Current status of FR type Thought experiments

[AAAdrian]

Recently I've seen many fixed FR(or Wigner's friend) thought experiments.For example, https://arxiv.org/pdf/2011.12716.pdf is a zero-cited paper that presents a strenthened version of FR ,and claims to have solved its weaknesses.The author claims that it supports the idea that the "C"(for short,agreed facts in quantum systems)assumption should be given up.
But as far as I know,the field of WF type experiments often shows self-contrary results,and careful analyses of these papers often discovers hidden problematic aspects of these papers(eg.hidden assumptions,misunderstandings) and the community seems to ignore,nowadays,conclusions drawn from there areas of research.Original authors of FR changes their opinions frequently as well.
So what are your opinions on this paper and the whole WF research?

 

[PeterDonis]

what are your opinions on this paper and the whole WF research?

All of these thought experiments involve what, to me, seems like an inconsistent set of premises. In particular, they include the following two things:

(1) One human observer (Wigner's Friend) makes a measurement on a quantum system and observes some definite outcome.

(2) A second observer (Wigner) represents the combined quantum state of the first observer (the friend) and the system the first observer measures, as a superposition of different possible outcomes based on unitary evolution.

But #1 and #2 cannot both be true. If #1 happens, the joint observer+system state decoheres and interference effects can no longer be observed. But the only way for the second observer in #2 to show that the unitary evolution they are claiming has happened (i.e., that the observer in #1 did not observe a single definite outcome but is in a superposition of different possible outcomes) is to observe interference effects in the joint observer+system of #1--which can't be done if #1 has decohered.

 

[AAAdrian]

All of these thought experiments involve what, to me, seems like an inconsistent set of premises. In particular, they include the following two things:

(1) One human observer (Wigner's Friend) makes a measurement on a quantum system and observes some definite outcome.

(2) A second observer (Wigner) represents the combined quantum state of the first observer (the friend) and the system the first observer measures, as a superposition of different possible outcomes based on unitary evolution.

But #1 and #2 cannot both be true. If #1 happens, the joint observer+system state decoheres and interference effects can no longer be observed. But the only way for the second observer in #2 to show that the unitary evolution they are claiming has happened (i.e., that the observer in #1 did not observe a single definite outcome but is in a superposition of different possible outcomes) is to observe interference effects in the joint observer+system of #1--which can't be done if #1 has decohered.

This paper claims to have simplified the thought experiment,and do not involve some steps you mentioned above,so I guess it might be more convincing and posted it here!

 

[Morbert]

All of these thought experiments involve what, to me, seems like an inconsistent set of premises. In particular, they include the following two things:

(1) One human observer (Wigner's Friend) makes a measurement on a quantum system and observes some definite outcome.

(2) A second observer (Wigner) represents the combined quantum state of the first observer (the friend) and the system the first observer measures, as a superposition of different possible outcomes based on unitary evolution.

But #1 and #2 cannot both be true. If #1 happens, the joint observer+system state decoheres and interference effects can no longer be observed. But the only way for the second observer in #2 to show that the unitary evolution they are claiming has happened (i.e., that the observer in #1 did not observe a single definite outcome but is in a superposition of different possible outcomes) is to observe interference effects in the joint observer+system of #1--which can't be done if #1 has decohered.

The experiment is teasing at the slight ambiguity of "irreversibility". For us humans, observer (1)'s measurement indeed causes decoherence, establishing an intersubjective reliability of the measurement outcome for all human observers. But an alien "superobserver" with an apparatus many times larger than the observable universe may assert that the system has not decohered. The absurdity of such a superobserver, even before we introduce constraints of relativity, is presumably why people do not worry too much about such thought experiments.

 

[Demystifier]

What the Frauchiger-Renner (FR) theorem rules out is one particular class of Copenhagen-like interpretations, that is the objective Copenhagen interpretation. The objective Copenhagen interpretation is an interpretation in which both of the two statements are true:
(i) The observation-induced collapse of |ψ⟩ is objective.
and
(ii) The level on which this collapse happens (the level of Wigner or the level of his friend) is subjective.
The FR theorem says that (i) and (ii) are not consistent with each other, i.e. that the objective Copenhagen interpretation is inconsistent. In other words, the theorem states that it is inconsistent to treat the collapse as both objective and subjective. When put in this form, the theorem looks rather intuitive and hardly surprising. Perhaps the only surprising aspect of this is that the actual proof of this intuitive statement (that the collapse cannot be both objective and subjective) is technically quite complicated.

 

[AAAdrian]

The experiment is teasing at the slight ambiguity of "irreversibility". For us humans, observer (1)'s measurement indeed causes decoherence, establishing an intersubjective reliability of the measurement outcome for all human observers. But an alien "superobserver" with an apparatus many times larger than the observable universe may assert that the system has not decohered. The absurdity of such a superobserver, even before we introduce constraints of relativity, is presumably why people do not worry too much about such thought experiments.

Actually the mentioned paper claims to have solved the irreversibility problem by modifying the setup.Not sure how plausible that was.

 

[Demystifier]

Actually the mentioned paper claims to have solved the irreversibility problem by modifying the setup.Not sure how plausible that was.

The paper uses Peres-Mermin proof of contextuality and interprets it as a logical inconsistency, not as a physical change of state due to the act of measurement. In this way there is no collapse, and hence no irreversibility, in the analysis. This logical inconsistency is called the "quantum Rashomon effect" in the paper. In my opinion, any interpretation that accepts logical inconsistencies, no matter how they are called, is unacceptable. To avoid the logical inconsistency, one must accept that quantum contextuality corresponds to physical changes of state due to the act of measurement. Effectively, this not only brings back collapse and irreversibility, but also makes all such FR type of thought experiments pretty much pointless.

 

[Demystifier]

Original authors of FR changes their opinions frequently as well. So what are your opinions on this paper and the whole WF research?

Let me paraphrase Bohr. If Wigner friend thought experiment confuses you more than the rest of quantum mechanics, then you have not understood the rest of quantum mechanics.

 

[AAAdrian]

Let me paraphrase Bohr. If Wigner friend thought experiment confuses you more than the rest of quantum mechanics, then you have not understood the rest of quantum mechanics.

Thanks for reading my thread and for givin insights.Also advertise here for my thread which so far had no reply and you might be interested in.

 

[PeterDonis]

This paper claims to have simplified the thought experiment,and do not involve some steps you mentioned above,

It claims to have simplified the original Frauchiger-Renner thought experiment, but it neither claims to, nor does, eliminate the inconsistent items I listed.

 

[RUTA]

Here's our take on it https://academic.oup.com/book/44484/chapter/376468182 . See Section 4.