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B Quantum Entanglement and events

  1. Mar 28, 2016 #1
    Dear PF Forum,
    I have a question regarding Quantum Entanglement.
    I don't know much about QE, but there's something that I want to know.
    Two particles are entangled.
    Blue line is the world line of Particle A

    See Pic 02
    Particle B travels according to Green line.
    And at E1 Particle B's spin is changed (is that how we say it in QM?) so the "change" is reflected in Particle A, right.
    A: Is it true that E1's "now" is at E2, according to SR?

    Now Particle B accelerates a little so its word line is the Red Line. See Pic 02
    And at E3 Particle B's spin is changed, now the change (Pic 03) is reflected at E2
    B: Is it true that E3's "now" is at E2 ,according to SR?

    C: Can, at E2, particle A is changed according to two different events?

    Thank you very much
  2. jcsd
  3. Mar 28, 2016 #2

    Simon Bridge

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    The "event" would properly be the event that the spin was measured.
  4. Mar 28, 2016 #3


    Staff: Mentor

    The usual way would be to say that a measurement is made of particle B's spin, which, at least according to some interpretations of QM, forces particle A's spin to change "instantaneously". But no measurement is being made of particle A's spin, so the question of "when" particle A's spin changes is physically meaningless--indeed, the claim that "particle A's spin changes" is itself physically meaningless.

    No. SR says that the concept of "now" has no physical meaning.

    No; particle B's spin is measured a second time, but just as above, since no measurement is being made of particle A's spin, the question of "when" particle A's spin changes is physically meaningless, as is, indeed, the claim that particle A's spin "changes" at all.

    There is also another factor you are overlooking: even on the interpretations of QM where the first measurement of particle B's spin "changes" particle A's spin "instantaneously", via entanglement, that process also breaks the entanglement. So on this interpretation, when the second measurement of particle B's spin is made, it no longer does anything to particle A's spin because the two are no longer entangled.

    No. See above.

    No. See above.

    What this scenario is really illustrating is the limitations of the interpretation of QM that says that a measurement of one entangled particle "instantaneously" changes a property of the other entangled particle. The best way to deal with this is to drop that interpretation.
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