Copenhagen - What qualifies as "measurement" and "observer"?

[bhobba]

But that again seems to promote the observer to special status, because if the wave function is not real, then the criterion for reality is again subjective.

I knew it was going down this path. It really is philosophy which I find rather useless semantic sophistry. The key point is if an object always has a certain property if you observe it there is no difference to it actually having that property. Because its assumed to be a proper mixed state it is indistinguishable from actually having that property and being part of an objective world existing independent of observation

Thanks
Bill

 

[atyy]

I knew it was going down this path. It really is philosophy which I find rather useless semantic sophistry. The key point is if an object always has a certain property if you observe it there is no difference to it actually having that property. Because its assumed to be a proper mixed state it is indistinguishable from actually having that property and being part of an objective world existing independent of observation.

But could you clearly answer the question: doesn't what you are saying give the observer special status, exactly the same as in Copenhagen?

 

[bhobba]

But could you clearly answer the question: doesn't what you are saying give the observer special status, exactly the same as in Copenhagen?

No - and obviously so.

Thanks
Bill

 

[atyy]

No - and obviously so.

It doesn't seem obvious to me. The basic problem is if the wave function is subjective - how can objective reality emerge from something subjective?

 

[bhobba]

It doesn't seem obvious to me. The basic problem is if the wave function is subjective - how can objective reality emerge from something subjective?

The properties revealed by observation are very real. If a state is in an eigenstate of an observable it has that property regardless of observation. Since it is assumed to be in a proper mixed state it actually has that property prior to observation regardless of if you observe it or not.

Thanks
Bill

 

[atyy]

The properties revealed by observation are very real. If a state is in an eigenstate of an observable it has that property regardless of observation. Since it is assumed to be in a proper mixed state it actually has that property prior to observation regardless of if you observe it or not.

But in that case, isn't the wave function real? Also, isn't collapse real?

 

[bhobba]

But in that case, isn't the wave function real? Also, isn't collapse real?

All the state is is a codification of the results of observations. If it is in an eigenstate it actually has that property - that's what the state tells us.

Please use state instead of wavefunction - its much clearer.

Thanks
Bill

 

[Jimster41]

But the consensus is 1 and 2 are basically solved. Its only on this forum you get these long threads about this fringe stuff like the factorisation and similar issues. The book above acknowledges them but puts them in much better perspective.

From this somewhat confusing thread.
https://www.physicsforums.com/threads/experimental-verification-of-matter-waves.824377/

...the following paper seems less confusing, and seems to state pretty clearly that 2 is not proved? But maybe I'm misunderstanding how linear superposition of unobserved states, when measured, results in non-linear histories.
http://arxiv.org/abs/1410.0270

Testing the limits of quantum mechanical superpositions
Markus Arndt, Klaus Hornberger
(Submitted on 1 Oct 2014)
Quantum physics has intrigued scientists and philosophers alike, because it challenges our notions of reality and locality--concepts that we have grown to rely on in our macroscopic world. It is an intriguing open question whether the linearity of quantum mechanics extends into the macroscopic domain. Scientific progress over the last decades inspires hope that this debate may be decided by table-top experiments.
Comments: 16 pages, 4 Figures; published version differs by minor editorial changes
Subjects: Quantum Physics (quant-ph)
Journal reference: Nature Physics 10, 271 (2014)
DOI: http://arxiv.org/ct?url=http%3A%2F%2Fdx.doi.org%2F10%252E1038%2Fnphys2863&v=986986e6
Cite as: arXiv:1410.0270 [quant-ph]
(or arXiv:1410.0270v1 [quant-ph] for this version)

 

[bhobba]

...the following paper seems less confusing, and seems to state pretty clearly that 2 is not proved?

Note the use of reasonable. They don't think its reasonable - fine - I won't argue semantically about reasonable. Others, me included, think its reasonable.

If you want to understand the use of reasonable here then study the reference I quoted which is a standard text on the issue - IMHO THE standard text.

You are wanting a definitive answer to a subtle issue - sorry by the definition of subtle you won't get it.

Thanks
Bill

 

[atyy]

All the state is is a codification of the results of observations. If it is in an eigenstate it actually has that property - that's what the state tells us.

Please use state instead of wavefunction - its much clearer.

So the state is not real, collapse is not real, but the outcome after collapse is real?

OK, perhaps it is just semantics. What I would say is that if the outcome is real and the observer is not privileged, then the state is real (eg. GRW, CSL). On the other hand, if the outcome is real and the observer is privileged, then the state is not necessarily real (most forms of Copenhagen).

I think this is shown by the interesting thing that one can get very similar equations from real collapse interpretations like GRW or CSL, as well as the continuous measurement formalism from within Copenhagen.

 

[bhobba]

OK, perhaps it is just semantics.

That's all much of this is IMHO.

Thanks
Bill

 

[atyy]

That's all much of this is IMHO.

I'm not sure. If you agree with my analogy with GRW/CSL versus Copenhagen Continuous Measurement, then in the former we can have a state of the universe including the observer (ie. quantum mechanics without observers), while in the latter there is no meaning to the state of the universe.

Also, GRW and CSL and Bohmian Mechanics, as I understand, do eventually lead to deviations from quantum mechanics, which I think can be in principle tested (maybe even in practice as discussed by the link http://arxiv.org/abs/1410.0270 posted by Jimster41).

 

[bhobba]

It is an intriguing open question whether the linearity of quantum mechanics extends into the macroscopic domain.

The key point of assuming its a proper mixture is there is some process that makes it a proper mixture - that linearity breaks down is one way to explain it - but not the only one.

Thanks
Bill

 

[atyy]

The key point of assuming its a proper mixture is there is some process that makes it a proper mixture - that linearity breaks down is one way to explain it - but not the only one.

The idea is that so far assuming one world, if one is to seriously solve the factorization problem as BM, GRW and CSL try to do, then the linearity does break down. This is why the factorization problem is stressed by some for the emergence of classical reality without a privileged status for observers.

 

[Jimster41]

I'm not sure. If you agree with my analogy with GRW/CSL versus Copenhagen Continuous Measurement, then in the former we can have a state of the universe including the observer (ie. quantum mechanics without observers), while in the latter there is no meaning to the state of the universe.

Also, GRW and CSL and Bohmian Mechanics, as I understand, do eventually lead to deviations from quantum mechanics, which I think can be in principle tested (maybe even in practice as discussed by the link http://arxiv.org/abs/1410.0270 posted by Jimster41).

Just want to mention that @bhobba (as well as others) pointed me to that paper.