Bob and Alice, Wigner and his friend

In summary: This is a problem in the literature known as "Wigner's friend paradox".In summary, there is a problem in the literature known as "Wigner's friend paradox" where Wigner and his friend make measurements on a particle, but the friend's measurement has no effect on Wigner's subsequent measurement, leading to the question of whether the particle's state remains unchanged or becomes entangled with the measurement results. This is similar to the situation with Alice and Bob, but with the added complication of a friend making a measurement.
  • #1
Heidi
411
40
Hi Pfs,
I wrote in another threas that when a source emits maximally entangled
pairs of photons with nul global momentum and null global angular momentum, there is no local properies for the photons shared by Alice and Bob. i said that the the source only emits correlations. it has no sense to ask why the get the same result. No need to ftl signal.
there is another problem in the litterature: Wigner and his friend
Wigner makes a measurement on a particle and ger a result. then his friend who ignores the result considere the Wigner + particle system. For him there is still a superposition of different
states "Wigner + particle" all with given probabilities. If hi makes
the same measurement that Wigner on the particle he will get the
same result. Why
it seems to me (maybe falsely) that we have the same situation than with Bob and
Alice.
we could think that the first measurement gave a property to the partice (ignored by
his friend) and later his frienl will measure it. this suppose that le particle will keep it continuously during that time. but if space and time are discrete we will get to states at different moments just like Bon was not at the samd place than Alice.
It seems that we have here also a creation of correlations between a result now and a future result. We would get the same result in repeated measurements not because the measured state would stay unchanged but because of an entanglement between same measurements.
I wonder if this point of view is consistent :)
 
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  • #2
One common response: No physically realistic friend is capable of any measurement that resolves a pure state of the macroscopic system (i.e. microscopic system + Wigner + lab environment). So epistemically speaking, we can safely use our classical intuition (Wigner knows something his friend doesn't)
 
  • #3
Heidi said:
said that the the source only emits correlations
This makes no sense unless "correlations" means "quantum objects that can be detected by Alice and Bob". Which doesn't seem right.

Heidi said:
Wigner makes a measurement on a particle and ger a result. then his friend who ignores the result considere the Wigner + particle system.
Actually it's the other way around in the literature: Wigner's friend makes a measurement on a particle, then Wigner considers the friend + particle system.

Heidi said:
it seems to me (maybe falsely) that we have the same situation than with Bob and
Alice.
No, because neither Bob nor Alice are looking at a combined system composed of the other (Alice or Bob) plus the particle they are measuring.
 

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