ChadGPT
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- Does Wigner change what the friend saw simply by entering the lab and asking? And how is it possible for Wigner to erase the memory of the friend simply by performing an interference measurement on the lab? And what is an example of a clear case that violates LF inequalities?
Brukner, Renner, and Cavalcanti recently won the Paul Ehrenfest award for their work deriving new no-go theorems involving an Extended Wigner's Friend Scenario, but I'm having trouble understanding certain aspects of how this thought experiment works. I'm trying to understand it conceptually. Here are my questions:
1) Brukner seems to say in some of his lectures that when Wigner opens the lab and asks the friend what he saw, that this can actually change what the friend saw. For instance, say the friend measures the particle along a certain axis and gets "spin up." Then, afterwards, Wigner opens the lab and asks the friend what he saw. This means that Wigner's wave function which includes the lab+friend+particle is projected into a definite state, with equal probability of finding that the friend says that he measured "spin up" and "spin down." So, sometimes, Wigner will ask the friend what he saw and the friend will say he remembers measuring "spin down" even though he actually originally measured "spin up." Since this particle is entangled with the particle that is being measured in the other lab, this change produced by Wigner opening the lab can affect what the friend in the other lab sees. Am I getting this right?
2) Also, instead of opening the lab and asking the friend what he measured, Wigner has the alternative option to perform a reverse unitary measurement on the entire lab+friend+particle, which has the effect of coherently erasing the friend's memory and returning the entire lab+friend+particle to a superposition. Wigner can then proceed to measure the particle directly, and may measure it "spin up" even if the friend measured it "spin down" originally. Is this right? And is it correct that just performing a certain kind of measurement from outside of the lab Wigner can actually cause the friend's memory to be erased? This is surprising to me.
3) Finally, I'm struggling to conceptualize a case in which the Local Friendliness assumptions are clearly violated. For instance, let's say Friend A measures the particle as "spin up", and Friend B measures the particle as "Spin down." This has to be the case due to the entanglement, right? At least when the two friends measure along the same axis. So then what can the Wigner's do to violate the inequalities? Perhaps Wigner A asks Friend A what was measured and Friend A says "Spin down," (changing his result) and then Wigner B performs a reverse unitary transformation on Lab B, erasing Friend B's memory, and then measures Particle B to be "spin up." The results of the two Wigners would then contradict the results of the two friends, but would still be consistent with the entangled state. Is something like that what is going on? Or something else.
Thank you in advance for your help and patience. :)
1) Brukner seems to say in some of his lectures that when Wigner opens the lab and asks the friend what he saw, that this can actually change what the friend saw. For instance, say the friend measures the particle along a certain axis and gets "spin up." Then, afterwards, Wigner opens the lab and asks the friend what he saw. This means that Wigner's wave function which includes the lab+friend+particle is projected into a definite state, with equal probability of finding that the friend says that he measured "spin up" and "spin down." So, sometimes, Wigner will ask the friend what he saw and the friend will say he remembers measuring "spin down" even though he actually originally measured "spin up." Since this particle is entangled with the particle that is being measured in the other lab, this change produced by Wigner opening the lab can affect what the friend in the other lab sees. Am I getting this right?
2) Also, instead of opening the lab and asking the friend what he measured, Wigner has the alternative option to perform a reverse unitary measurement on the entire lab+friend+particle, which has the effect of coherently erasing the friend's memory and returning the entire lab+friend+particle to a superposition. Wigner can then proceed to measure the particle directly, and may measure it "spin up" even if the friend measured it "spin down" originally. Is this right? And is it correct that just performing a certain kind of measurement from outside of the lab Wigner can actually cause the friend's memory to be erased? This is surprising to me.
3) Finally, I'm struggling to conceptualize a case in which the Local Friendliness assumptions are clearly violated. For instance, let's say Friend A measures the particle as "spin up", and Friend B measures the particle as "Spin down." This has to be the case due to the entanglement, right? At least when the two friends measure along the same axis. So then what can the Wigner's do to violate the inequalities? Perhaps Wigner A asks Friend A what was measured and Friend A says "Spin down," (changing his result) and then Wigner B performs a reverse unitary transformation on Lab B, erasing Friend B's memory, and then measures Particle B to be "spin up." The results of the two Wigners would then contradict the results of the two friends, but would still be consistent with the entangled state. Is something like that what is going on? Or something else.
Thank you in advance for your help and patience. :)