Psi-ontic view, macro objects and decoherence

[fanieh]

I don't want to create a new thread about Collapse so let me put it here as the topic is related...
I couldn't reply to this thread https://www.physicsforums.com/threads/how-does-qft-handle-non-locality.849972/page-3

atyy wrote in message number 58:

"When A measures u, then the state will immediately collapse to |uu>, so B will measure u with certainty. But can B tell that A made a measurement? He cannot, because if A always measures before B, A will collapse the state to |uu> half the time and to |dd> the other half of the time. But if A measures after B, then B will measure u half the time and d half the time. So although taking collapse as reality will violate relativistic causality as something real, collapse does not lead to any superluminal communication. This is why collapse is consistent with "no superluminal signalling".

The quantum correlations are a subset of no-signalling, and the relativistic causality correlations are a subset of the quantum correlations. Quantum mechanics including collapse violates relativistic causality as something real, but it does not violate no signaling"

Does this mean it is perfectly all right to treat wave function collapse as objective as long as it doesn't violate no signaling meaning "no superluminal signalling" and yet it is ok to violate relativistic causality where causality means measurements at spacelike separated events must commute (meaning their results cannot depend on the order in which they are performed). So the probabilistic nature of the collapse can violate causality yet doesn't violate "no superluminal signaling"..

In other words. It is ok to violate causality as long as there is no information transferred? What is the consensus about this? Is this how the justification for those who want to treat collapse as objective?

This is all very confusing. The thread is left hanging. If collapse can be real.. how do you deal with Wigner's Friend situation. Maybe they meant that in QFT, there is no Wigner Friend situation occurring? So only QFT can have collapse?

 

[Nugatory]

This is all very confusing. The thread is left hanging. If collapse can be real.. how do you deal with Wigner's Friend situation?

Wigner's Friend illustrates a problem with the idea that consciousness causes collapse, not with collapse. Collapse can be real but not caused by consciousness, and then there is no problem with Wigner's Friend.

 

[fanieh]

Wigner's Friend illustrates a problem with the idea that consciousness causes collapse, not with collapse. Collapse can be real but not caused by consciousness, and then there is no problem with Wigner's Friend.

Let's say Wigner friend is doing Gerlach-Stern experiment.. if the electron is spin up. He is happy. If electron is spin down. He is sad. So he is in superposition of spin up/happy and spin down/sad. Now Wigner watches his friend doing the experiment. Will Wagner see the superposition. If yes. Does it mean Wigner friend is in real superposition.. but Wigner friend doesn't experience any superposition. So Wigner friend is not related to the idea of consciousness cause collapse. Or to use wiki https://en.wikipedia.org/wiki/Wigner's_friend :

"The thought experiment posits a friend of Wigner who performs the Schrödinger's cat experiment after Wigner leaves the laboratory. Only when he returns does Wigner learn the result of the experiment from his friend, that is, whether the cat is alive or dead. The question is raised: was the state of the system a superposition of "dead cat/sad friend" and "live cat/happy friend," only determined when Wigner learned the result of the experiment, or was it determined at some previous point?"

So Collapse can't be real. Unless you take into account Objective Collapse where superposition won't grow in size.. "According to objective collapse theories, wave function collapse occurs when a superposed systems reaches a certain objective threshold of size, complexity etc. Objective collapse proponents would expect a system as macroscopic as a cat to have collapsed before the box was opened, so the question of observation-of-observers does not arise for them."
Or you take MWI. This means Collapse can't be real unless there is either Objective Collapse or MWI, right? But if collapse can still be real. Without objective collapse and MWI.. how do you deal with the Wigner friend situation?

 

[PeterDonis]

Thread closed for moderation.