Is there still a Wigner's friend paradox?

[Heidi]

Happy new year to all the forumers.
The Wigner's friend paradox appeared in the early years of quantum mechanics:
Wigner knows that his friend looked inside the box to see if the cat is dead or alive.
He has two cells in his mind to put what his friend saw.
As he ignores the result, he may assign a probability to each cell or he may see "friend + cat" as a quantum system and assigne amplitudes which have to be summed.

Then theorists invented a "cut". Macroscopic devices are classical and particles obey to quantum mechanics rules.
They also said that quantum systems evolve unitarily but that when measured by a
macroscopic device there are random non unitary projections.

We often read now that all systems are in a quantum state (or with a given density matrix)
There is also a purification theorem which allows to add degrees of freedom to get a greater pure state from a decohered one. What Bob and Alice see are given by partial trace on a pure state.
We are also used to assign amplitudes do Feynman diagrams not probabilities.
And when one tried to add probability to describe quantum experiments it gave
Bell inequalities.

So i wonder if the "Wigner's friend paradox" will not be as "the age of the twins paradox"

 

[abdiaziz]

Yeah

 

[Nugatory]

So i wonder if the "Wigner's friend paradox" will not be as "the age of the twins paradox"

There's really no comparison. The twin paradox was never a paradox (it appears and is resolved in Einstein's 1905 paper for crissakes!) and is a valuable teaching tool. In contrast, Wigner's friend is a genuine paradox, invented to put teeth in Schrödinger's problem - the cat is bizarre but not inherently paradoxical, whereas the situation with Wigner's friends is.

But to answer the question in the title: Wigner's paradox goes away when we include decoherence in a non-collapse interpretation. Resolving it is no longer a significant foundational problem; I'd consider that as satisfactorily resolved so no longer a paradox.

 

[Demystifier]

Bur some still consider a version of it to be a paradox.
https://en.wikipedia.org/wiki/Wigner's_friend#An_extension_of_the_Wigner's_friend_experiment

 

[PeroK]

Bur some still consider a version of it to be a paradox.
https://en.wikipedia.org/wiki/Wigner's_friend#An_extension_of_the_Wigner's_friend_experiment

Wigner's friend isn't Kurt Goedel by any chance?

 

[gentzen]

Bur some still consider a version of it to be a paradox.
https://en.wikipedia.org/wiki/Wigner's_friend#An_extension_of_the_Wigner's_friend_experiment

I don't like Renner's "follow-ups" to his paper, but I have to admit that he involuntarily proved "a fact": the inability of the physics interpretation community to separate intriguing contributions from misguided fundationalism, and to suitably ignore misguided stuff. Mathematicians were more successful in ignoring attacks against complex numbers or infinity, and computer scientists are better at dealing with the deluge of amateur P vs NP proofs or the graph isomorphism desease. Therefore, I looked-up what exactly NB wrote to me: "Ja, die Quantenmechanik kann einen schon auf die Palme bringen, vor allem wenn man sie gegen den Strich streichelt. Und für mich wären das die ganzen Interpretationsfragen, das führt für mich und mein Verständnis aus nicht so recht zu irgendwas, insofern zerbreche ich mir darüber nicht den Kopf."

What is worse, mathematicians or computer scientists don't need to warn people against reading popular science accounts of their field. Of course, you might not learn much in that way, and of course some bad and counter-productive accounts do exist too. But it is not like with quantum physics that you are less likely to learn correct information than to learn wrong or misguided stuff.

Just like von Neumann's book, Wigner article itself is deep and intriguing:

The second argument to support the existence of an influence of the consciousness on the physical world is based on the observation that we do not know of any phenomenon in which one subject is influenced by another without exerting an influence thereupon. This appears convincing to this writer.

And it triggered deep and intriguing reactions like
Ballentine, Leslie E. (2019). "A Meeting with Wigner". Foundations of Physics. 49 (8): 783–785:

According to my notes I should then have then criticized CCC, but there was a widespread legend that Eugene Wigner was a supporter of CCC. The origin of that popular legend is found in the so-called “Wigner’s Friend” argument [3]. Since Prof. Wigner was seated in front of me I asked him whether he did indeed believe CCC. His reply was very interesting. He said that he did not believe that CCC was physically true, but he believed that it followed logically from the QM Principles that formed the hypotheses which he had used in the argument [3] that led him to CCC.
I responded by pointing out that his argument had the logical form of a reductio-ad-absurdum. From his chosen QM hypotheses he had logically deduced CCC, which he had reason to believe is not physically true. To avoid this contradiction we should revise some of his initial QM postulates. Prof. Wigner accepted my characterization of his argument that led to CCC as being a Reductio-ad-Absurdum.

3. Wigner, E.P.: Remarks on the mind-body question. In: Good, I.J. (ed.) The Scientist Speculates, pp. 284–303. Heineman, London (1961)

My feeling is that Ballentine came quite close to a satisfactory understanding of the "situation".

 

[Heidi]

I had to search on google the meaning of CCC
And i found consciousness causes collapse:
https://en.wikipedia.org/wiki/Von_Neumann–Wigner_interpretation