Suppose we have a pair of entangled photons, one photon sent to Alice and the other to Bob. A and B are at rest in the same reference frame. When A receives and measures the polarization of her photon, the photon sent to B is still traveling and he measures its polarization later. Does B's photon acquire its correlated polarization instantaneously when A measures her photon, or when B makes his measurement later on?

  • 3
    $\begingroup$ How would you tell the difference? $\endgroup$
    – tparker
    Nov 24, 2019 at 4:19
  • $\begingroup$ @ReneKail as far as we know, the collapse is instantaneous for the entangled photon. $\endgroup$
    – lineage
    Nov 24, 2019 at 4:41
  • $\begingroup$ see this answer physics.stackexchange.com/questions/446671/… $\endgroup$
    – anna v
    Nov 24, 2019 at 7:48

1 Answer 1


As tparker points out in the comment I just upvoted, there is more than one observationally equivalent way to model this. But according to standard interpretations, your measurement collapses the pair into a new state, the new state is unentangled, and therefore you can say that from that moment on the second particle has a state of its own. Of course you can't expect that state to necessarily be an eigenstate of some measurement that nobody's yet made.

  • $\begingroup$ Yes, this corresponds to the conclusion I came upon after some reflection. $\endgroup$
    – Rene Kail
    Nov 24, 2019 at 9:00
  • $\begingroup$ It's worth clarifying that by "standard interpretations" you specifically mean the Copenhagen interpretation. In the many-worlds interpretation, the wave function collapse is not instantaneous but gets "transmitted" at or below the speed of light. $\endgroup$
    – tparker
    Nov 26, 2019 at 14:34

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