Entanglement

Discussion in 'Physics & Math' started by Willem, Apr 19, 2019.

  1. Willem Banned Banned

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    If two particles is entangled and far from each other, and one of them is annihilated, what happens to the other one?
     
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  3. NotEinstein Valued Senior Member

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    My guess would be that the act of annihilation (i.e. interacting with another particle in order to annihilate) counts as an observation, thus unentangling the particle pair before/during annihilation.
     
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  5. James R Just this guy, you know? Staff Member

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    I agree with NotEinstein. One thing that definitely doesn't happen is the "other one" being automatically annihilated when the first one is annihilated.
     
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  7. Q-reeus Valued Senior Member

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    Forget about the answers you have been given so far. It depends. On what kind of particles. And the degree of entanglement in initial preparation. And the kind of interaction that counts as 'annihilation'. And the detector settings.

    If Bosons e.g. photon pairs, prepared with perfect co-linear polarizations, then annihilation of one partner via say absorption by an orthogonally polarized film, that is a measurement and the other partner remains entangled until also measured. Polarization measurement of which is perfectly correlated to that of the first measured photon - assuming the same orientation of polarizing film. Otherwise, with arbitrary film orientations, Malus's Law is invoked.

    Supposing instead we have Fermions say an electron-positron pair prepared with perfectly anti-correlated spins. Annihilation of one partner via destructive interaction with a third, independently prepared anti-particle, does NOT count as a measurement per se, since no permanent record results. Unless perhaps each partner of the subsequent gamma ray pair is measured (k vector and polarization) by a macroscopic device(s). AND the initial state of that third anti-particle was known. Only then could one in principle reconstruct the initial spin polarization of the first partner particle. Without that recorded info, any subsequent measurement of spin of second partner particle is moot since no determination of spin correlation with the first partner particle is possible. Thus ab initio effectively uncorrelated.

    I'm no expert in QM but believe the above is valid.
     
    Last edited: May 1, 2019
  8. NotEinstein Valued Senior Member

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    Indeed so. Most indeededly.
     
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  9. Q-reeus Valued Senior Member

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    Thanks. Actually my first given scenario was deficient in that, while detection of photons is inherently an annihilation process, absorptive polarizing film may or may not leave an irreversible record i.e. 'spot'. If not it merely acts as a filter in which case e.g. a CCD detector placed beyond the film is needed to provide true detection capability. The weird thing about such a detector is it's mere presence is sufficient even if no detection 'click' occurs - as per what happens in two slit interference experiments. A null result is still there a record. Other arrangements might use instead e.g. polarizing plate splitters + photo-detectors.

    Your argument about annihilation of one particle destroying entanglement at that point is afaik provisionally a valid example of environmental decoherence - subject to the details of the annihilation process.
     
  10. el es Registered Senior Member

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    "what happens to the other one ?"

    IMHO:

    Consider the annihilation an observation. Wave function collapse occurs and the other one is no longer in a quantum superposition state.
     
  11. Vmedvil Registered Member

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    This is correct it would break entanglement if the annihilation occurs thus it would not destroy both particles they would just go to different states. You must remember that only the superposition data is transmitted not the other properties.
     
  12. Write4U Valued Senior Member

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    This may illustrate.

    Please Register or Log in to view the hidden image!


    https://waves.neocities.org/superposition.html
     
  13. Q-reeus Valued Senior Member

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  14. Write4U Valued Senior Member

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    Thank you for responding and the link. What basic research?

    I was not posting on a whim but in the most general term of what waves and quantum mechanics have in common, not what sets them apart. What did strike me is the similarity between your link and Schrodinger's cat.

    I don't know if entanglement even has anything to do with superposition, i.e. if they occur at the same level. If they do it sounds much like the probability superposition in Schrodinger's cat.

    From your link:
    And no matter how you look at it, when we open the box, the cat collapses to a state of either dead ot alive (probability wave-function collapse). But is that an example of entanglement or is the wave collapse of a probability superposition a more general function?
     
    Last edited: Jun 10, 2019
  15. Q-reeus Valued Senior Member

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    Uh huh. Accompanying commentary to that effect - back in the relevant post - would have saved me responding as I have.
    Depends how you define 'at the same level'. You could argue a connection of sorts but they are distinct all the same:
    https://backreaction.blogspot.com/2016/03/dear-dr-b-what-is-difference-between.html
    Entanglement is as descibed there basis independent thus more fundamental than superposition.
    Yes it is the source of many debates. But no my #6 was not another example of Schrodinger's cat.
    See link given above. In some interpretations, wavefunction collapse never occurs. I'm not qualified to adjudicate.
     
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  16. Write4U Valued Senior Member

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    From your link;

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