside of our time dimension etc.
     
  4. Aug 25, 2012 #3
    Could you elaborate? Since the measurements are being taken at the same time with respect to the reference frame of the source, aren't the spins of both the particles still indefinite at the instant before the measurements? As a result, isn't there a probability that the particles will have the same measured spin? I think I'm misunderstanding something.
     
    Last edited: Aug 25, 2012
  5. Aug 25, 2012 #4
    The short answer is we would get opposite spins if measured both of them instantly from what we believe. I don't believe we ever could measure them instantly how ever.

    If a zero spin particle decays to two entangled particles - their spin total when combined has to equal the zero spin because thats where they came from.

    So they always have to be opposite (and im assuming here even during the uncertain non measured state - both of them are still opposite). But as we can never know that without measuring which collapses the uncertainty we can only assume logically they stay opposite to keep every thing balanced to zero spin. But weather they are up or down spin is completely random.


    As far as we know - when you measure one or both, the spins are set "instantly" no time has past when this occurs - so if our computers really could measure instantly (which they probably cannot even if they difference is so small it appears instant to us mere mortals........) they would still be opposite spins.


    Also the distance between the particles does not change this "instant" action. They could span the universe and appear to change instantly...we have tested such things on quite large distances on Earth with no delay.

    This is why its "spooky".
     
  6. Aug 26, 2012 #5

    mfb

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    It does not matter at which time you measure the spins.
    In addition, "at the same time" cannot be exact, the measurement itself requires some finite time.

    No. It depends on your favourite interpretation of QM, but they all have some way to explain this.
     
  7. Aug 26, 2012 #6

    Bill_K

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    How does the Copenhagen interpretation explain it?
     
  8. Aug 26, 2012 #7

    mfb

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    As soon as you measure (at least) one particle, the wave function of both collapses into a state corresponding to your measurement.


    Many worlds gives you two worlds - if you measure "horizontal"/"vertical", for example, one world ("A horizontal, B vertical") and one ("A vertical, B horizontal").
     
  9. Aug 26, 2012 #8

    Bill_K

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    Sure. Now what does "as soon as" mean, in a relativistic context? In which rest frame are they simultaneous. If I measure particle A, how does the Copenhagen interpretation decide when to affect particle B?
     
  10. Aug 26, 2012 #9

    mfb

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    It does not matter in which frame you look at the system. One frame might say "the measurement of A came first and collapsed the system", an another frame could say "the measurement of B came first and collapsed the system".

    As I never saw some equation handling those collapses, I do not like the Copenhagen interpretation, but it is somehow well-defined what happens.
     
  11. Aug 26, 2012 #10
    What do you mean no - its still entirely possible they communicate with some kind of energy unknown to us, or are connected in higher dimensions and so the wave collapses much faster than light.

    Can't rule those out.
     
  12. Aug 26, 2012 #11

    Bill_K

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    Right, or the collapse fairies. :wink:

    Ok, thanks, mfb.
     
  13. Aug 26, 2012 #12
    Mock if you must but theres no real reason to suggest what I'm saying is any more wrong/right than what you believe. We know zero about how it happens instantly so its any ones guess.
     
  14. Aug 27, 2012 #13

    mfb

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    Well, Bill_K is right: You can invent anything, but as long as there is no experimental test for this, you should be careful with words with a fixed meaning. Why would you call that "energy" or "connection in higher dimensions", if you have absolutely no hint that it is in any way related to energy or spacetime geometry?
     
  15. Aug 27, 2012 #14
    Okay good point, but is there really any good ideas that can be experimented regarding how it happens ?
     
  16. Aug 27, 2012 #15

    DrChinese

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    Experiments have been performed to measure them as close in time as possible. It makes no difference to the outcome how you order the observations (according to both theory and within experimental limits). A before B, B before A, or same time.
     
  17. Aug 27, 2012 #16

    mfb

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    If there would be an experiment showing collapse at all (independent of the mechanism), collapse interpretations would be a theory, not interpretations.
    In other words: There is no single experiment which cannot be explained without collapses of wave functions. To avoid triple negation: All experiments can be explained without collapses.
     
  18. Aug 27, 2012 #17
    EDIT:

    So we have no real explanation how their spins become set instantly regardless of distance between them upon a measurement of either, or both??
     
  19. Aug 27, 2012 #18
    I've also heard a suggestion that the "first" measurement may send a signal backwards in time to when the entangled particles were first produced, ensuring that the spins are always opposite.

    This seems to solve the issue about the ordering of the measurements, and removes a bit of the "spookiness" involved with the particles being spatially separated.
    I'm not sure what other issues this might create though. And I can't think of an experiment which could test for this.

    Interesting though ...
     
  20. Aug 27, 2012 #19
    Time Symmetric Quantum Mechanics. Beautiful interpretation. Search it through the Forum. In some threads you will find links to very interesting papers. It preserves locality but ruins causality.
    I really like not only the preservation of locality but also leaving causality aside because it makes you think as time being much more similar to space than what one usually does (in this interpretation the arrow of time problem sort of does not occur in microscopic world and arises in the macroscopic human world -by the way my thought about it is that it is a human illusion which arises as a consequence of the fact that we, human beings, are consumers of ordered energy and producers of entropy, and, because of this, we order our life from situation of low entropy -past- to situations of high entropy -future--)

    Ps: Remember that all that I said is just an interpretation which probably will never be tested! (Nevertheless I have faith that someone much more intelligent than me will do it!)
     
  21. Aug 28, 2012 #20
    Thanks the_pulp, I'll take a look at those
     
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