GR/StatMech/QM foundations, epistemic views only please

[atyy]

atyy's last point might relate to what I don't understand about QBism. The problem I pointed out was that Alice and Bob see "nonlocality" when they construct the M4 depiction of the experiment after exchanging measurement outcomes. That problem exists in the realm "we are agents." If Bob, say, rather sticks to "I am an agent" and records only what he observes (to include his observation of Alice's results sent to him in null or time-like fashion), and he doesn't bother to put her results in an M4 depiction to give credence to her as an agent, then he has no ontological basis for nonlocality. Am I on the right track?

Yes. I believe that QBism can in some sense consistently assert locality, as long as it expunges statements like "We are agents". In the technical sense, Bell nonlocality has to do with P(a,b|λ), where a and b are classical outcomes of measurements. If one denies that b "exists" far away, then one cannot form the Bell inequality. Now one also cannot then define locality by satisfaction of the inequality. However, one can use other definitions like the existence of a classical M4. So the solution is one agent, one classical M4, one quantum wavefunction for everything else except the agent. I think this is consistent.

 

[RUTA]

But then can you consistently say why the nonlocality in GR is not an ontological problem while that in quantum mechanics or de Broglie-Bohm theory is?

I assume you mean the nonlocality of Newtonian gravity. But, yes, the nonlocality of dBB is a theoretical problem, not an ontological problem, exactly in the same fashion. If I said otherwise, let me correct it now. The instrumentalists who aren't concerned with theoretical integrity would only care about dBB predictions that vary from standard QM. Again, their leit motif is "If the theory predicts what we observe, then I'm happy. Just tell me where it's applicable and how to use it."

 

[RUTA]

Yes. I believe that QBism can in some sense consistently assert locality, as long as it expunges statements like "We are agents". In the technical sense, Bell nonlocality has to do with P(a,b|λ), where a and b are classical outcomes of measurements. If one denies that b "exists" far away, then one cannot form the Bell inequality. Now one also cannot then define locality by satisfaction of the inequality. However, one can use other definitions like the existence of a classical M4. So the solution is one agent, one classical M4, one quantum wavefunction for everything else except the agent. I think this is consistent.

Dude, I think you nailed it. What does marcus have to say?

 

[atyy]

I assume you mean the nonlocality of Newtonian gravity. But, yes, the nonlocality of dBB is a theoretical problem, not an ontological problem, exactly in the same fashion. If I said otherwise, let me correct it now. The instrumentalists who aren't concerned with theoretical integrity would only care about dBB predictions that vary from standard QM. Again, their leit motif is "If the theory predicts what we observe, then I'm happy. Just tell me where it's applicable and how to use it."

Ah ok, that makes sense, I was confused about what you thought about the ontological status of nonlocality in dBB.

I guess the part where we might still disagree is that I consider the achievement of dBB to be the solution of the measurement problem. It is true that Bell nonlocality in the orthodox interpretation of QM is operationally defined, but it inherits that from the classical/quantum split which is within QM logically prior to the Bell nonlocality of QM. I don't think the violation of the Bell inequality requires instrumentalism, since dBB also violates Bell inequalities.

 

[atyy]

Yes. I believe that QBism can in some sense consistently assert locality, as long as it expunges statements like "We are agents". In the technical sense, Bell nonlocality has to do with P(a,b|λ), where a and b are classical outcomes of measurements. If one denies that b "exists" far away, then one cannot form the Bell inequality. Now one also cannot then define locality by satisfaction of the inequality. However, one can use other definitions like the existence of a classical M4. So the solution is one agent, one classical M4, one quantum wavefunction for everything else except the agent. I think this is consistent.

Dude, I think you nailed it. What does marcus have to say?

Well, I'm glad you agree for the moment. I should point out this was all said by Einstein many years ago. http://arxiv.org/abs/quant-ph/0509061 (section 6 on p9)

I should also say that it has been argued that Einstein preferred an epistemic interpretation of the wave function. Of course, an epistemic interpretation does not have to deny the existence of hidden variables. In fact one class of epistemic interpretations is defined using hidden variables.
http://arxiv.org/abs/0706.2661

 

[RUTA]

I guess the part where we might still disagree is that I consider the achievement of dBB to be the solution of the measurement problem.

I think dBB provides a solution to the MP. There are others, e.g., Many Worlds. There's just no consensus among foundationalists as to which is best.

I don't think the violation of the Bell inequality requires instrumentalism, since dBB also violates Bell inequalities.

I agree. If I led you to believe otherwise, let me correct that now. In general, foundationalists abhor instrumentalism. In fact, I frequently have to defend my own interpretation against the claim that it's "merely instrumentalism."

 

[RUTA]

Well, I'm glad you agree for the moment. I should point out this was all said by Einstein many years ago. http://arxiv.org/abs/quant-ph/0509061 (section 6 on p9)

I hadn't seen this view put quite this way. The way I've always seen it stated is a form of "non-separability." That is, the pair of detectors with their space-like separated outcomes are both real, but not truly ... distinct from one another. The correlated, space-like separated outcomes evidence the manner by which the two detectors are "not separate" or "connected" by the source. But, both are equally real. It's interesting to consider Einstein as a QBist

 

[marcus]

Hi RUTA, Atyy,
I've just been rehearsing a great pecs of choral music (Mendelsohn "Elijah") that took much of the afternoon.

Yes. I believe that QBism can in some sense consistently assert locality, as long as it expunges statements like "We are agents"...

Just got back. I think Atyy you can take a statement like "We are all agents" (i.e. in the same application of QM, the app that is being interpreted) as either EXPUNGED or as never having been made.

Talking informally in ordinary English you could say something like "Each of us fills the role of agent in our separate individual application of QM. But that is not reflected in the formal structure of the model. In the formal structure, which is what we are talking about and interpreting, there is one agent.

 

[marcus]

This paper came out recently and may be relevant to the QBist (quantum bayesian) discussion:

http://arxiv.org/abs/1403.1146
Why I am not a QBist
Louis Marchildon
(Submitted on 5 Mar 2014)
Quantum Bayesianism, or QBism, is a recent development of the epistemic view of quantum states, according to which the state vector represents knowledge about a quantum system, rather than the true state of the system. QBism explicitly adopts the subjective view of probability, wherein probability assignments express an agent's personal degrees of belief about an event. QBists claim that most if not all conceptual problems of quantum mechanics vanish if we simply take a proper epistemic and probabilistic perspective. Although this judgement is largely subjective and logically consistent, I explain why I do not share it...

 

[RUTA]

I have the same complaint with QBism -- how can it be used to advance theoretical physics? If you rather assume there is a physical state that needs to be found, then you're at least looking. As Weinberg said about unification -- "It may be there is no simple underlying theory, but if we don't at least assume such a theory exists, we'll never find it if it does."

 

[strangerep]

I notice that QBists / epistemists use phrases like "the wave function corresponds to (lack of) knowledge" in an agent's mind.

But... what then is the Hilbert space? Is it the set of possible configurations of one's ignorance?

If so, then why do we measure only half-integral values for the spins of elementary particles? Surely my ignorance didn't change the classical continuous spectrum into the discrete half-integral spectrum of QM.

IMHO, this fact makes QBist/epistemic interpretations untenable.

 

[atyy]

I notice that QBists / epistemists use phrases like "the wave function corresponds to (lack of) knowledge" in an agent's mind.

But... what then is the Hilbert space? Is it the set of possible configurations of one's ignorance?

If so, then why do we measure only half-integral values for the spins of elementary particles? Surely my ignorance didn't change the classical continuous spectrum into the discrete half-integral spectrum of QM.

IMHO, this fact makes QBist/epistemic interpretations untenable.

In some epistemic interpretations, the physical/epistemic distinction is made as follows.

If the wave function is such that knowing the microscopic state of hidden variables determines the the wave function uniquely, then the wave function is "physical" or "ontic".

But if the wave function is such that the microscopic state of hidden variables does not determine it uniquely, then the wave function is "epistemic".

There are tight constraints on epistemic theories, and there are no hidden variable theories such that the wave function is maximally epistemic. However strong arguments suggest that there are hidden variable theories in which the wave function is epistemic. http://arxiv.org/abs/1303.2834

Rather interestingly, Ballentine has just written a paper defending epistemic views http://arxiv.org/abs/1402.5689.

 

[RUTA]

I notice that QBists / epistemists use phrases like "the wave function corresponds to (lack of) knowledge" in an agent's mind.

But... what then is the Hilbert space? Is it the set of possible configurations of one's ignorance?

If so, then why do we measure only half-integral values for the spins of elementary particles? Surely my ignorance didn't change the classical continuous spectrum into the discrete half-integral spectrum of QM.

IMHO, this fact makes QBist/epistemic interpretations untenable.

IMO, there's nothing wrong with an epistemic view as long as it comes with an ontic view. QBism doesn't say anything about a corresponding ontic view, so I just don't find it interesting per se.

 

[strangerep]

In some epistemic interpretations, the physical/epistemic distinction is made as follows.

If the wave function is such that knowing the microscopic state of hidden variables determines the the wave function uniquely, then the wave function is "physical" or "ontic".

But if the wave function is such that the microscopic state of hidden variables does not determine it uniquely, then the wave function is "epistemic".

Hmm,... but,... I don't see how this really addresses my point: which was about the range of possible values of those physical variables which are not hidden.

Or is the spectrum of values of non-hidden variables determined by a conspiracy among the hidden variables?

There are tight constraints on epistemic theories, and there are no hidden variable theories such that the wave function is maximally epistemic. However strong arguments suggest that there are hidden variable theories in which the wave function is epistemic. http://arxiv.org/abs/1303.2834

Rather interestingly, Ballentine has just written a paper defending epistemic views http://arxiv.org/abs/1402.5689.

I read Ballentine's paper shortly after it appeared and was surprised by the initial tone, which did indeed seem to be (partially) defending epistemic views. But, as I got further into it, his tone seemed to change. I got the impression he was more interested in exploring rigorous models within which such questions can be discussed more transparently, and also to clarify distinctions between (e.g.,) "epistemic" and "subjective" -- cf. the diagram in fig 2 on p8.

Indeed, he writes:

For the record, my own writings on this subject are firmly in the classes of ensemble and objective. So far, I maintain an open mind regarding ontic versus epistemic.

I take this to mean that he has not had a fundamental change of mind, but rather is clarifying some subtleties in the terminology to make them more accessible to physicists who cannot tolerate the usual waffle of philosophy.

IMO, there's nothing wrong with an epistemic view as long as it comes with an ontic view. [...]

If Ballentine's diagram is correct, at least this means the interpretation is necessarily objective.

 

[atyy]

If Ballentine's diagram is correct, at least this means the interpretation is necessarily objective.

I don't think so. In dBB the wave function is ontic, but because dBB is probabilistic it can be subjective (Ballentine's diagram is missing a line from ontic to subjective).

 

[strangerep]

I don't think so. In dBB the wave function is ontic, but because dBB is probabilistic it can be subjective (Ballentine's diagram is missing a line from ontic to subjective).

You should email Ballentine and let him know.