Assumptions of the Bell theorem

[vanhees71]

This is where I know we have different views. And I also find your position somehow incomplete, but perhaps(?) for a different reason than Demystifier, I can not tell for sure.

As I see it, the born rule defines the agents expectations of a "potential" future measurement. And if you (like me) are into the agent picture, this reflects the actual state of the agent, even WITHOUT measurments.

That's just an interpretation of probabilities, maybe more in a Bayesian than a frequentist approach, but whether you call it "probability" or "expectation", it's about results of measurements. If you don't perform a measurement it's just a number you can believe or not.

I am sure this would have observable consequences in terms of causality, as the agents actions are tuned as per the born rule, BEFORE the measurement takes place, and BEFORE the information update.

Why are the agents' actions tuned as per the Born rule? No matter, how I prepare a system, I can measure whatever observable I like, and the Born rule predicts a probability/expectation for the outcome of this measurement, no more no less.

But I see our perspective too, as the agent view is a specific interpretation, mixed up with idea of howto modiy QM, so i agree it does not belong to the minimal QM. But this is why the minimal itnerpretation seems to be as "fine" in a FAPP sense, yet conceptually incomplete. This is why i find the minimal theory deeply insatisfactory.

Mvh
/Fredrik

I don't understand, what makes it insatisfactory for you.

 

[vanhees71]

I agree. But the discussion should not be restricted to "thermodynamically irreversible" events. Also the emission of a 21 cm photon is an event that QM should be able to describe without introducing "frozen terminology".

QM describes the emission of that photon by predicting a probability for that emission to happen. To measure the photon it has to interact with some device (radio antenna) to be detected as some signal.

 

[gentzen]

Physicists know very well what constitutes a measurement. It's only too philosophy inclined theoreticians that don't agree ;-). SCNR.

I have to admit that I am positively surprised by your sudden coherent account of how you interpret quantum mechanics. I thought I had read many previous posts on this forum, and I somehow had the impression that you just didn't get what everybody else is worried about.

But now suddenly you defend a fully coherent interpretation, basically the orthodox interpretation, but well argued and defended. Did I just miss this before?

Of course it may well be that phycisists know very well what constitutes a measurement. But as long as they don't explicitly lay down and explain what constitutes a measurement (as you do above), how should anybody know whether they really know it, or just believe that they do?

 

[vanhees71]

I always defended the minimal statistical interpretation. Maybe I expressed myself not clear enough before, but I don't know what I say different now. The statistical interpretation is not far from the orthodox view anyway, maybe closest to Bohr's version, but it's hard to tell for me what Bohr really thought, because his writings are so enigmatic and qualitatively philosophical. The only difference to some flavor of Copenhagen (mostly Heisenberg's view, but he is even more enigmatic an philosophical than Bohr) is that it takes the quantum state as a probabilistic and only probabilistic description for the outcome of measurements without the necessity for any collapse nor a "quantum-classical" cut.

Where I strongly differ with the orthodox/minimal view (aka the 7 rules agreed on by the majority in this forum) is only in refusing the collapse/projection postulate as a fundamental generally valid postulate.

 

[Kolmo]

Of course it may well be that phycisists know very well what constitutes a measurement. But as long as they don't explicitly lay down and explain what constitutes a measurement (as you do above), how should anybody know whether they really know it, or just believe that they do?

It's in a good few conceptual texts like Peres or texts on modern measurement theory such as Busch's.

It's just that the theory uses a notion, i.e. "measurement" or "event", that you need early on pedagogically but can only be explained much later in full detail. A full model of measurement unfortunately is hard to give in an undergraduate course. It's a bit like how you discuss the event horizon in an undergraduate GR class, but you need a difficult analysis such as in Wald to properly define the notion.

 

[vanhees71]

But I don't think that you need to bother with all this subtleties if you are not particularly interested in this topic. It's pretty clear what a measurement is. That's why all physics students have to suffer (a theory inclined student really suffers ;-)) the beginners and advanced labs.

 

[Kolmo]

But I don't think that you need to bother with all this subtleties if you are not particularly interested in this topic. It's pretty clear what a measurement is. That's why all physics students have to suffer (a theory inclined student really suffers ;-)) the beginners and advanced labs.

Certainly, the above is only if one wanted to see a full model of measurement. For most it is such a basic idea it doesn't matter.

 

[Demystifier]

The answer to this, in my opinion, is that a projection can of course not be described by a unitary time evolution, and indeed a filter meausurement (or rather a filter preparation) needs a "filter" and thus you have to use the description in terms of an open quantum system to understand the said transition.

The bold part is what confuses me. You are saying that there is something that we can't understand with a full closed system, but can understand with the open system. That's strange, because the closed system contains all the information that the open system does, plus some more. There is nothing in the open system that, in principle, cannot also be understood with the closed system. I guess you are saying that with open system, which contains less degrees of freedom, it is simpler to extract relevant information in practice. I'm fine with that, but I am interested in what can be done in principle. So in principle, is it possible to understand filter measurement in the closed system? If yes, how? If no, then something seems to be missing in the closed system, which is a problem in principle (even if not a problem in practice).

 

[Demystifier]

Physicists know very well what constitutes a measurement. It's only too philosophy inclined theoreticians that don't agree ;-). SCNR.

Physicists know very well that wave function collapses by measurement. It's only too philosophy inclined theoreticians that don't agree ;-). SCNR.

 

[vanhees71]

Physicists know very well that wave function collapses by measurement. It's only too philosophy inclined theoreticians that don't agree ;-). SCNR.

I've never heard an experimentalist saying, "I collapsed the wave function" when measuring something.

 

[Fra]

Why are the agents' actions tuned as per the Born rule?

Not surprisingly I don't know the answer. But it's one of the question that I expect to get explained as a part of a future modified intrinsic measurement theory. In current QM, it's just an axiom/postulate, and thus not begging an explanation.

But I think you are asking for some presumed reason following from the general reasoning, then I expect an answer along the lines of quantum informtion motivation; that it likely represents the optimal inference system of an agent of that complexity - but while standard QM, is the optimal system for "classical agents" (corresponding to the qm/classical cut), one may expect a modificarion of this of course for more general agents.

Meaning the born rule might be seen as emergent, in the sense that while other crazy systems are not banned by nature - agents implementing them are not stable.

The whole "point" of this is then not to reconstruct stadnard QM from an alternative set of axiom - that would be a big effort with minimal added value - the point would be that such a general theory should make progress on other open questions as well, such as unifying all forces including gravity and understand spacetime in the Planck scale, by considering "plack scale" agents, which would be a start contrats to a classical agent, and it's here i also quesion how lighly uncountable numbers are pull into the early reconstruction of a probability theory. Ariel, Cox and other do that as well and i can not digest it.

I don't understand, what makes it insatisfactory for you.

If it's not clear from my previous scattered posts, it's hard to convey perspectives, as we constantly witness on here. My internal motivation makes use to some conjectures about equivalence between laws of physics and rules of inferenece, that is a mix of qbist intepretation + information theoretic approaches but in an evolutionary perspective.

So you see the tip of a diamond, and you are convinced there is a whole iceberg of diamond to be revealed, that is thet kind of dissatisfaction I feel about QM. But perhaps the tip is all there is to it, what do i know.

/Fredrik

 

[Demystifier]

I've never heard an experimentalist saying, "I collapsed the wave function" when measuring something.

I've never heard an experimentalist saying, "There is no collapse, because it contradicts locality of QFT."

 

[vanhees71]

The bold part is what confuses me. You are saying that there is something that we can't understand with a full closed system, but can understand with the open system. That's strange, because the closed system contains all the information that the open system does, plus some more. There is nothing in the open system that, in principle, cannot also be understood with the closed system. I guess you are saying that with open system, which contains less degrees of freedom, it is simpler to extract relevant information in practice. I'm fine with that, but I am interested in what can be done in principle. So in principle, is it possible to understand filter measurement in the closed system? If yes, how? If no, then something seems to be missing in the closed system, which is a problem in principle (even if not a problem in practice).

I think you completely misunderstand me. I don't know, how I can describe my point of view differently than I have done already. For sure I seem not to understand what you mean by "measurement problem", because you insist that it should be solvable by treating a closed system.

 

[vanhees71]

Not surprisingly I don't know the answer. But it's one of the question that I expect to get explained as a part of a future modified intrinsic measurement theory. In current QM, it's just an axiom/postulate, and thus not begging an explanation.

But you must know, what you want to say. I wanted to know you mean by "the agents' actions tuned as per the Born rule". I've no clue what this means. I guess by "agent" you mean "experimentalist" doing a measurement. Why should he be tuned somehow by Born's rule?

 

[Fra]

I've never heard an experimentalist saying, "I collapsed the wave function" when measuring something.

I think they say "Eureka", but it means the same thing.
/Fredrik

 

[Demystifier]

For sure I seem not to understand what you mean by "measurement problem", because you insist that it should be solvable by treating a closed system.

That's because you think about measurement from a practical point of view, while I think from "in principle" point of view. But I wouldn't insist on using "in principle" perspective if you didn't use "in principle" perspective in another related topic, which is the collapse. You use "in principle" argument (locality of QFT) that collapse cannot exist, but practical argument (open system) that filtering exists. What frustrates me that you can't talk about both from the same perspective. But of course, my frustration is my problem, not yours.

 

[Fra]

But you must know, what you want to say. I wanted to know you mean by "the agents' actions tuned as per the Born rule". I've no clue what this means. I guess by "agent" you mean "experimentalist" doing a measurement. Why should he be tuned somehow by Born's rule?

Yes sorry, I try to strike a balance here to avoid violating the forum rules. Things would have been easier if my perspective was cleanly fitting into one of the major approches.

What I meant by tuned agent is this:

In my view and interpretation, the agent, is the abstraction for the system that encodes and processes the information from interactions, and from what infers an expectation of the future, which in turn is used to guide the agents actions/decisions. Here the "guiding probability" is the interpretation of probability from qbism, where the causal implication of the expectations, is on the agents own actions. This means that, from the perspective of another agent, the first agents "reaction" to a perturbation, would be dependent on it's own "prejudices" PRIOR to the interaction; and this expectation would then be in compliance with born rule. Essentially I bundle all the properties of a generalized probability theory, into the agent structure, except for the measurement part - which involves interactions between other observers. (All the abov can be phrased in a somewhat mathematical way, except it's not my focus, so i find it useless to go anal at this point when concepts are still beeing pondered on)

For example, Alice shold be able to infer what Bob's prjudices/priors are, from observing his choice of preparations. When it comes to HUMAN observes, things obviously get complex, so its' not a good example, but a "natural" agent, should CHOOSE his next measuerements for maximal inferences! Ie. you ask the question that will gain you the most. This is why an agents choices are not independent of it's priors. That a human observer has freedom to make apparently free choices is a different story. The more realistic example involving human agents in this case would not be physicists, it would be a gaming setup, say agents that place bets and compete against othre agents in economic games. No strategy is forbidden, but "naturally abundant" strategies are bound to follow some emergent rules. So the "freedom of choice" comes at a price, if you want to stay in the game, you can not make stupid choices.

Then what the agent means in practice is, for regular QM, it would mean any classical system with the ability to react leaving an imprint for others to read off. It could be an experimenter, or a macroscopic chunk of matter. In my extended interpretation OTOH, it can be any physical system that can encode information (need not be macroscopic). The way to characterise the state spaces for such agents is of course highly speculative.

I'm not sure if that made it clear? but in thise sene the logic containing the born rule, is as i see it manifest inside agents microstructure, even in between measurements. The key I think is to distinguish between the "guiding probability" and the "observed probability". For a theorist they are of course supposed to match. But in my view, for a real agent (seen as participating in interactions) the whole points, that gives non-trivial phenomena is that they will NOT coincide. The constant information updates and guiding probabilites seem to have the promise to generate interactions rules on their own. This the the tip of the diamond I see, and I think here is more to unravel.

/Fredrik

 

[PeterDonis]

A neutral hydrogen atom in interstellar space is the best example of a closed quantum system.

Not if it radiates. See below.

Do we really need to introduce a radio astronomer and his dish to discuss the 21 cm radiation?

No, but you do need to introduce the electromagnetic field, which makes your neutral hydrogen atom an open system, not a closed one.

 

[PeterDonis]

the closed system contains all the information that the open system does, plus some more.

Depends on which closed system and which open system. To use the example I just responded to in my previous post, a neutral hydrogen atom in interstellar space, considered as a closed system, contains less information than that atom, considered as an open system, radiating 21 cm radiation (and therefore interacting with the EM field).

 

[Demystifier]

Depends on which closed system and which open system.

Of course, I meant closed system including the measuring apparatus and the environment.

 

[vanhees71]

That's because you think about measurement from a practical point of view, while I think from "in principle" point of view. But I wouldn't insist on using "in principle" perspective if you didn't use "in principle" perspective in another related topic, which is the collapse. You use "in principle" argument (locality of QFT) that collapse cannot exist, but practical argument (open system) that filtering exists. What frustrates me that you can't talk about both from the same perspective. But of course, my frustration is my problem, not yours.

I indeed don't see what one issue as to do with the other. Of course you can describe open quantum systems also within QFT. I also don't see, why you insist that open quantum systems are less "fundamental" than closed ones.

 

[vanhees71]

Of course, I meant closed system including the measuring apparatus and the environment.

But that is precisely which cannot solve the "measurement problem" in its usual form, i.e., why you get unique measurement results when measuring an observable that's not determined by the state preparation.

 

[Demystifier]

I also don't see, why you insist that open quantum systems are less "fundamental" than closed ones.

Because the open system is a subsystem of the full closed system. Hence the properties of the open system can be derived from the properties of the closed system, and not the other way around.

 

[vanhees71]

Yes, but you cannot describe it in all detail, and trying so misses the solution of the measurement problem you look for. To understand macroscopic objects you need coarse graining to get an adequate description. This holds also true within classical physics. There's no way to understand phenomena like friction and dissipation, approach of thermodynamic equilibrium, the "arrow time" and all such phenomena by considering only "closed systems". As Anderson famously put it: "more is different".

 

[Demystifier]

But that is precisely which cannot solve the "measurement problem" in its usual form

Great, we finally agree that a closed system cannot resolve the measurement problem. What we disagree is that you think that an open system (which, in my understanding, is a subsystem of the full closed system) can resolve it.

 

[vanhees71]

Ok, then you think quantum theory is incomplete and you need some extension of it to solve the measurement problem. That's a legitimate opinion but there's no empirical evidence whatsoever hinting at the necessity of such an extension, let alone a clue in which direction one should seek for it.

 

[Demystifier]

There's no way to understand phenomena like friction and dissipation, approach of thermodynamic equilibrium, the "arrow time" and all such phenomena by considering only "closed systems".

I disagree.

As Anderson famously put it: "more is different".

He didn't say "coarse grained is different".

 

[vanhees71]

Well, if condensed matter physicists wouldn't "coarse grain", they couldn't understand anything of their subject ;-).

 

[Demystifier]

Ok, then you think quantum theory is incomplete and you need some extension of it to solve the measurement problem. That's a legitimate opinion but there's no empirical evidence whatsoever hinting at the necessity of such an extension, let alone a clue in which direction one should seek for it.

I agree that there is no empirical evidence, but I think there is a logical evidence. It's the logic that if something cannot be explained by considering the full closed system (on which we agree), then it also cannot be explained by considering its open subsystem.

 

[Demystifier]

Well, if condensed matter physicists wouldn't "coarse grain", they couldn't understand anything of their subject ;-).

Something they would. For instance, energy conservation in a closed system is a general theorem which is valid for any number of atoms and does not depend on coarse graining. Likewise, the principle that closed system QM cannot produce definite outcomes is another such general theorem.

 

[vanhees71]

Of course, if you draw wrong conclusions, you are in danger to fight against windmills!

 

[Kolmo]

Something they would. For instance, energy conservation in a closed system is a general theorem which is valid for any number of atoms and does not depend on coarse graining. Likewise, the principle that closed system QM cannot produce definite outcomes is another such general theorem.

Yes but measurements by necessity involve open systems of a certain form so that you get an irreversible process resulting in a measurement result/event that quantum theory assigns a probability to.

 

[Demystifier]

Yes but measurements by necessity involve open systems

Not necessarily. If you include the measuring apparatus as a part of the closed system, then measurement can be described by the closed system.

 

[Kolmo]

Not necessarily. If you include the measuring apparatus as a part of the closed system, then measurement can be described by the closed system.

Then when you look at the macroscopic coarse grained collective coordinates within that closed system you see they don't show interference terms and that the evolution is irreversible and thus you have a measurement result.

*Not due to some "breakdown" of quantum theory though, before anybody starts suggesting that.

 

[Demystifier]

Then when you look at the macroscopic coarse grained collective coordinates within that closed system you see they don't show interference terms and that the evolution is irreversible and thus you have a measurement result.

Coarse graining indeed explains how the interference terms apparently disappear and how the evolution becomes apparently irreversible. However, and this is the crucial point, it does not explain how the measurement results appear. See e.g. my http://thphys.irb.hr/wiki/main/images/5/50/QFound3.pdf pages 21-22.