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I'm guessing the answer is no ;) but figured I'd ask anyway. Suppose I have one partner in an entangled pair and my friend on Alpha C has the other. I measure an observable (spin?) and the wave function collapses to a single eigenvalue. Does this cause the wave function for that observable to collapse for the remote partner, and if so, can the wave function collapse event itself be used as a superluminal signal? I assume the answer is no simply because he can't tell if the wave function has collapsed without making a measurement . . . which would cause the wave function to collapse. The whole idea falls apart if wave function collapse per se is not something that can be observed directly but is only inferred from measurement, I suppose.

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  • $\begingroup$ PS it also occurs to me I am not sure if my measurement collapses the wave function of the remote partner, or simply creates a condition such that when the remote partner is measured the necessary correlation is seen. $\endgroup$
    – JForster
    Sep 24, 2013 at 22:26
  • $\begingroup$ Your basic problem here is that you can't observe a wavefunction collapse. You can just measure or not measure. When you measure you get a result, but you can't tell if it was required because the measured system was in a pure state or just came up by chance from among the allowed results. $\endgroup$ Sep 25, 2013 at 1:28
  • $\begingroup$ @dmckee thanks very much; this is what I suspected. This clarifies the problem. $\endgroup$
    – JForster
    Sep 25, 2013 at 2:15
  • $\begingroup$ Even if the entanglement holds (no interactions of the particle running to the stars) your measurement on earth allows you to know what the entangled pair's eigenvalue is, but the fellow on the star, unless he also measures the particle arriving there, will still be in the dark about its value, and will know your value only if he measures, so no communication. Just a knowledge of state. $\endgroup$
    – anna v
    Oct 27, 2015 at 17:52

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There is no communication because the sender has no way of influencing the eigenvalue i.e. they can influence the collapse, but they cannot influence the subsequent state of "our" entangled particle, and this not the state of the particle on Alpha C

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