Extended Wigner’s Friend Scenario and the existence of the reality

[Spathi]

https://backreaction.blogspot.com/2022/02/has-quantum-mechanics-proved-that.html

Physicists have shown that objective reality doesn’t exist. This is allegedly an insight derived from quantum mechanics. And not only this, it’s been experimentally confirmed. Really? How do you prove that reality doesn’t exist? Has it really been done? And do we have to stop saying “really” now? That’s what we’ll talk about today.

Okay, so that was Wigner’s friend in the 1960s. You can’t experimentally test this, but in 2016 Daniela Frauchinger and Renato Renner proposed another thought experiment that moved physicists closer to experimental test. This has been dubbed the “Extended Wigner’s Friend Scenario.”

In this thought experiment you have two Wigners, each of whom has a friend. We will call these the Wigners and the Alices. The Alices each measure one of a pair of entangled particles. As a quick reminder, entangled particles share some property but you don’t know which particle has which share. You may know for example that the particles spins must add up to zero, but you don’t know whether the left particle has spin plus one and the right particle spin minus one, or the other way round.

So the Alices each measure an entangled particle. Now the thing with entangled particles is that if their measurements don’t collapse the wave-function, then now the two Alices are entangled. Either the left one thinks the spin was up and the right one thinks it’s down, or the other way round. And then there’s the two Wigners, each of which goes to ask their friend something about their measurement. Formally this “asking” just means they do another measurement. Frauchinger and Renner then show that there are combinations of measurements in which the two Alices cannot agree with the two Wigners on what the measurement outcomes were.

It is unclear for me, why from these experiments the tabloids made the conclusion that "the reality does not exist"? Does the essence of this experiment lie in the fact that it confirmed the Wigner's friend paradox, or is it more about something fundamentally new? I understand the Wigner's friend's paradox in such a way that when a scientist opens a box with a cat, a superposition is formed of two scientists observing a living or dead cat, in other words, two "parallel worlds". It seems there is nothing here too strange, that “refutes the existence of objective reality”. Or maybe the main idea is that simply that there are two realities in this case?

 

[Heidi]

Take a look at Relational Quantum Mechanics:
https://plato.stanford.edu/entries/qm-relational/#FrauRennExpeLoca

 

[PeterDonis]

It is unclear for me, why from these experiments the tabloids made the conclusion that "the reality does not exist"?

Because they are (a) making overblown claims about what the experiment actually did, and (b) making overblown claims about what doing that means.

See further comments below.

Does the essence of this experiment lie in the fact that it confirmed the Wigner's friend paradox

No, because it didn't. Doing an actual "Wigner's friend" experiment would require doing it on humans. This experiment only did it on qubits. Qubits are not humans. Not just in the obvious sense, but in the much stronger sense that humans have astronomically more degrees of freedom than qubits, and that makes a difference.

The idea behind Wigner's friend experiments is that the friend, inside his lab, can make some measurement and obtain a definite result, A or B, whereas WIgner, outside the lab, can treat friend + lab as a quantum system and make a measurement on it that shows it to be in an entangled state with nonzero amplitude for both results, A and B. And the fact that Wigner can (it is claimed) do this means, according to the hype, that the "reality" the friend thinks they have measured, i.e., a definite result of their measurement, doesn't actually "exist", because Wigner can show that the friend's measurement didn't have a single definite result--both possible results are "still there", because Wigner's measurement can show interference effects between them. Thus, according to the hype, reality doesn't exist.

But in the actual experiment that was actually done, there were no humans involved; it was just qubits. And of course it's easy to make measurements that show interference effects on qubits, and it's easy to reverse unitary operations on qubits, which is what the "Wigner" measurement amounts to--reversing the unitary operation that took place when the friend made their measurement (and therefore, according to the hype, "undoing" the measurement and making the result the friend thought they measured "not exist"). But humans are not qubits; when a human makes a measurement and perceives a result, something like $10^{28}$ degrees of freedom are involved, not just one or two, and decoherence is involved, which it isn't in the case of the qubits, so doing the "Wigner" measurement on the friend would require being able to reverse decoherence. But we can't. So doing the experiment with qubits says nothing about doing it with humans.

 

[PeroK]

https://backreaction.blogspot.com/2022/02/has-quantum-mechanics-proved-that.htmlIt is unclear for me, why from these experiments the tabloids made the conclusion that "the reality does not exist"? Does the essence of this experiment lie in the fact that it confirmed the Wigner's friend paradox, or is it more about something fundamentally new? I understand the Wigner's friend's paradox in such a way that when a scientist opens a box with a cat, a superposition is formed of two scientists observing a living or dead cat, in other words, two "parallel worlds". It seems there is nothing here too strange, that “refutes the existence of objective reality”. Or maybe the main idea is that simply that there are two realities in this case?

I'd treat everything on Hossenfelders channel with a degree of suspicion. She's found her niche in catering to the sceptics, doubters and crackpots.

 

[PeterDonis]

I'd treat everything on Hossenfelders channel with a degree of suspicion.

In this article, Hossenfelder is not supporting the claims of "reality does not exist", she is debunking them.

 

[PeterDonis]

She's found her niche in catering to the sceptics, doubters and crackpots.

I think this is way too strong. Her articles, like this one, are often a useful antidote to overblown claims.