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

[atyy]

What's in dispute is the role of conciousness. I say its exactly the same as in any theory - theories require a reasonably intelligent conscious entity to apply them - Atty thinks for QM it goes deeper than that for reasons I don't quite follow.

Well, the part I don't follow is why you say that the measurement problem is "why is there a definite outcome?". If that is the problem, why not just postulate that there is a definite outcome and the measurement problem will be solved?

 

[bhobba]

Well, the part I don't follow is why you say that the measurement problem is "why is there a definite outcome?". If that is the problem, why not just postulate that there is a definite outcome and the measurement problem will be solved?

That's what I do - with the twist of decoherence that its legitimate to view it as being these prior to observation because its a mixed state.

Thanks
Bill

 

[stevendaryl]

I believe it is. In Copenhagen, the observer does 3 things:
1) choose a factorization, which is usually classical/quantum
2) choose a preferred basis
3) determine when an observation has occurred.

In Copenhagen, these are all subjective. bhobba and I do seem to have a disagreement as to how the measurement problem is stated. To me, the measurement problem is how do you get rid of the observer, but bhobba usually says it is "why is there a definite outcome"?

I see the measurement problem differently. Here's the way I would say it:

Suppose you have a system described by a state |\psi\rangle and you have some variable A. Then you can rewrite |\psi\rangle in terms of states with definition values for A as follows:

|\psi\rangle = \sum_\alpha C_\alpha |\psi_{A \alpha}\rangle

where |\psi_{A \alpha}\rangle is the normalized projection of |\psi\rangle onto the subspace in which A has eigenvalue \alpha.

Then having written |\psi\rangle this way, we would like to say that variable A for that system has value \alpha with probability |C_\alpha|^2. That's the Born rule, essentially.

However, we can also choose a different observable B and write

|\psi\rangle = \sum_\beta D_\beta |\psi_{B \beta}\rangle

where \psi_{B \beta} is a normalized state in which B has value \beta.

However, various no-go results such as theorems by Kochen-Specker and Bell show that it is not consistent to suppose that EVERY physical variable has a value simultaneously. So, the Born probability rule can't be taken to simultaneously give the probabilities of all possible variables. It only applies to one variable at a time (or a collection of commuting observables). So somehow some variable is singled out.

According to the Bohm theory, what is singled out is location in configuration space. According to Copenhagen, what is singled out is whichever variable we chose to measure. I don't particularly like that way of putting it, because to me, a measurement is just an interaction like any other, and the measuring device is a physical component like any other.

Rather than separating the physical situation into observed system and measuring device, and saying that the measuring device measures some observable of the observed system, it seems to me that you get the same effect if you just say that

There is a composite system (which includes the observed system and the measuring device). This composite system is described by a composite state. The variable that is singled out is the "pointer values" of the measuring device (if the measuring device has a pointer--otherwise, it's whatever macroscopic quantity the device records).

So to me, there is a quantum mystery, which is: Why are some physical variables singled out to have definite values? But I don't see why observers and measurements and so forth need to be special in QM.

 

[atyy]

That's what I do - with the twist of decoherence that its legitimate to view it as being these prior to observation because its a mixed state.

But it that's what you do, then your interpretation would solve the measurement problem. Yet in other posts you say it doesn't solve the measurement problem.

Which is it?

 

[atyy]

However, various no-go results such as theorems by Kochen-Specker and Bell show that it is not consistent to suppose that EVERY physical variable has a value simultaneously. So, the Born probability rule can't be taken to simultaneously give the probabilities of all possible variables. It only applies to one variable at a time (or a collection of commuting observables). So somehow some variable is singled out.

I think that is only part (2) of the measurement problem - the choice of preferred basis. Before that, one has to choose the Hilbert space, which is equivalent to choosing a classical/quantum cut. After one choose the preferred basis, one has to say when the measurement occurred.

 

[bhobba]

But it that's what you do, then your interpretation would solve the measurement problem. Yet in other posts you say it doesn't solve the measurement problem.

Its obvious. I simply assume it - I don't explain why. BM explains why for example - I don't.

The issue is whether explaining why or not worries you - it doesn't worry me. Axiomatically it makes no difference - but some like certain axioms - others don't.

Thanks
Bill

 

[atyy]

Its obvious. I simply assume it - I don't explain why. BM explains why for example - I don't.

The issue is whether explaining why or not worries you - it doesn't worry me. Axiomatically it makes no difference - but some like certain axioms - others don't.

I don't agree with that. Why should we need to explain why?

The point is to be able to answer questions like - as you sometime state the measurement problem - how does classical reality arise from the laws of quantum mechanics? Practically, can we have a theory of the whole universe, including the observers, since the observers are presumably part of the universe?

If we define the measurement problem in this way, does your interpretation solve it?

 

[bhobba]

I don't agree with that. Why should we need to explain why?

You don't need to explain why. But by the definition of solving the measurement problem you need to explain the three parts I have mentioned in past posts not simply assume it. Decoherence explains the first two - well at least its reasonable to assume it does - there are issues - but not the third part. I simply assume it therefore I have not explained the measurement problem. BM for instance explains all three. Its even easier to see when you understand my interpretation is entirely compatible with BM - as is the statistical interpretation of Ballentine.

Thanks
Bill

 

[atyy]

You don't need to explain why. But by the definition of solving the measurement problem you need to explain the three parts I have mentioned in past posts not simply assume it. Decoherence explains the first two - well at least its reasonable to assume it does - there are issues - but not the third part. I simply assume it therefore I have not explained the measurement problem. BM for instance explains all three. Its even easier to see when you understand my interpretation is entirely compatible with BM - as is the statistical interpretation of Ballentine.

I don't agree. The traditional statement of the measurement problem is "How can we have quantum mechanics without observers?"

 

[bhobba]

I don't agree. The traditional statement of the measurement problem is "How can we have quantum mechanics without observers?"

You are using a different definition than me. I am using the definition I have posted many many times from the following standard text:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

It has three parts:
1. The preferred basis problem.
2. The non observability of interference effects
3. Why we get any outcomes at all - technically how does an improper mixture become a proper one.

Again, as stated clearly in that text, decoherence can reasonably explain the first two. It stands powerless before the third. I simply assume an improper mixture is a proper one - I don't explain it.

Thanks
Bill

 

[atyy]

You are using a different definition than me. I am using the definition I have posted many many times from the following standard text:
https://www.amazon.com/dp/3540357734/?tag=pfamazon01-20

It has three parts:
1. The preferred basis problem.
2. The non observability of interference effects
3. Why we get any outcomes at all - technically how does an improper mixture become a proper one.

Again, as stated clearly in that text, decoherence can reasonably explain the first two. It stands powerless before the third. I simply assume an improper mixture is a proper one - I don't explain it.

That is one of the less standard aspects of the Schlosshauer's text.

One can state the measurement problem in a traditional way with less emphasis on observers, eg. Landau & Lifshitz, Tsirelson, Weinberg: quantum mechanics assumes the existence of a classical world, so classical physics is not a less fundamental theory from which quantum mechanics is derived. In other words, a purely quantum theory appears meaningless or not the most fundamental. What is the more fundamental theory underlying quantum mechanics and the assumed classical reality?

 

[bhobba]

existence of a classical world, so classical physics is not a less fundamental theory from which quantum mechanics is derived.

That's a blemish in Copenhagen and others - not what I call the measurement problem.

That said if you can explain issue 3 then you have explained the emergence of the classical world.

Also its not the only way to fix that blemish.

Thanks
Bill

 

[atyy]

That's a blemish in Copenhagen and others - not what I call the measurement problem.

That said if you can explain issue 3 then you have explained the emergence of the classical world.

Also its not the only way to fix that blemish.

Yes, I do state the measurement problem relative to Copenhagen, because Copenhagen is the only interpretation of quantum mechanics that is universally agreed to work, so it is the definition of quantum mechanics.

Explaining issue 3 is not sufficient to explain the emergence of the classical world. You have to show that the factorization and deviation from diagonal are objective, not subjective.

 

[atyy]

That's a blemish in Copenhagen and others - not what I call the measurement problem.

That said if you can explain issue 3 then you have explained the emergence of the classical world.

Also its not the only way to fix that blemish.

Doesn't your interpretation have this "blemish"?

 

[bhobba]

Doesn't your interpretation have this "blemish"?

No. By interpreting a mixed state as a proper mixed state it exists independent of observation. The classical world emerges from the quantum.

Thanks
Bill

 

[atyy]

No. By interpreting a mixed state as a proper mixed state it exists independent of observation. The classical world emerges from the quantum.

That's what I thought. However, I am skeptical of the claim.

(1) The classical world is objective. I don't think you have shown that the classical world is independent of the choice of F (Factoriation), T (Deviation from diagonal).

(2) In your interpretation, the quantum state is not necessarily real. The classical world is real. How can reality emerge from something subjective?

 

[bhobba]

I don't think you have shown that the classical world is independent of the choice of F (Factoriation), T (Deviation from diagonal).

I was very careful to quote Schlosshauer exactly, and its my view as well. It's REASONABLE to assume decoherence solves 1 and 2. There are issues that need to be resolved like the factorisation problem. There are others as well. If you want to delve further into it check out:
https://www.amazon.com/dp/0691004358/?tag=pfamazon01-20

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.

If you want to say they are a deal breaker - be my guest. I won't argue about it - we have had tons and tons of threads doing that and there is nothing to gain by going over it again and again.

Thanks
Bill

 

[bhobba]

In your interpretation, the quantum state is not necessarily real. The classical world is real. How can reality emerge from something subjective?

But the observation is very real.

Thanks
Bill

 

[atyy]

I was very careful to quote Schlosshauer exactly, and its my view as well. It's REASONABLE to assume decoherence solves 1 and 2. There are issues that need to be resolved like the factorisation problem. There are others as well. If you want to delve further into it check out:
https://www.amazon.com/dp/0691004358/?tag=pfamazon01-20

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.

If you want to say they are a deal breaker - be my guest. I won't argue about it - we have had tons and tons of threads doing that and there is nothing to gain by going over it again and again.

OK, but I would stress a few points.

(1) Your interpretation is not minimal. The criterion of the predictability sieve is already beyond standard quantum mechanics.

(2) The factorization problem is not a fringe issue, and is considered a serious question by eg. Bohmian Mechanics. It is also not considered a fringe problem by Zurek, as quoted by Schlosshauer:

http://arxiv.org/abs/quant-ph/0312059 (p8)
However, the assumption of a decomposition of the universe into subsystems—as necessary as it appears to be for the emergence of the measurement problem and for the definition of the decoherence program—is definitely nontrivial. By definition, the universe as a whole is a closed system, and therefore there are no “unobserved degrees of freedom” of an external environment which would allow for the application of the theory of decoherence to determine the space of quasiclassical observables of the universe in its entirety. Also, there exists no general criterion for how the total Hilbert space is to be divided into subsystems, while at the same time much of what is called a property of the system will depend on its correlation with other systems. This problem becomes particularly acute if one would like decoherence not only to motivate explanations for the subjective perception of classicality (as in Zurek’s “existential interpretation,” see Zurek, 1993, 1998, 2003b, and Sec. IV.C below), but moreover to allow for the definition of quasiclassical “macrofacts.” Zurek (1998, p. 1820) admits this severe conceptual difficulty: "In particular, one issue which has been often taken for granted is looming big, as a foundation of the whole decoherence program. It is the question of what are the “systems” which play such a crucial role in all the discussions of the emergent classicality. (. . . ) [A] compelling explanation of what are the systems—how to define them given, say, the overall Hamiltonian in some suitably large Hilbert space—would be undoubtedly most useful."

 

[atyy]

But the observation is very real.

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.

 

[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.