anuttarasammyak said:
Say A observes up spin. Decoherence for A takes place then. A judges that B's spin is down. A sends this result to B by light speed signal (slower signal admitted.). B gets it at time T.
Case 1: B observes spin down at t<T. B judges that A's spin is up. He confirms it later at time T by news from B.
You mean news from A: A sent the news of his observation of spin up to B, it arrived at B at time T.
anuttarasammyak said:
Decoherence for B takes place at time t
For this case, yes. But see notes at the end of this post.
anuttarasammyak said:
Case 2: After A got news from B at time T
A didn't get news from B at time T. B received the news from A at time T. That's what your description above says.
anuttarasammyak said:
he observes spin down at t<T and confirm the entangled result.
B can't go back in time and make his own measurement at t < T after he receives a message from A at time T. Either B already made a measurement at t < T (which is Case 1 above), or he didn't (which is I assume what this Case 2 is supposed to be).
anuttarasammyak said:
Decoherence for B takes place at time T
For the case where B makes no measurement prior to time T, yes, B will decohere because of becoming entangled with the information arriving from A. But note that B's *particle* itself will *not* decohere, since B has not measured it. (We're assuming the particles are kept isolated until they are measured.) Also see further notes at the end of this post.
anuttarasammyak said:
Case 3: A got news from B at time T but does no observation. He is sure that spin is down with no need of observation.
You keep confusing A with B. B gets the news from A at time T. There is no Case 3 for B: either he makes a measurement on his own particle before he receives the news from A, or he doesn't. Those are Cases 1 and 2 above, and they exhaust the possibilities.
anuttarasammyak said:
From this I understand that decoherence is not universal but observer specific.
Decoherence is a local process: it happens when a quantum system gets entangled with another system that has a large number of degrees of freedom that can't be individually tracked, like a measuring device, or even the environment in general. The entanglement interactions involved are local.
anuttarasammyak said:
B's decoherence takes place at earlier times of the two: B's observation and B's receipt of A's observation result.
Yes, this is basically true. But see notes at the end of this post.
anuttarasammyak said:
symmetrically
A's decoherence takes place at earlier times of the two: A's observation and A's receipt of B's observation result.
Yes, this is basically true. But see notes at the end of this post.
anuttarasammyak said:
In other words one's decoherence timing is determined when he got the knowledge by his own observation or news from his partner.
It's not "getting the knowledge" that causes the decoherence, it's becoming entangled with a system that has a large number of degrees of freedom that can't be individually tracked. Since B himself (the observer--you used A and B to refer to the observers, not the particles themselves) is such a system, he is basically continually decohering himself. The "decoherence" specifically related to the measurement results happens as soon as he becomes entangled with any degree of freedom that is itself entangled with one of those results--either his own particle and measuring device, if he makes his own measurement first, or the signal arriving from A that is entangled with A and A's particle and measuring device, if B receives the signal from A first.